FileSink: return per-chunk byte count when a write flushes buffered data

src/io/PipeWriter.rs

@@ -887,10 +887,13 @@ impl<Parent: PosixStreamingWriterParent> PosixStreamingWriter<Parent> {
             return WriteResult::Wrote(buf_len);
         }
 
-        self.try_write_newly_buffered_data()
+        self.try_write_newly_buffered_data(buf_len)
     }
 
-    fn try_write_newly_buffered_data(&mut self) -> WriteResult {
+    /// `chunk_len` is the caller's chunk, already appended to `outgoing`.
+    /// `Wrote(n)` reports bytes of *that chunk* accepted, not bytes drained
+    /// from the buffer (older chunks included) — that is `flush()`'s accounting.
+    fn try_write_newly_buffered_data(&mut self, chunk_len: usize) -> WriteResult {
         debug_assert!(!self.is_done);
 
         // Borrow `self.outgoing` only for the syscall. `try_write` takes `&self`
@@ -903,20 +906,28 @@ impl<Parent: PosixStreamingWriterParent> PosixStreamingWriter<Parent> {
 
         match rc {
             WriteResult::Wrote(amt) => {
-                if amt == self.outgoing.size() {
-                    self.outgoing.reset();
-                    self.parent_on_write(amt, WriteStatus::Drained);
-                } else {
+                if amt < self.outgoing.size() {
                     self.outgoing.wrote(amt);
                     self.parent_on_write(amt, WriteStatus::Pending);
                     Self::register_poll(self);
                     return WriteResult::Pending(amt);
                 }
+
+                self.outgoing.reset();
+                self.parent_on_write(amt, WriteStatus::Drained);
+                return WriteResult::Wrote(chunk_len);
             }
             WriteResult::Done(amt) => {
+                // `amt` drains the whole buffer (older chunks first); report
+                // only the portion that belongs to this chunk, like `Wrote`.
+                let old_buffered = self.outgoing.size().saturating_sub(chunk_len);
                 self.outgoing.reset();
                 self.parent_on_write(amt, WriteStatus::EndOfFile);
+                return WriteResult::Done(amt.saturating_sub(old_buffered));
             }
+            // `Pending` returns a promise whose resolution value is accounted
+            // for separately (see `FileSink::bytes_accepted`), so it is left as
+            // the drained count here.
             WriteResult::Pending(amt) => {
                 self.outgoing.wrote(amt);
                 self.parent_on_write(amt, WriteStatus::Pending);
@@ -957,7 +968,7 @@ impl<Parent: PosixStreamingWriterParent> PosixStreamingWriter<Parent> {
                 return WriteResult::Err(sys::Error::oom());
             }
 
-            return self.try_write_newly_buffered_data();
+            return self.try_write_newly_buffered_data(buf.len());
         }
 
         let rc = self.try_write(self.force_sync, buf);

test/js/bun/util/filesink.test.ts

@@ -1,6 +1,6 @@
 import { createSocketPair, fileSinkInternals } from "bun:internal-for-testing";
 import { describe, expect, it } from "bun:test";
-import { bunEnv, bunExe, fileDescriptorLeakChecker, isPosix, isWindows, tmpdirSync } from "harness";
+import { bunEnv, bunExe, fileDescriptorLeakChecker, isPosix, isWindows, tempDir, tmpdirSync } from "harness";
 import { mkfifo } from "mkfifo";
 import { join } from "node:path";
 
@@ -273,6 +273,41 @@ it.skipIf(!isPosix)("a backpressured string write() resolves to its encoded byte
   expect(received).toBe(size);
 });
 
+// A chunk big enough to flush the bytes buffered by earlier writes must still
+// report its own size. Writes are issued without `await` in between so the
+// auto-flush microtask cannot drain the buffer first. Skipped on Windows: there
+// every write to a file-backed sink returns a (shared) promise instead.
+it.skipIf(!isPosix)("write result is not cumulative when the chunk flushes buffered bytes", async () => {
+  using dir = tempDir("filesink-cumulative", {});
+  const filename = path.join(String(dir), "test.bin");
+  const writer = Bun.file(filename).writer();
+
+  const ascii = Buffer.alloc(505, "a").toString();
+  const bytes = Buffer.alloc(64 * 1024, "c");
+  const latin1 = Buffer.alloc(40_000, "é").toString();
+  const utf16 = Buffer.alloc(20_000, "😀").toString();
+
+  const results = [
+    writer.write(ascii), // buffered
+    writer.write(bytes), // flushes `ascii` + itself
+    writer.write(Buffer.alloc(10, "d")), // buffered
+    writer.write(latin1), // flushes the 10 bytes + itself
+    writer.write("e"), // buffered
+    writer.write(utf16), // flushes the 1 byte + itself
+  ];
+  await writer.end();
+
+  expect(results).toEqual([
+    Buffer.byteLength(ascii),
+    bytes.byteLength,
+    10,
+    Buffer.byteLength(latin1),
+    1,
+    Buffer.byteLength(utf16),
+  ]);
+  expect(results.reduce((sum, n) => sum + n, 0)).toBe(fs.statSync(filename).size);
+});
+
 if (isWindows) {
   it("ENOENT, Windows", () => {
     expect(() => Bun.file("A:\\this-does-not-exist.txt").writer()).toThrow(