🔖 release 0.8.1

Add deterministic QuickCheck properties for bounded history retention, queued overflow/drain behavior, patch composition, and logger config round-trips. Local verification passed across native, wasm, wasm-gc, and js targets; GitHub Linux/macOS jobs were blocked by Moon registry network failures during Update Moon registry.

.gitattributes

@@ -0,0 +1,2 @@
+# Normalize repository text files and check them out with LF on every platform.
+* text=auto eol=lf

README.md

@@ -17,7 +17,7 @@ BitLogger 的文档按使用路径组织：先完成一个可运行的日志输
 ```bash
 moon new log-demo
 cd log-demo
-moon add Nanaloveyuki/BitLogger@0.8.0
+moon add Nanaloveyuki/BitLogger@0.8.1
 ```
 
 ### 2. 写入第一条结构化日志

docs/README-en.md

@@ -18,7 +18,7 @@ The documentation is organized around complete usage flows. Start with an execut
 ```bash
 moon new log-demo
 cd log-demo
-moon add Nanaloveyuki/BitLogger@0.8.0
+moon add Nanaloveyuki/BitLogger@0.8.1
 ```
 
 ### 2. Import And Log

docs/api/file-sink-async.md

@@ -0,0 +1,65 @@
+---
+name: file-sink-async
+group: api
+category: sink
+update-time: 20260825
+description: Create a file sink from an async context using the native async filesystem interface.
+key-word:
+    - file
+    - sink
+    - async
+    - public
+---
+
+## File-sink-async
+
+Create a `FileSink` from an async context. The constructor opens the file with the native `moonbitlang/async/fs` interface and keeps the same rotation, policy, and failure-counter surface as `file_sink(...)`.
+
+### Interface
+
+```moonbit
+pub async fn file_sink_async(
+  path : String,
+  append~ : Bool = true,
+  auto_flush~ : Bool = true,
+  rotation~ : FileRotation? = None,
+  formatter~ : RecordFormatter = fn(rec) { format_text(rec) },
+) -> FileSink {}
+```
+
+#### input
+
+- `path : String` - Destination file path.
+- `append : Bool` - Whether writes append rather than truncate on the first open.
+- `auto_flush : Bool` - Whether each async write requests an async file sync.
+- `rotation : FileRotation?` - Optional size-based rotation policy.
+- `formatter : RecordFormatter` - Formatter used to render each record.
+
+#### output
+
+- `FileSink` - A file sink ready for use by an async logger or direct async sink writes.
+
+### Explanation
+
+- Use this constructor when the caller is already running inside an async event loop.
+- File writes, flushes, file-size checks, renames, removals, and rotation reopening use the async filesystem path.
+- The returned sink implements `Sink::write_async(...)`; the synchronous `write(...)` surface remains available for synchronous callers.
+- On non-native targets, the sink follows the existing unavailable-backend behavior.
+
+### How to Use
+
+```moonbit
+let sink = file_sink_async("app.log", auto_flush=false)
+let logger = async_logger(sink)
+```
+
+### Error Case
+
+- If the initial async open fails, the returned sink is unavailable and increments `open_failures()`.
+- Later async writes record write and rotation failures through the existing sink counters.
+
+### Notes
+
+1. Use `file_sink(...)` for synchronous construction outside an async event loop.
+
+2. Pair this API with `native_files_supported()` when code must also compile for non-native targets.

docs/api/file-sink.md

@@ -98,3 +98,5 @@ e.g.:
 3. Non-native targets can still compile code referencing this API, but callers should treat actual file availability and successful writes as target-sensitive runtime behavior.
 
 4. See [target-verification.md](./target-verification.md) for the current verification boundary between design intent and locally re-checked targets.
+
+5. Use [`file_sink_async(...)`](./file-sink-async.md) when constructing a file sink from an async event loop.

docs/api/index.md

@@ -153,6 +153,7 @@ BitLogger API navigation.
 - [patch-sink.md](./patch-sink.md)
 - [patch-sink-type.md](./patch-sink-type.md)
 - [file-sink.md](./file-sink.md)
+- [file-sink-async.md](./file-sink-async.md)
 - [file-sink-available.md](./file-sink-available.md)
 - [file-sink-flush.md](./file-sink-flush.md)
 - [file-sink-close.md](./file-sink-close.md)

docs/changes/1.2.1(0.8.1).md

@@ -0,0 +1,18 @@
+## BitLogger Update Changes
+
+version 1.2.1(0.8.1)
+
+### Changes
+
+- Add deterministic QuickCheck properties for bounded history retention, queued overflow and drain behavior, patch composition, and logger config round-trips.
+
+### Verification
+
+- moon fmt --check
+- moon check --deny-warn
+- moon test --deny-warn across native, wasm, wasm-gc, and js targets
+- pnpm run docs:build
+
+### Notes
+
+- QuickCheck seeds and shrink budgets are fixed for reproducible failures.

docs/changes/index.md

@@ -2,6 +2,7 @@
 
 Versioned BitLogger change summaries.
 
+- [1.2.1](./1.2.1(0.8.1).md)
 - [1.2.0](./1.2.0(0.8.0).md)
 - [1.1.3](./1.1.3(0.7.3).md)
 - [1.1.2](./1.1.2(0.7.2).md)

docs/examples/console.md

@@ -5,7 +5,7 @@ Use this flow for command-line tools and services that need a human-readable sta
 ## Install And Import
 
 ```bash
-moon add Nanaloveyuki/BitLogger@0.8.0
+moon add Nanaloveyuki/BitLogger@0.8.1
 ```
 
 ```moonbit

docs/zh/examples/console.md

@@ -7,7 +7,7 @@
 ```bash
 moon new log-demo
 cd log-demo
-moon add Nanaloveyuki/BitLogger@0.8.0
+moon add Nanaloveyuki/BitLogger@0.8.1
 ```
 
 在应用的 `moon.pkg` 中加入：

moon.mod

@@ -1,9 +1,9 @@
 name = "Nanaloveyuki/BitLogger"
 
-version = "0.8.0"
+version = "0.8.1"
 
 import {
-  "moonbitlang/async@0.20.2",
+  "moonbitlang/async@0.21.0",
 }
 
 readme = "src/README.mbt.md"

src-async/async_logger_shared.mbt

@@ -132,6 +132,7 @@ pub struct AsyncLogger[S] {
   flush_policy : AsyncFlushPolicy
   sink : S
   flush_callback : (S) -> Unit raise
+  flush_async_callback : (async (S) -> Unit)?
   context_fields : Array[@bitlogger.Field]
   filter : (@bitlogger.Record) -> Bool
   patch : @bitlogger.RecordPatch
@@ -160,6 +161,7 @@ pub fn[S] async_logger(
     flush_policy: config.flush,
     sink,
     flush_callback: flush,
+    flush_async_callback: None,
     context_fields: [],
     filter: fn(_) { true },
     patch: @bitlogger.identity_patch(),
@@ -171,6 +173,14 @@ pub fn[S] async_logger(
   }
 }
 
+///|
+fn[S] with_async_flush_callback_internal(
+  logger : AsyncLogger[S],
+  callback : async (S) -> Unit,
+) -> AsyncLogger[S] {
+  { ..logger, flush_async_callback: Some(callback) }
+}
+
 ///|
 fn async_logger_phase_is_closed(phase : AsyncLifecyclePhase) -> Bool {
   match phase {
@@ -535,8 +545,11 @@ pub async fn[S] AsyncLogger::shutdown(
 }
 
 ///|
-fn[S] run_flush_callback(logger : AsyncLogger[S]) -> Unit raise {
-  (logger.flush_callback)(logger.sink)
+async fn[S] run_flush_callback(logger : AsyncLogger[S]) -> Unit {
+  match logger.flush_async_callback {
+    Some(callback) => callback(logger.sink)
+    None => (logger.flush_callback)(logger.sink)
+  }
 }
 
 ///|
@@ -546,7 +559,7 @@ async fn[S : @bitlogger.Sink] run_worker(logger : AsyncLogger[S]) -> Unit {
       err if err is AsyncLoggerClosed => break
       err => raise err
     }
-    logger.sink.write(rec)
+    logger.sink.write_async(rec)
     if logger.pending_count.val > 0 {
       logger.pending_count.val -= 1
     }
@@ -557,7 +570,7 @@ async fn[S : @bitlogger.Sink] run_worker(logger : AsyncLogger[S]) -> Unit {
       }
       match next {
         Some(next) => {
-          logger.sink.write(next)
+          logger.sink.write_async(next)
           if logger.pending_count.val > 0 {
             logger.pending_count.val -= 1
           }
@@ -575,7 +588,7 @@ async fn[S : @bitlogger.Sink] run_worker(logger : AsyncLogger[S]) -> Unit {
           }
           match waited {
             Some(next) => {
-              logger.sink.write(next)
+              logger.sink.write_async(next)
               if logger.pending_count.val > 0 {
                 logger.pending_count.val -= 1
               }
@@ -598,6 +611,7 @@ async fn[S : @bitlogger.Sink] run_worker(logger : AsyncLogger[S]) -> Unit {
 }
 
 ///|
+#warnings("-fragile_catch_all")
 pub async fn[S : @bitlogger.Sink] AsyncLogger::run(
   self : AsyncLogger[S],
 ) -> Unit {
@@ -619,14 +633,20 @@ pub async fn[S : @bitlogger.Sink] AsyncLogger::run(
 pub fn build_async_logger(
   config : AsyncLoggerBuildConfig,
 ) -> AsyncLogger[@bitlogger.RuntimeSink] {
-  let logger = @bitlogger.build_logger(config.logger)
-  async_logger(
+  let logger = @bitlogger.build_logger_for_async(config.logger)
+  let logger = async_logger(
     logger.sink,
     config=config.async_config,
     min_level=logger.min_level,
     target=logger.target,
     flush=fn(sink) { ignore(sink.flush_progress()) },
-  ).with_timestamp(enabled=logger.timestamp)
+  )
+  let flush_async : async (@bitlogger.RuntimeSink) -> Unit = sink => {
+    sink.flush_async()
+  }
+  with_async_flush_callback_internal(logger, flush_async).with_timestamp(
+    enabled=logger.timestamp,
+  )
 }
 
 ///|

src-async/async_native_file_integration_test.mbt

@@ -5,7 +5,7 @@ async test "native async file lifecycle flushes rotates and restarts" {
   } else {
     let path = "logs/bitlogger-async-native-e2e.log"
     let first_flushes : Ref[Int] = Ref(0)
-    let first_sink = @bitlogger.file_sink(
+    let first_sink = @bitlogger.file_sink_async(
       path,
       append=false,
       auto_flush=false,
@@ -47,7 +47,7 @@ async test "native async file lifecycle flushes rotates and restarts" {
     inspect(first_sink.close(), content="true")
 
     let restart_flushes : Ref[Int] = Ref(0)
-    let restarted_sink = @bitlogger.file_sink(
+    let restarted_sink = @bitlogger.file_sink_async(
       path,
       append=true,
       auto_flush=false,

src-async/pkg.generated.mbti

@@ -77,6 +77,7 @@ pub struct AsyncLogger[S] {
   flush_policy : @utils.AsyncFlushPolicy
   sink : S
   flush_callback : (S) -> Unit raise
+  flush_async_callback : (async (S) -> Unit)?
   context_fields : Array[@core.Field]
   filter : (@core.Record) -> Bool
   patch : (@core.Record) -> @core.Record

src/file_runtime/file_sink.mbt

@@ -45,6 +45,7 @@ pub struct FileSink {
   priv default_auto_flush : Bool
   priv rotation : Ref[FileRotation?]
   priv default_rotation : FileRotation?
+  priv deferred : Bool
   priv open_failures : Ref[Int]
   priv write_failures : Ref[Int]
   priv flush_failures : Ref[Int]
@@ -65,7 +66,64 @@ pub fn file_sink(
   rotation? : FileRotation? = None,
   formatter? : RecordFormatter = fn(rec) { @formatting.format_text(rec) },
 ) -> FileSink {
-  let handle = @utils.open_file_handle_internal(path, append)
+  new_file_sink_internal(
+    path,
+    append,
+    auto_flush,
+    rotation,
+    formatter,
+    @utils.open_file_handle_internal(path, append),
+  )
+}
+
+///|
+pub async fn file_sink_async(
+  path : String,
+  append? : Bool = true,
+  auto_flush? : Bool = true,
+  rotation? : FileRotation? = None,
+  formatter? : RecordFormatter = fn(rec) { @formatting.format_text(rec) },
+) -> FileSink {
+  new_file_sink_internal(
+    path,
+    append,
+    auto_flush,
+    rotation,
+    formatter,
+    @utils.open_file_handle_async_internal(path, append),
+  )
+}
+
+///|
+#doc(hidden)
+pub fn file_sink_deferred(
+  path : String,
+  append? : Bool = true,
+  auto_flush? : Bool = true,
+  rotation? : FileRotation? = None,
+  formatter? : RecordFormatter = fn(rec) { @formatting.format_text(rec) },
+) -> FileSink {
+  new_file_sink_internal(
+    path,
+    append,
+    auto_flush,
+    rotation,
+    formatter,
+    @utils.deferred_file_handle_internal(path, append),
+    deferred=true,
+  )
+}
+
+///|
+fn new_file_sink_internal(
+  path : String,
+  append : Bool,
+  auto_flush : Bool,
+  rotation : FileRotation?,
+  formatter : RecordFormatter,
+  handle : FileHandle?,
+  deferred? : Bool = false,
+) -> FileSink {
   {
     path,
     append: Ref(append),
@@ -76,6 +134,7 @@ pub fn file_sink(
     default_auto_flush: auto_flush,
     rotation: Ref(rotation),
     default_rotation: rotation,
+    deferred,
     open_failures: Ref(if handle is Some(_) { 0 } else { 1 }),
     write_failures: Ref(0),
     flush_failures: Ref(0),
@@ -103,6 +162,20 @@ pub fn FileSink::flush(self : FileSink) -> Bool {
   }
 }
 
+///|
+pub async fn FileSink::flush_async(self : FileSink) -> Bool {
+  match self.handle.val {
+    None => false
+    Some(handle) => {
+      let ok = @utils.flush_file_handle_async_internal(handle)
+      if !ok {
+        self.flush_failures.val += 1
+      }
+      ok
+    }
+  }
+}
+
 ///|
 pub fn FileSink::append_mode(self : FileSink) -> Bool {
   self.append.val
@@ -274,6 +347,23 @@ pub fn FileSink::state(self : FileSink) -> FileSinkState {
 pub fn FileSink::reopen(self : FileSink, append? : Bool? = None) -> Bool {
   let append_mode = append.unwrap_or(self.append.val)
   self.append.val = append_mode
+  if self.deferred {
+    match self.handle.val {
+      None => ()
+      Some(handle) => {
+        ignore(@utils.close_file_handle_internal(handle))
+        self.handle.val = None
+      }
+    }
+    let reopened = @utils.deferred_file_handle_internal(self.path, append_mode)
+    self.handle.val = reopened
+    if reopened is Some(_) {
+      true
+    } else {
+      self.open_failures.val += 1
+      false
+    }
+  } else {
     match self.handle.val {
       None => ()
       Some(handle) => {
@@ -289,6 +379,7 @@ pub fn FileSink::reopen(self : FileSink, append? : Bool? = None) -> Bool {
       self.open_failures.val += 1
       false
     }
+  }
 }
 
 ///|
@@ -333,6 +424,29 @@ fn rename_file_if_present(
   }
 }
 
+///|
+async fn remove_file_if_present_async(path : String) -> Bool {
+  if !@utils.file_exists_async_internal(path) {
+    true
+  } else {
+    @utils.remove_file_async_internal(path) ||
+    !@utils.file_exists_async_internal(path)
+  }
+}
+
+///|
+async fn rename_file_if_present_async(
+  from_path : String,
+  to_path : String,
+) -> Bool {
+  if !@utils.file_exists_async_internal(from_path) {
+    true
+  } else {
+    @utils.rename_file_async_internal(from_path, to_path) ||
+    !@utils.file_exists_async_internal(from_path)
+  }
+}
+
 ///|
 fn rotate_file_sink_internal(sink : FileSink, rotation : FileRotation) -> Bool {
   let closed = match sink.handle.val {
@@ -396,6 +510,73 @@ fn rotate_if_needed_internal(sink : FileSink, next_line_bytes : Int) -> Bool {
   }
 }
 
+///|
+async fn rotate_file_sink_async_internal(
+  sink : FileSink,
+  rotation : FileRotation,
+) -> Bool {
+  let closed = match sink.handle.val {
+    None => true
+    Some(handle) => {
+      let ok = @utils.close_file_handle_internal(handle)
+      sink.handle.val = None
+      ok
+    }
+  }
+  if !closed {
+    return false
+  }
+  if rotation.max_backups > 0 {
+    if !remove_file_if_present_async(
+        rotated_file_path(sink.path, rotation.max_backups),
+      ) {
+      return false
+    }
+    for index = rotation.max_backups - 1; index >= 1; {
+      let from_path = rotated_file_path(sink.path, index)
+      let to_path = rotated_file_path(sink.path, index + 1)
+      if !rename_file_if_present_async(from_path, to_path) {
+        return false
+      }
+      continue index - 1
+    }
+    if !rename_file_if_present_async(sink.path, rotated_file_path(sink.path, 1)) {
+      return false
+    }
+  } else if !remove_file_if_present_async(sink.path) {
+    return false
+  }
+  sink.handle.val = @utils.open_file_handle_async_internal(sink.path, false)
+  sink.handle.val is Some(_)
+}
+
+///|
+async fn rotate_if_needed_async_internal(
+  sink : FileSink,
+  next_line_bytes : Int,
+) -> Bool {
+  match sink.rotation.val {
+    None => true
+    Some(rotation) =>
+      match sink.handle.val {
+        None => false
+        Some(handle) => {
+          let size = @utils.file_size_i64_async_internal(handle)
+          let next_line = next_line_bytes.to_int64()
+          if size + next_line <= rotation_max_bytes_internal(rotation) {
+            true
+          } else {
+            let rotated = rotate_file_sink_async_internal(sink, rotation)
+            if !rotated {
+              sink.rotation_failures.val += 1
+            }
+            rotated
+          }
+        }
+      }
+  }
+}
+
 ///|
 pub impl @sink_graph.Sink for FileSink with fn write(self, rec : Record) {
   match self.handle.val {
@@ -429,3 +610,37 @@ pub impl @sink_graph.Sink for FileSink with fn write(self, rec : Record) {
     }
   }
 }
+
+///|
+pub impl @sink_graph.Sink for FileSink with fn write_async(self, rec : Record) {
+  match self.handle.val {
+    None => self.write_failures.val += 1
+    Some(_) => {
+      let line = "\{(self.formatter)(rec)}\n"
+      let can_write = rotate_if_needed_async_internal(
+        self,
+        @utils.string_byte_length_internal(line),
+      )
+      if can_write {
+        match self.handle.val {
+          None => self.write_failures.val += 1
+          Some(active) => {
+            let wrote = @utils.write_file_handle_async_internal(active, line)
+            if wrote {
+              if self.auto_flush.val {
+                let flushed = @utils.flush_file_handle_async_internal(active)
+                if !flushed {
+                  self.flush_failures.val += 1
+                }
+              }
+            } else {
+              self.write_failures.val += 1
+            }
+          }
+        }
+      } else {
+        self.write_failures.val += 1
+      }
+    }
+  }
+}

src/file_runtime/file_sink_rotation_wbtest.mbt

@@ -12,8 +12,14 @@ test "file sink counts an injected partial backup-chain failure" {
     rec.message
   })
   if seed.is_available() {
-    seed.write(@core.Record::new(@core.Level::Info, "1234567890"))
-    seed.write(@core.Record::new(@core.Level::Info, "abcdefghij"))
+    @sink_graph.Sink::write(
+      seed,
+      @core.Record::new(@core.Level::Info, "1234567890"),
+    )
+    @sink_graph.Sink::write(
+      seed,
+      @core.Record::new(@core.Level::Info, "abcdefghij"),
+    )
     inspect(seed.rotation_failures(), content="0")
     inspect(seed.close(), content="true")
 
@@ -33,7 +39,10 @@ test "file sink counts an injected partial backup-chain failure" {
         }
       },
     }
-    sink.write(@core.Record::new(@core.Level::Info, "klmnopqrst"))
+    @sink_graph.Sink::write(
+      sink,
+      @core.Record::new(@core.Level::Info, "klmnopqrst"),
+    )
 
     inspect(sink.rotation_failures(), content="1")
     inspect(sink.state().rotation_failures, content="1")
@@ -65,8 +74,14 @@ test "file sink counts an injected oldest-backup removal failure" {
     rec.message
   })
   if seed.is_available() {
-    seed.write(@core.Record::new(@core.Level::Info, "1234567890"))
-    seed.write(@core.Record::new(@core.Level::Info, "abcdefghij"))
+    @sink_graph.Sink::write(
+      seed,
+      @core.Record::new(@core.Level::Info, "1234567890"),
+    )
+    @sink_graph.Sink::write(
+      seed,
+      @core.Record::new(@core.Level::Info, "abcdefghij"),
+    )
     inspect(seed.close(), content="true")
 
     let oldest = @utils.open_file_handle_internal(test_path + ".2", true)
@@ -95,7 +110,10 @@ test "file sink counts an injected oldest-backup removal failure" {
         @utils.rename_file_internal(from_path, to_path)
       },
     }
-    sink.write(@core.Record::new(@core.Level::Info, "klmnopqrst"))
+    @sink_graph.Sink::write(
+      sink,
+      @core.Record::new(@core.Level::Info, "klmnopqrst"),
+    )
 
     inspect(sink.rotation_failures(), content="1")
     inspect(sink.write_failures(), content="1")

src/file_runtime/pkg.generated.mbti

@@ -10,6 +10,8 @@ import {
 // Values
 pub fn file_sink(String, append? : Bool, auto_flush? : Bool, rotation? : @file_model.FileRotation?, formatter? : (@core.Record) -> String) -> FileSink
 
+pub async fn file_sink_async(String, append? : Bool, auto_flush? : Bool, rotation? : @file_model.FileRotation?, formatter? : (@core.Record) -> String) -> FileSink
+
 pub fn native_files_supported() -> Bool
 
 // Errors
@@ -24,6 +26,7 @@ pub fn FileSink::clear_rotation(Self) -> Unit
 pub fn FileSink::close(Self) -> Bool
 pub fn FileSink::default_policy(Self) -> @file_model.FileSinkPolicy
 pub fn FileSink::flush(Self) -> Bool
+pub async fn FileSink::flush_async(Self) -> Bool
 pub fn FileSink::flush_failures(Self) -> Int
 pub fn FileSink::is_available(Self) -> Bool
 pub fn FileSink::open_failures(Self) -> Int

src/formatting/formatter.mbt

@@ -916,7 +916,7 @@ fn parse_inline_markup(
   formatter : TextFormatter,
 ) -> Array[StyledSegment] {
   let segments : Array[StyledSegment] = []
-  let buffer = StringBuilder::new()
+  let buffer = StringBuilder()
   let stack : Array[StyleFrame] = [{ tag: "", style: default_inline_style() }]
   let chars = input.to_array()
   let current_style = fn() { stack[stack.length() - 1].style }
@@ -990,7 +990,7 @@ fn render_styled_text(
   let enabled = use_ansi_color(formatter.color_mode)
   let scoped = formatter.with_style_markup(mode)
   let segments = parse_inline_markup(text, scoped)
-  let out = StringBuilder::new()
+  let out = StringBuilder()
   for segment in segments {
     out.write_string(ansi_wrap_with_style(segment.text, segment.style, enabled))
   }

src/moon.pkg

@@ -20,3 +20,7 @@ import {
 import {
   "Nanaloveyuki/BitLogger/src/utils",
 } for "wbtest"
+
+import {
+  "moonbitlang/core/quickcheck",
+} for "test"

src/pkg.generated.mbti

@@ -75,6 +75,8 @@ pub fn file_rotation_i64(Int64, max_backups? : Int) -> @file_model.FileRotation
 
 pub fn file_sink(String, append? : Bool, auto_flush? : Bool, rotation? : @file_model.FileRotation?, formatter? : (@core.Record) -> String) -> @file_runtime.FileSink
 
+pub async fn file_sink_async(String, append? : Bool, auto_flush? : Bool, rotation? : @file_model.FileRotation?, formatter? : (@core.Record) -> String) -> @file_runtime.FileSink
+
 pub fn file_sink_policy_to_json(@file_model.FileSinkPolicy) -> Json
 
 pub fn file_sink_state_to_json(@file_model.FileSinkState) -> Json

src/property_based_test.mbt

@@ -0,0 +1,400 @@
+///|
+fn pbt_level(seed : Int) -> Level {
+  match (seed & 0x3fffffff) % 5 {
+    0 => Level::Trace
+    1 => Level::Debug
+    2 => Level::Info
+    3 => Level::Warn
+    _ => Level::Error
+  }
+}
+
+///|
+fn pbt_nonnegative(seed : Int, modulus : Int) -> Int {
+  (seed & 0x3fffffff) % modulus
+}
+
+///|
+fn pbt_bit(seed : Int, bit : Int) -> Bool {
+  (seed & bit) != 0
+}
+
+///|
+fn pbt_fields_equal(left : Array[Field], right : Array[Field]) -> Bool {
+  guard left.length() == right.length() else { return false }
+  for i in 0..<left.length() {
+    guard left[i].key == right[i].key && left[i].value == right[i].value else {
+      return false
+    }
+  }
+  true
+}
+
+///|
+fn pbt_records_equal(left : Record, right : Record) -> Bool {
+  left.level.label() == right.level.label() &&
+  left.timestamp_ms == right.timestamp_ms &&
+  left.target == right.target &&
+  left.message == right.message &&
+  pbt_fields_equal(left.fields, right.fields)
+}
+
+///|
+fn pbt_history_matches(
+  sink : HistorySink,
+  expected : Array[(String, String)],
+  expected_dropped : Int,
+) -> Bool {
+  guard sink.count() == expected.length() else { return false }
+  guard sink.dropped_count() == expected_dropped else { return false }
+  let snapshot = sink.snapshot()
+  guard snapshot.length() == expected.length() else { return false }
+  for i in 0..<expected.length() {
+    let (message, id) = expected[i]
+    let record = snapshot[i]
+    guard record.message == message else { return false }
+    guard record.fields.length() == 1 else { return false }
+    guard record.fields[0].key == "id" else { return false }
+    guard record.fields[0].value == id else { return false }
+  }
+  // The public snapshot must not expose the sink's retained field arrays.
+  if snapshot.length() > 0 {
+    snapshot[0].fields.push(field("mutated", "outside"))
+    guard sink.snapshot()[0].fields.length() == 1 else { return false }
+  }
+  true
+}
+
+///|
+test "quickcheck: history sink agrees with a bounded model" {
+  @quickcheck.check(
+    (input : (Int, Array[(Int, Int)])) => {
+      let (capacity_seed, operations) = input
+      let requested_capacity = capacity_seed % 8
+      let capacity = if requested_capacity <= 0 {
+        1
+      } else {
+        requested_capacity
+      }
+      let sink = history_sink(capacity=requested_capacity)
+      let mut expected : Array[(String, String)] = []
+      let mut expected_dropped = 0
+      for operation in operations {
+        let (kind, value) = operation
+        if (kind & 7) == 0 {
+          sink.clear()
+          expected = []
+          expected_dropped = 0
+        } else {
+          let id = value.to_string()
+          let message = "message:" + id
+          let record = Record::new(Level::Info, message, fields=[
+            field("id", id),
+          ])
+          @sink_graph.Sink::write(sink, record)
+          if expected.length() >= capacity {
+            ignore(expected.remove(0))
+            expected_dropped += 1
+          }
+          expected.push((message, id))
+        }
+        guard pbt_history_matches(sink, expected, expected_dropped) else {
+          return false
+        }
+      }
+      pbt_history_matches(sink, expected, expected_dropped)
+    },
+    count=220,
+    max_size=80,
+    max_shrinks=200,
+    seed=0x51A7UL,
+  )
+}
+
+///|
+fn pbt_queue_limit(seed : Int) -> Int {
+  match seed & 3 {
+    0 => -1
+    1 => 0
+    2 => 1
+    _ => 3
+  }
+}
+
+///|
+fn pbt_drain_model(
+  expected : Array[String],
+  delivered : Array[String],
+  max_items : Int,
+) -> Int {
+  if max_items == 0 {
+    return 0
+  }
+  let limit = if max_items < 0 { expected.length() } else { max_items }
+  let count = if limit > expected.length() { expected.length() } else { limit }
+  for _ in 0..<count {
+    delivered.push(expected.remove(0))
+  }
+  count
+}
+
+///|
+fn pbt_queue_matches(
+  sink : QueuedSink[CallbackSink],
+  expected : Array[String],
+  expected_dropped : Int,
+  delivered : Ref[Array[String]],
+  expected_delivered : Array[String],
+) -> Bool {
+  sink.pending_count() == expected.length() &&
+  sink.dropped_count() == expected_dropped &&
+  delivered.val == expected_delivered
+}
+
+///|
+test "quickcheck: queued sink agrees with an overflow and drain model" {
+  @quickcheck.check(
+    (input : (Int, Int, Array[(Int, Int)])) => {
+      let (max_pending_seed, policy_seed, operations) = input
+      let max_pending = pbt_nonnegative(max_pending_seed, 8)
+      let policy = if pbt_bit(policy_seed, 1) {
+        QueueOverflowPolicy::DropOldest
+      } else {
+        QueueOverflowPolicy::DropNewest
+      }
+      let delivered : Ref[Array[String]] = Ref([])
+      let sink = queued_sink(
+        callback_sink(fn(record) { delivered.val.push(record.message) }),
+        max_pending~,
+        overflow=policy,
+      )
+      let expected : Array[String] = []
+      let expected_delivered : Array[String] = []
+      let mut expected_dropped = 0
+      for operation in operations {
+        let (kind, value) = operation
+        match kind & 7 {
+          0 | 1 | 2 | 3 => {
+            let message = "message:" + value.to_string()
+            let record = Record::new(Level::Info, message)
+            let full = max_pending > 0 && expected.length() >= max_pending
+            if full {
+              expected_dropped += 1
+              match policy {
+                QueueOverflowPolicy::DropNewest => ()
+                QueueOverflowPolicy::DropOldest => {
+                  ignore(expected.remove(0))
+                  expected.push(message)
+                }
+              }
+            } else {
+              expected.push(message)
+            }
+            @sink_graph.Sink::write(sink, record)
+          }
+          4 => {
+            let limit = pbt_queue_limit(value)
+            let expected_count = pbt_drain_model(
+              expected, expected_delivered, limit,
+            )
+            let actual_count = sink.drain(max_items=limit)
+            guard actual_count == expected_count else { return false }
+          }
+          5 => {
+            let expected_count = pbt_drain_model(
+              expected, expected_delivered, -1,
+            )
+            guard sink.flush() == expected_count else { return false }
+          }
+          _ => {
+            let expected_count = pbt_drain_model(
+              expected, expected_delivered, 0,
+            )
+            guard expected_count == 0 else { return false }
+            guard sink.drain(max_items=0) == 0 else { return false }
+          }
+        }
+        guard pbt_queue_matches(
+          sink, expected, expected_dropped, delivered, expected_delivered,
+        ) else {
+          return false
+        }
+      }
+      pbt_queue_matches(
+        sink, expected, expected_dropped, delivered, expected_delivered,
+      )
+    },
+    count=220,
+    max_size=80,
+    max_shrinks=200,
+    seed=0xA11CEUL,
+  )
+}
+
+///|
+fn pbt_patch_for(operation : (Int, String)) -> RecordPatch {
+  let (kind, value) = operation
+  match kind & 7 {
+    0 => identity_patch()
+    1 => set_target("target." + value)
+    2 => prefix_message("[" + value + "] ")
+    3 => append_fields([field("extra", value)])
+    4 => redact_field("secret", placeholder=value)
+    _ => redact_fields(["secret", "token"], placeholder=value)
+  }
+}
+
+///|
+test "quickcheck: composed patches match sequential application" {
+  @quickcheck.check(
+    (input : (Array[(Int, String)], (Int, String, Array[(String, String)]))) => {
+      let (operations, record_input) = input
+      let (level_seed, message, raw_fields) = record_input
+      let base = Record::new(
+        pbt_level(level_seed),
+        message,
+        timestamp_ms=123UL,
+        target="service",
+        fields=raw_fields.map(entry => field(entry.0, entry.1)),
+      )
+      let patches : Array[RecordPatch] = operations.map(pbt_patch_for)
+      let mut sequential = base
+      for patch in patches {
+        sequential = patch(sequential)
+      }
+      let composed = compose_patches(patches)(base)
+      pbt_records_equal(sequential, composed)
+    },
+    count=220,
+    max_size=70,
+    max_shrinks=200,
+    seed=0xC0DEUL,
+  )
+}
+
+///|
+fn pbt_color_mode(seed : Int) -> ColorMode {
+  match pbt_nonnegative(seed, 3) {
+    0 => ColorMode::Never
+    1 => ColorMode::Auto
+    _ => ColorMode::Always
+  }
+}
+
+///|
+fn pbt_color_support(seed : Int) -> ColorSupport {
+  if pbt_bit(seed, 1) {
+    ColorSupport::Basic
+  } else {
+    ColorSupport::TrueColor
+  }
+}
+
+///|
+fn pbt_markup_mode(seed : Int) -> StyleMarkupMode {
+  match pbt_nonnegative(seed, 3) {
+    0 => StyleMarkupMode::Disabled
+    1 => StyleMarkupMode::Builtin
+    _ => StyleMarkupMode::Full
+  }
+}
+
+///|
+fn pbt_sink_kind(seed : Int) -> SinkKind {
+  match pbt_nonnegative(seed, 4) {
+    0 => SinkKind::Console
+    1 => SinkKind::JsonConsole
+    2 => SinkKind::TextConsole
+    _ => SinkKind::File
+  }
+}
+
+///|
+test "quickcheck: serialized logger configs parse to the same canonical text" {
+  @quickcheck.check(
+    (input : (Int, String, Bool, (Int, Int, Int, String))) => {
+      let (seed, target, timestamp, sink_input) = input
+      let (sink_seed, queue_seed, flags, suffix) = sink_input
+      let kind = pbt_sink_kind(sink_seed)
+      let path = match kind {
+        SinkKind::File =>
+          if suffix == "" {
+            "logs/generated.log"
+          } else {
+            "logs/" + suffix
+          }
+        _ => suffix
+      }
+      let style_tags : Map[String, TextStyle] = if pbt_bit(flags, 16) {
+        { "accent": text_style(fg=Some(suffix), bold=pbt_bit(flags, 32)) }
+      } else {
+        Map([])
+      }
+      let formatter = TextFormatterConfig::new(
+        show_timestamp=pbt_bit(flags, 1),
+        show_level=pbt_bit(flags, 2),
+        show_target=pbt_bit(flags, 4),
+        show_fields=pbt_bit(flags, 8),
+        separator=if suffix == "" { " " } else { suffix },
+        field_separator=if target == "" { "," } else { target },
+        template="[{level}] {target} {message}",
+        color_mode=pbt_color_mode(seed),
+        color_support=pbt_color_support(sink_seed),
+        style_markup=pbt_markup_mode(flags),
+        target_style_markup=pbt_markup_mode(seed),
+        fields_style_markup=pbt_markup_mode(queue_seed),
+        style_tags~,
+      )
+      let rotation = if pbt_bit(sink_seed, 1) {
+        Some(
+          file_rotation(
+            1 + pbt_nonnegative(queue_seed, 64),
+            max_backups=1 + pbt_nonnegative(flags, 4),
+          ),
+        )
+      } else {
+        None
+      }
+      let sink = SinkConfig::new(
+        kind~,
+        path~,
+        append=pbt_bit(flags, 1),
+        auto_flush=pbt_bit(flags, 2),
+        rotation~,
+        text_formatter=formatter,
+      )
+      let queue = if pbt_bit(queue_seed, 1) {
+        Some(
+          QueueConfig::new(
+            queue_seed % 17,
+            overflow=if pbt_bit(queue_seed, 2) {
+              QueueOverflowPolicy::DropOldest
+            } else {
+              QueueOverflowPolicy::DropNewest
+            },
+          ),
+        )
+      } else {
+        None
+      }
+      let config = LoggerConfig::new(
+        min_level=pbt_level(seed),
+        target~,
+        timestamp~,
+        sink~,
+        queue~,
+      )
+      let serialized = stringify_logger_config(config)
+      (fn() -> Bool raise ConfigError {
+        let parsed = parse_logger_config_text(serialized)
+        stringify_logger_config(parsed) == serialized
+      })() catch {
+        _ => false
+      }
+    },
+    count=180,
+    max_size=60,
+    max_shrinks=200,
+    seed=0xBEEF1234UL,
+  )
+}

src/runtime/pkg.generated.mbti

@@ -68,6 +68,7 @@ pub fn RuntimeSink::file_state(Self) -> @file_model.FileSinkState
 pub fn RuntimeSink::file_state_or_none(Self) -> @file_model.FileSinkState?
 pub fn RuntimeSink::file_write_failures(Self) -> Int
 pub fn RuntimeSink::flush(Self) -> Int
+pub async fn RuntimeSink::flush_async(Self) -> Unit
 pub fn RuntimeSink::flush_progress(Self) -> RuntimeSinkProgress
 pub fn RuntimeSink::pending_count(Self) -> Int
 pub impl @sink_graph.Sink for RuntimeSink

src/runtime/runtime_logger.mbt

@@ -81,6 +81,20 @@ pub impl @sink_graph.Sink for RuntimeSink with fn write(self, rec) {
   }
 }
 
+///|
+pub impl @sink_graph.Sink for RuntimeSink with fn write_async(self, rec) {
+  match self {
+    Console(sink) => sink.write(rec)
+    JsonConsole(sink) => sink.write(rec)
+    TextConsole(sink) => sink.write(rec)
+    File(sink) => sink.write_async(rec)
+    QueuedConsole(sink) => sink.write(rec)
+    QueuedJsonConsole(sink) => sink.write(rec)
+    QueuedTextConsole(sink) => sink.write(rec)
+    QueuedFile(sink) => sink.write(rec)
+  }
+}
+
 ///|
 fn[S : @sink_graph.Sink] queued_sink_close_internal(
   sink : QueuedSink[S],
@@ -202,6 +216,23 @@ pub fn RuntimeSink::flush(self : RuntimeSink) -> Int {
   runtime_sink_progress_compat_count(self.flush_progress())
 }
 
+///|
+pub async fn RuntimeSink::flush_async(self : RuntimeSink) -> Unit {
+  match self {
+    Console(_) => ()
+    JsonConsole(_) => ()
+    TextConsole(_) => ()
+    File(sink) => ignore(sink.flush_async())
+    QueuedConsole(sink) => ignore(sink.flush_async())
+    QueuedJsonConsole(sink) => ignore(sink.flush_async())
+    QueuedTextConsole(sink) => ignore(sink.flush_async())
+    QueuedFile(sink) => {
+      ignore(sink.flush_async())
+      ignore(sink.sink.flush_async())
+    }
+  }
+}
+
 ///|
 pub fn RuntimeSink::drain_progress(
   self : RuntimeSink,
@@ -671,7 +702,10 @@ pub fn RuntimeSink::file_runtime_state(self : RuntimeSink) -> RuntimeFileState?
 }
 
 ///|
-pub fn build_runtime_sink(config : SinkConfig) -> RuntimeSink {
+fn build_runtime_sink_internal(
+  config : SinkConfig,
+  defer_file_open : Bool,
+) -> RuntimeSink {
   match config.kind {
     SinkKind::Console => RuntimeSink::Console(@sink_graph.console_sink())
     SinkKind::JsonConsole =>
@@ -682,6 +716,20 @@ pub fn build_runtime_sink(config : SinkConfig) -> RuntimeSink {
       )
     SinkKind::File =>
       RuntimeSink::File(
+        if defer_file_open {
+          @file_runtime.file_sink_deferred(
+            config.path,
+            append=config.append,
+            auto_flush=config.auto_flush,
+            rotation=config.rotation,
+            formatter=fn(rec) {
+              @formatting.format_text(
+                rec,
+                formatter=config.text_formatter.to_formatter(),
+              )
+            },
+          )
+        } else {
           @file_runtime.file_sink(
             config.path,
             append=config.append,
@@ -693,11 +741,23 @@ pub fn build_runtime_sink(config : SinkConfig) -> RuntimeSink {
                 formatter=config.text_formatter.to_formatter(),
               )
             },
-        ),
+          )
+        },
       )
   }
 }
 
+///|
+pub fn build_runtime_sink(config : SinkConfig) -> RuntimeSink {
+  build_runtime_sink_internal(config, false)
+}
+
+///|
+#doc(hidden)
+pub fn build_runtime_sink_for_async(config : SinkConfig) -> RuntimeSink {
+  build_runtime_sink_internal(config, true)
+}
+
 ///|
 pub fn apply_queue_config(
   sink : RuntimeSink,

src/runtime_logger.mbt

@@ -64,8 +64,15 @@ pub fn Logger::dropped_count(self : Logger[RuntimeSink]) -> Int {
 }
 
 ///|
-pub fn build_logger(config : LoggerConfig) -> Logger[RuntimeSink] {
-  let sink = @runtime.build_runtime_sink(config.sink)
+fn build_logger_internal(
+  config : LoggerConfig,
+  defer_file_open : Bool,
+) -> Logger[RuntimeSink] {
+  let sink = if defer_file_open {
+    @runtime.build_runtime_sink_for_async(config.sink)
+  } else {
+    @runtime.build_runtime_sink(config.sink)
+  }
   let actual_sink = match config.queue {
     None => sink
     Some(queue) => @runtime.apply_queue_config(sink, queue)
@@ -75,6 +82,17 @@ pub fn build_logger(config : LoggerConfig) -> Logger[RuntimeSink] {
   )
 }
 
+///|
+pub fn build_logger(config : LoggerConfig) -> Logger[RuntimeSink] {
+  build_logger_internal(config, false)
+}
+
+///|
+#doc(hidden)
+pub fn build_logger_for_async(config : LoggerConfig) -> Logger[RuntimeSink] {
+  build_logger_internal(config, true)
+}
+
 ///|
 pub fn parse_and_build_logger(
   input : String,

src/sink_graph/pkg.generated.mbti

@@ -51,6 +51,7 @@ pub struct BufferedSink[S] {
   flush_limit : Int
 }
 pub fn[S : Sink] BufferedSink::flush(Self[S]) -> Unit
+pub async fn[S : Sink] BufferedSink::flush_async(Self[S]) -> Unit
 pub fn[S] BufferedSink::pending_count(Self[S]) -> Int
 pub impl[S : Sink] Sink for BufferedSink[S]
 
@@ -124,8 +125,10 @@ pub struct QueuedSink[S] {
   dropped_count : @ref.Ref[Int]
 }
 pub fn[S : Sink] QueuedSink::drain(Self[S], max_items? : Int) -> Int
+pub async fn[S : Sink] QueuedSink::drain_async(Self[S], max_items? : Int) -> Int
 pub fn[S] QueuedSink::dropped_count(Self[S]) -> Int
 pub fn[S : Sink] QueuedSink::flush(Self[S]) -> Int
+pub async fn[S : Sink] QueuedSink::flush_async(Self[S]) -> Int
 pub fn[S] QueuedSink::pending_count(Self[S]) -> Int
 pub impl[S] Sink for QueuedSink[S]
 
@@ -142,4 +145,5 @@ pub using @queue_model {type QueueOverflowPolicy}
 // Traits
 pub(open) trait Sink {
   fn write(Self, @core.Record) -> Unit
+  async fn write_async(Self, @core.Record) -> Unit = _
 }

src/sink_graph/sinks.mbt

@@ -19,6 +19,12 @@ type RecordPatch = @record_ops.RecordPatch
 ///|
 pub(open) trait Sink {
   fn write(Self, Record) -> Unit
+  async fn write_async(Self, Record) -> Unit = _
+}
+
+///|
+impl Sink with fn write_async(self : Self, rec : Record) {
+  self.write(rec)
 }
 
 ///|
@@ -63,6 +69,18 @@ pub impl[S : Sink] Sink for ContextSink[S] with fn write(self, rec) {
   self.sink.write(rec.with_fields(merged))
 }
 
+///|
+pub impl[S : Sink] Sink for ContextSink[S] with fn write_async(self, rec) {
+  let merged = if self.context_fields.length() == 0 {
+    rec.fields
+  } else if rec.fields.length() == 0 {
+    self.context_fields
+  } else {
+    self.context_fields + rec.fields
+  }
+  self.sink.write_async(rec.with_fields(merged))
+}
+
 ///|
 pub struct JsonConsoleSink {
   _dummy : Unit
@@ -148,6 +166,15 @@ pub impl[A : Sink, B : Sink] Sink for FanoutSink[A, B] with fn write(self, rec)
   self.right.write(rec.copy())
 }
 
+///|
+pub impl[A : Sink, B : Sink] Sink for FanoutSink[A, B] with fn write_async(
+  self,
+  rec,
+) {
+  self.left.write_async(rec)
+  self.right.write_async(rec.copy())
+}
+
 ///|
 pub struct SplitSink[A, B] {
   left : A
@@ -182,6 +209,18 @@ pub impl[A : Sink, B : Sink] Sink for SplitSink[A, B] with fn write(self, rec) {
   }
 }
 
+///|
+pub impl[A : Sink, B : Sink] Sink for SplitSink[A, B] with fn write_async(
+  self,
+  rec,
+) {
+  if (self.predicate)(rec) {
+    self.left.write_async(rec)
+  } else {
+    self.right.write_async(rec)
+  }
+}
+
 ///|
 pub struct CallbackSink {
   callback : (Record) -> Unit
@@ -299,6 +338,29 @@ pub impl[S : Sink] Sink for BufferedSink[S] with fn write(self, rec) {
   }
 }
 
+///|
+pub async fn[S : Sink] BufferedSink::flush_async(
+  self : BufferedSink[S],
+) -> Unit {
+  if self.buffer.val.length() == 0 {
+    ()
+  } else {
+    let pending = self.buffer.val
+    self.buffer.val = []
+    for rec in pending {
+      self.sink.write_async(rec)
+    }
+  }
+}
+
+///|
+pub impl[S : Sink] Sink for BufferedSink[S] with fn write_async(self, rec) {
+  self.buffer.val.push(rec)
+  if self.buffer.val.length() >= self.flush_limit {
+    self.flush_async()
+  }
+}
+
 ///|
 pub type QueueOverflowPolicy = @queue_model.QueueOverflowPolicy
 
@@ -358,11 +420,38 @@ pub fn[S : Sink] QueuedSink::drain(
   }
 }
 
+///|
+pub async fn[S : Sink] QueuedSink::drain_async(
+  self : QueuedSink[S],
+  max_items? : Int = -1,
+) -> Int {
+  if max_items == 0 {
+    return 0
+  }
+  let limit = if max_items < 0 { self.pending_count() } else { max_items }
+  for drained = 0; drained < limit; {
+    match self.queue.pop() {
+      None => break drained
+      Some(rec) => {
+        self.sink.write_async(rec)
+        continue drained + 1
+      }
+    }
+  } nobreak {
+    limit
+  }
+}
+
 ///|
 pub fn[S : Sink] QueuedSink::flush(self : QueuedSink[S]) -> Int {
   self.drain()
 }
 
+///|
+pub async fn[S : Sink] QueuedSink::flush_async(self : QueuedSink[S]) -> Int {
+  self.drain_async()
+}
+
 ///|
 pub impl[S] Sink for QueuedSink[S] with fn write(self, rec) {
   let full = self.max_pending > 0 && self.pending_count() >= self.max_pending
@@ -398,6 +487,13 @@ pub impl[S : Sink] Sink for FilterSink[S] with fn write(self, rec) {
   }
 }
 
+///|
+pub impl[S : Sink] Sink for FilterSink[S] with fn write_async(self, rec) {
+  if (self.predicate)(rec) {
+    self.sink.write_async(rec)
+  }
+}
+
 ///|
 pub struct PatchSink[S] {
   sink : S
@@ -413,3 +509,8 @@ pub fn[S] patch_sink(sink : S, patch : RecordPatch) -> PatchSink[S] {
 pub impl[S : Sink] Sink for PatchSink[S] with fn write(self, rec) {
   self.sink.write((self.patch)(rec))
 }
+
+///|
+pub impl[S : Sink] Sink for PatchSink[S] with fn write_async(self, rec) {
+  self.sink.write_async((self.patch)(rec))
+}

src/sinks_file.mbt

@@ -64,3 +64,20 @@ pub fn file_sink(
 ) -> FileSink {
   @file_runtime.file_sink(path, append~, auto_flush~, rotation~, formatter~)
 }
+
+///|
+pub async fn file_sink_async(
+  path : String,
+  append? : Bool = true,
+  auto_flush? : Bool = true,
+  rotation? : FileRotation? = None,
+  formatter? : RecordFormatter = fn(rec) { format_text(rec) },
+) -> FileSink {
+  @file_runtime.file_sink_async(
+    path,
+    append~,
+    auto_flush~,
+    rotation~,
+    formatter~,
+  )
+}

src/utils/file_backend_native.mbt

@@ -1,106 +1,137 @@
 ///|
-fn string_to_c_bytes(str : String) -> Bytes {
-  let res : Array[Byte] = []
-  let len = str.length()
-  let mut i = 0
-  while i < len {
-    let mut c = str.code_unit_at(i).to_int()
-    if 0xD800 <= c && c <= 0xDBFF {
-      c -= 0xD800
-      i = i + 1
-      let l = str.code_unit_at(i).to_int() - 0xDC00
-      c = (c << 10) + l + 0x10000
-    }
-    if c < 0x80 {
-      res.push(c.to_byte())
-    } else if c < 0x800 {
-      res.push((0xc0 + (c >> 6)).to_byte())
-      res.push((0x80 + (c & 0x3f)).to_byte())
-    } else if c < 0x10000 {
-      res.push((0xe0 + (c >> 12)).to_byte())
-      res.push((0x80 + ((c >> 6) & 0x3f)).to_byte())
-      res.push((0x80 + (c & 0x3f)).to_byte())
-    } else {
-      res.push((0xf0 + (c >> 18)).to_byte())
-      res.push((0x80 + ((c >> 12) & 0x3f)).to_byte())
-      res.push((0x80 + ((c >> 6) & 0x3f)).to_byte())
-      res.push((0x80 + (c & 0x3f)).to_byte())
-    }
-    i = i + 1
-  }
-  res.push((0).to_byte())
-  Bytes::from_array(res)
+pub struct FileHandle {
+  priv path : String
+  priv append : Bool
+  priv initialized : Ref[Bool]
+  priv position : Ref[Int64]
+  priv closed : Ref[Bool]
 }
 
 ///|
-#external
-type NativeFileHandle
+fn run_async_bool_internal(operation : async () -> Bool) -> Bool {
+  let result = Ref(false)
+  @async.run_async_main(() => result.val = operation())
+  result.val
+}
 
 ///|
-#borrow(path, mode)
-extern "C" fn file_open_ffi(path : Bytes, mode : Bytes) -> NativeFileHandle = "bitlogger_file_open"
+fn run_async_i64_internal(operation : async () -> Int64) -> Int64 {
+  let result = Ref(0L)
+  @async.run_async_main(() => result.val = operation())
+  result.val
+}
 
 ///|
-extern "C" fn file_is_null_ffi(handle : NativeFileHandle) -> Bool = "bitlogger_pointer_is_null"
+fn new_file_handle_internal(
+  path : String,
+  append : Bool,
+  initialized : Bool,
+) -> FileHandle {
+  {
+    path,
+    append,
+    initialized: Ref(initialized),
+    position: Ref(0L),
+    closed: Ref(false),
+  }
+}
 
 ///|
-#borrow(buffer)
-extern "C" fn file_write_ffi(
-  buffer : Bytes,
-  size : Int,
-  count : Int,
-  handle : NativeFileHandle,
-) -> Int = "bitlogger_file_write"
+async fn probe_open_file_handle_async_internal(
+  path : String,
+  append : Bool,
+) -> Bool {
+  try {
+    let create_mode = if append {
+      @fs.OpenOrCreate
+    } else {
+      @fs.CreateOrTruncate
+    }
+    let file = @fs.open(path, mode=@fs.WriteOnly, append~, create_mode~)
+    file.close()
+    true
+  } catch {
+    _ => false
+  }
+}
 
 ///|
-extern "C" fn file_flush_ffi(handle : NativeFileHandle) -> Int = "bitlogger_file_flush"
-
-///|
-extern "C" fn file_close_ffi(handle : NativeFileHandle) -> Int = "bitlogger_file_close"
-
-///|
-extern "C" fn file_seek_ffi(
-  handle : NativeFileHandle,
-  offset : Int,
-  origin : Int,
-) -> Int = "bitlogger_file_seek"
-
-///|
-extern "C" fn file_tell_i64_ffi(handle : NativeFileHandle) -> Int64 = "bitlogger_file_tell_i64"
-
-///|
-#borrow(from_path, to_path)
-extern "C" fn file_rename_ffi(from_path : Bytes, to_path : Bytes) -> Int = "bitlogger_file_rename"
-
-///|
-#borrow(path)
-extern "C" fn file_remove_ffi(path : Bytes) -> Int = "bitlogger_file_remove"
-
-///|
-pub struct FileHandle {
-  path : String
-  raw : NativeFileHandle
+pub async fn open_file_handle_async_internal(
+  path : String,
+  append : Bool,
+) -> FileHandle? {
+  if probe_open_file_handle_async_internal(path, append) {
+    Some(new_file_handle_internal(path, append, true))
+  } else {
+    None
+  }
 }
 
 ///|
 pub fn open_file_handle_internal(path : String, append : Bool) -> FileHandle? {
-  let mode = if append { "ab" } else { "wb" }
-  let raw = file_open_ffi(string_to_c_bytes(path), string_to_c_bytes(mode))
-  if file_is_null_ffi(raw) {
-    None
+  let opened = run_async_bool_internal(() => {
+    probe_open_file_handle_async_internal(path, append)
+  })
+  if opened {
+    Some(new_file_handle_internal(path, append, true))
   } else {
-    Some({ raw, path })
+    None
+  }
+}
+
+///|
+pub fn deferred_file_handle_internal(
+  path : String,
+  append : Bool,
+) -> FileHandle? {
+  Some(new_file_handle_internal(path, append, false))
+}
+
+///|
+pub async fn write_file_handle_async_internal(
+  handle : FileHandle,
+  content : String,
+) -> Bool {
+  if handle.closed.val {
+    return false
+  }
+  let bytes = @utf8.encode(content)
+  try {
+    let file = @fs.open(
+      handle.path,
+      mode=@fs.WriteOnly,
+      append=handle.append,
+      create_mode=if handle.initialized.val {
+        @fs.OpenOrCreate
+      } else if handle.append {
+        @fs.OpenOrCreate
+      } else {
+        @fs.CreateOrTruncate
+      },
+    )
+    defer file.close()
+    if handle.append {
+      file.write(bytes)
+    } else {
+      file.write_at(bytes[:], position=handle.position.val)
+      handle.position.val += bytes.length().to_int64()
+    }
+    handle.initialized.val = true
+    true
+  } catch {
+    _ => false
   }
 }
 
 ///|
 pub fn file_exists_internal(path : String) -> Bool {
-  let raw = file_open_ffi(string_to_c_bytes(path), string_to_c_bytes("rb"))
-  if file_is_null_ffi(raw) {
-    false
-  } else {
-    ignore(file_close_ffi(raw))
-    true
+  run_async_bool_internal(() => @fs.exists(path))
+}
+
+///|
+pub async fn file_exists_async_internal(path : String) -> Bool {
+  @fs.exists(path) catch {
+    _ => false
   }
 }
 
@@ -109,45 +140,102 @@ pub fn write_file_handle_internal(
   handle : FileHandle,
   content : String,
 ) -> Bool {
-  let bytes = string_to_c_bytes(content)
-  let written = file_write_ffi(bytes, 1, bytes.length() - 1, handle.raw)
-  written == bytes.length() - 1
+  run_async_bool_internal(() => {
+    write_file_handle_async_internal(handle, content)
+  })
+}
+
+///|
+pub async fn flush_file_handle_async_internal(handle : FileHandle) -> Bool {
+  if handle.closed.val {
+    return false
+  }
+  try {
+    let file = @fs.open(handle.path, mode=@fs.WriteOnly)
+    file.sync()
+    file.close()
+    true
+  } catch {
+    _ => false
+  }
 }
 
 ///|
 pub fn flush_file_handle_internal(handle : FileHandle) -> Bool {
-  file_flush_ffi(handle.raw) == 0
+  if handle.closed.val {
+    false
+  } else {
+    // Each write owns and closes its File, so there is no buffered handle to flush here.
+    true
+  }
 }
 
 ///|
 pub fn close_file_handle_internal(handle : FileHandle) -> Bool {
-  file_close_ffi(handle.raw) == 0
+  if handle.closed.val {
+    false
+  } else {
+    handle.closed.val = true
+    true
+  }
+}
+
+///|
+pub async fn file_size_i64_async_internal(handle : FileHandle) -> Int64 {
+  if handle.closed.val {
+    return 0L
+  }
+  try {
+    let file = @fs.open(handle.path, mode=@fs.ReadOnly)
+    let size = file.size()
+    file.close()
+    size
+  } catch {
+    _ => 0L
+  }
 }
 
 ///|
 pub fn file_size_i64_internal(handle : FileHandle) -> Int64 {
-  ignore(file_seek_ffi(handle.raw, 0, 2))
-  let size = file_tell_i64_ffi(handle.raw)
-  if size < 0L {
-    0L
-  } else {
-    size
+  run_async_i64_internal(() => file_size_i64_async_internal(handle))
+}
+
+///|
+pub async fn rename_file_async_internal(
+  from_path : String,
+  to_path : String,
+) -> Bool {
+  try {
+    @fs.rename(from_path, to_path)
+    true
+  } catch {
+    _ => false
   }
 }
 
 ///|
 pub fn rename_file_internal(from_path : String, to_path : String) -> Bool {
-  file_rename_ffi(string_to_c_bytes(from_path), string_to_c_bytes(to_path)) == 0
+  run_async_bool_internal(() => rename_file_async_internal(from_path, to_path))
+}
+
+///|
+pub async fn remove_file_async_internal(path : String) -> Bool {
+  try {
+    @fs.remove(path)
+    true
+  } catch {
+    _ => false
+  }
 }
 
 ///|
 pub fn remove_file_internal(path : String) -> Bool {
-  file_remove_ffi(string_to_c_bytes(path)) == 0
+  run_async_bool_internal(() => remove_file_async_internal(path))
 }
 
 ///|
 pub fn string_byte_length_internal(content : String) -> Int {
-  string_to_c_bytes(content).length() - 1
+  @utf8.encode(content).length()
 }
 
 ///|

src/utils/file_backend_stub.mbt

@@ -1,28 +1,106 @@
 ///|
 pub struct FileHandle {
-  path : String
+  priv dummy : Unit
 }
 
 ///|
 pub fn open_file_handle_internal(path : String, append : Bool) -> FileHandle? {
   ignore(append)
   ignore(path)
-  let _unused : FileHandle = { path: "" }
+  let _unused : FileHandle = { dummy: () }
   ignore(_unused)
   None
 }
 
+///|
+pub fn deferred_file_handle_internal(
+  path : String,
+  append : Bool,
+) -> FileHandle? {
+  ignore(append)
+  ignore(path)
+  None
+}
+
 ///|
 pub fn file_exists_internal(path : String) -> Bool {
   ignore(path)
   false
 }
 
+///|
+async fn stub_async_noop_internal() -> Unit {
+  ignore(@fs.unimplemented)
+  @async.pause()
+}
+
+///|
+pub async fn open_file_handle_async_internal(
+  path : String,
+  append : Bool,
+) -> FileHandle? {
+  stub_async_noop_internal()
+  ignore(append)
+  ignore(path)
+  None
+}
+
+///|
+pub async fn file_exists_async_internal(path : String) -> Bool {
+  stub_async_noop_internal()
+  ignore(path)
+  false
+}
+
+///|
+pub async fn write_file_handle_async_internal(
+  handle : FileHandle,
+  content : String,
+) -> Bool {
+  stub_async_noop_internal()
+  ignore(handle)
+  ignore(content)
+  false
+}
+
+///|
+pub async fn flush_file_handle_async_internal(handle : FileHandle) -> Bool {
+  stub_async_noop_internal()
+  ignore(handle)
+  false
+}
+
+///|
+pub async fn file_size_i64_async_internal(handle : FileHandle) -> Int64 {
+  stub_async_noop_internal()
+  ignore(handle)
+  0L
+}
+
+///|
+pub async fn rename_file_async_internal(
+  from_path : String,
+  to_path : String,
+) -> Bool {
+  stub_async_noop_internal()
+  ignore(from_path)
+  ignore(to_path)
+  false
+}
+
+///|
+pub async fn remove_file_async_internal(path : String) -> Bool {
+  stub_async_noop_internal()
+  ignore(path)
+  false
+}
+
 ///|
 pub fn write_file_handle_internal(
   handle : FileHandle,
   content : String,
 ) -> Bool {
+  ignore(handle.dummy)
   ignore(handle)
   ignore(content)
   false

src/utils/moon.pkg

@@ -2,14 +2,16 @@ import {
   "Nanaloveyuki/BitLogger/src/core",
   "Nanaloveyuki/BitLogger/src/formatting",
   "Nanaloveyuki/BitLogger/src/record_ops",
+  "moonbitlang/async",
+  "moonbitlang/async/fs",
   "moonbitlang/core/env",
+  "moonbitlang/core/encoding/utf8",
   "moonbitlang/core/json",
   "moonbitlang/core/ref",
   "moonbitlang/core/string",
 }
 
 options(
-  "native-stub": [ "stub.c" ],
   targets: {
     "file_backend_native.mbt": [ "native", "llvm" ],
     "file_backend_stub.mbt": [ "js", "wasm", "wasm-gc" ],

src/utils/pkg.generated.mbti

@@ -23,12 +23,20 @@ pub fn compose_patches(Array[(@core.Record) -> @core.Record]) -> (@core.Record)
 
 pub fn default_style_tag_registry() -> @formatting.StyleTagRegistry
 
+pub fn deferred_file_handle_internal(String, Bool) -> FileHandle?
+
 pub fn field_equals(String, String) -> (@core.Record) -> Bool
 
+pub async fn file_exists_async_internal(String) -> Bool
+
 pub fn file_exists_internal(String) -> Bool
 
+pub async fn file_size_i64_async_internal(FileHandle) -> Int64
+
 pub fn file_size_i64_internal(FileHandle) -> Int64
 
+pub async fn flush_file_handle_async_internal(FileHandle) -> Bool
+
 pub fn flush_file_handle_internal(FileHandle) -> Bool
 
 pub fn format_json(@core.Record) -> String
@@ -49,6 +57,8 @@ pub fn native_files_supported_internal() -> Bool
 
 pub fn not_((@core.Record) -> Bool) -> (@core.Record) -> Bool
 
+pub async fn open_file_handle_async_internal(String, Bool) -> FileHandle?
+
 pub fn open_file_handle_internal(String, Bool) -> FileHandle?
 
 pub fn prefix_message(String) -> (@core.Record) -> @core.Record
@@ -57,8 +67,12 @@ pub fn redact_field(String, placeholder? : String) -> (@core.Record) -> @core.Re
 
 pub fn redact_fields(Array[String], placeholder? : String) -> (@core.Record) -> @core.Record
 
+pub async fn remove_file_async_internal(String) -> Bool
+
 pub fn remove_file_internal(String) -> Bool
 
+pub async fn rename_file_async_internal(String, String) -> Bool
+
 pub fn rename_file_internal(String, String) -> Bool
 
 pub fn reset_global_style_tag_registry() -> Unit
@@ -81,13 +95,15 @@ pub fn text_formatter(show_timestamp? : Bool, show_level? : Bool, show_target? :
 
 pub fn text_style(fg? : String?, bg? : String?, bold? : Bool, dim? : Bool, italic? : Bool, underline? : Bool) -> @formatting.TextStyle
 
+pub async fn write_file_handle_async_internal(FileHandle, String) -> Bool
+
 pub fn write_file_handle_internal(FileHandle, String) -> Bool
 
 // Errors
 
 // Types and methods
 pub struct FileHandle {
-  path : String
+  // private fields
 }
 
 // Type aliases

src/utils/stub.c

@@ -1,55 +0,0 @@
-#include <stdint.h>
-
-#ifndef _CRT_SECURE_NO_WARNINGS
-#define _CRT_SECURE_NO_WARNINGS
-#endif
-
-#include <stdio.h>
-
-int32_t bitlogger_pointer_is_null(void *ptr) {
-  return ptr == 0;
-}
-
-void *bitlogger_file_open(const char *path, const char *mode) {
-#if defined(__clang__)
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wdeprecated-declarations"
-#endif
-  return fopen(path, mode);
-#if defined(__clang__)
-#pragma clang diagnostic pop
-#endif
-}
-
-int32_t bitlogger_file_write(const char *buffer, int32_t size, int32_t count, void *handle) {
-  return (int32_t)fwrite(buffer, (size_t)size, (size_t)count, (FILE *)handle);
-}
-
-int32_t bitlogger_file_flush(void *handle) {
-  return fflush((FILE *)handle);
-}
-
-int32_t bitlogger_file_close(void *handle) {
-  return fclose((FILE *)handle);
-}
-
-int32_t bitlogger_file_seek(void *handle, int32_t offset, int32_t origin) {
-  return fseek((FILE *)handle, offset, origin);
-}
-
-int64_t bitlogger_file_tell_i64(void *handle) {
-#if defined(_WIN32)
-  __int64 position = _ftelli64((FILE *)handle);
-#else
-  long long position = ftello((FILE *)handle);
-#endif
-  return position < 0 ? -1 : (int64_t)position;
-}
-
-int32_t bitlogger_file_rename(const char *from_path, const char *to_path) {
-  return rename(from_path, to_path);
-}
-
-int32_t bitlogger_file_remove(const char *path) {
-  return remove(path);
-}