s3: resolve streamed write/download with the byte count, not 0

src/runtime/webcore/Blob.rs

@@ -1663,12 +1663,14 @@ impl BlobExt for Blob {
                         return Ok(promise_value);
                     }
                     jsc::js_promise::Status::Fulfilled => {
+                        // SAFETY: `file_sink` holds our +1 ref; live until deref below.
+                        let written = unsafe { (*file_sink).written.get() };
                         // SAFETY: release our +1 ref on the sink.
                         unsafe { webcore::FileSink::deref(file_sink) };
                         readable_stream.done(global_this);
                         return Ok(JSPromise::resolved_promise_value(
                             global_this,
-                            JSValue::js_number(0.0),
+                            JSValue::js_number(written as f64),
                         ));
                     }
                     jsc::js_promise::Status::Rejected => {
@@ -1693,12 +1695,14 @@ impl BlobExt for Blob {
                 );
             }
         }
+        // SAFETY: `file_sink` holds our +1 ref; live until deref below.
+        let written = unsafe { (*file_sink).written.get() };
         // SAFETY: release our +1 ref on the sink.
         unsafe { webcore::FileSink::deref(file_sink) };
 
         Ok(JSPromise::resolved_promise_value(
             global_this,
-            JSValue::js_number(0.0),
+            JSValue::js_number(written as f64),
         ))
     }
 
@@ -5959,7 +5963,10 @@ pub fn on_file_stream_resolve_request_stream(
     if let Some(stream) = strong.get(global_this) {
         stream.done(global_this);
     }
-    this.promise.resolve(global_this, JSValue::js_number(0.0))?;
+    // SAFETY: `this.sink` is the +1 ref held until `Drop` below; live here.
+    let written = unsafe { (*this.sink).written.get() };
+    this.promise
+        .resolve(global_this, JSValue::js_number(written as f64))?;
     Ok(JSValue::UNDEFINED)
 }
 

src/runtime/webcore/s3/client.rs

@@ -445,12 +445,14 @@ pub(crate) fn writable_stream(
             let _exit_guard = unsafe { bun_jsc::event_loop::EventLoop::enter_scope(event_loop) };
             match result {
                 S3UploadResult::Success => {
+                    // flush() keeps its delta contract: 0 = nothing left pending.
                     if sink.flush_promise.has_value() {
                         sink.flush_promise
                             .resolve(global, JSValue::js_number(0.0))?;
                     }
                     if sink.end_promise.has_value() {
-                        sink.end_promise.resolve(global, JSValue::js_number(0.0))?;
+                        sink.end_promise
+                            .resolve(global, JSValue::js_number(sink.wrote as f64))?;
                     }
                 }
                 S3UploadResult::Failure(err) => {
@@ -498,6 +500,7 @@ pub(crate) fn writable_stream(
         available: IntegerBitSet::init_full(),
         current_part_number: 1,
         ref_count: core::cell::Cell::new(2), // +1 for the stream
+        uploaded: 0,
         ended: false,
         options,
         acl: None,
@@ -675,9 +678,10 @@ impl S3UploadStreamWrapper {
         match &result {
             S3UploadResult::Success => {
                 if self_.end_promise.has_value() {
+                    let uploaded = self_.task_mut().uploaded;
                     self_
                         .end_promise
-                        .resolve(&self_.global, JSValue::js_number(0.0))?;
+                        .resolve(&self_.global, JSValue::js_number(uploaded as f64))?;
                     self_.end_promise = bun_jsc::JSPromiseStrong::empty();
                 }
             }
@@ -852,6 +856,7 @@ pub fn upload_stream(
         available: IntegerBitSet::init_full(),
         current_part_number: 1,
         ref_count: core::cell::Cell::new(2), // +1 for the stream ctx (only deinit after task and context ended)
+        uploaded: 0,
         ended: false,
         options,
         acl,

src/runtime/webcore/s3/multipart.rs

@@ -129,6 +129,8 @@ pub struct MultiPartUpload {
 
     pub current_part_number: u16,
     pub ref_count: Cell<u32>, // intrusive refcount — see bun_ptr::IntrusiveRc
+    /// Server-acknowledged payload bytes; resolves `S3File.write()` / `.writer().end()`.
+    pub uploaded: u64,
     pub ended: bool,
 
     pub options: MultiPartUploadOptions,
@@ -308,6 +310,7 @@ impl UploadPart {
                     this.part_number
                 );
                 let sent = this.data().len();
+                ctx.uploaded += sent as u64;
                 this.free_allocated_slice();
                 // we will need to order this
                 ctx.multipart_etags.push(UploadPartResult {
@@ -459,8 +462,10 @@ impl MultiPartUpload {
             S3UploadResult::Success => {
                 scoped_log!(S3MultiPartUpload, "singleSendUploadResponse success");
 
+                let sent = this.buffered.size() as u64;
+                this.uploaded += sent;
                 if let Some(callback) = this.on_writable {
-                    callback(this, this.callback_context, this.buffered.size() as u64);
+                    callback(this, this.callback_context, sent);
                 }
                 this.done()
             }

src/runtime/webcore/streams.rs

@@ -2047,6 +2047,8 @@ pub struct NetworkSink {
     // JSC_BORROW: process-lifetime VM global; safe `Deref` via `BackRef`.
     pub global_this: Option<BackRef<JSGlobalObject>>,
     pub high_water_mark: BlobSizeType,
+    /// Mirrors `MultiPartUpload.uploaded`; survives `task` detaching before completion.
+    pub wrote: u64,
     pub flush_promise: JSPromiseStrong,
     pub end_promise: JSPromiseStrong,
     pub ended: bool,
@@ -2061,6 +2063,7 @@ impl Default for NetworkSink {
             signal: Signal::default(),
             global_this: None,
             high_water_mark: 2048,
+            wrote: 0,
             flush_promise: JSPromiseStrong::default(),
             end_promise: JSPromiseStrong::default(),
             ended: false,
@@ -2155,8 +2158,10 @@ impl NetworkSink {
             flushed,
             task.state as u8
         );
+        this.wrote = task.uploaded;
         if this.flush_promise.has_value() {
             let global = this.global_this.expect("global_this set at construction");
+            // flush() resolves with the per-call delta; only end() reports cumulative `wrote`.
             this.flush_promise
                 .resolve(&global, JSValue::js_number(flushed as f64))?;
         }
@@ -2274,27 +2279,21 @@ impl NetworkSink {
     }
 
     pub fn end_from_js(&mut self, _global_this: &JSGlobalObject) -> bun_sys::Result<JSValue> {
-        let _ = self.end(None);
         if self.end_promise.has_value() {
             // we are already waiting for the end
+            let _ = self.end(None);
             return bun_sys::Result::Ok(self.end_promise.value());
         }
         if self.task.is_some() {
-            // we need to wait for the task to end
+            // end() may complete synchronously; the promise must exist before it runs.
             self.end_promise = JSPromiseStrong::init(self.global_this());
             let value = self.end_promise.value();
-            if !self.ended {
-                self.ended = true;
-                // we need to send EOF
-                if let Some(task) = self.task_mut() {
-                    let _ = task.write_bytes(b"", true);
-                }
-                self.signal.close(None);
-            }
+            let _ = self.end(None);
             return bun_sys::Result::Ok(value);
         }
+        let _ = self.end(None);
         // task already detached
-        bun_sys::Result::Ok(JSValue::js_number(0.0))
+        bun_sys::Result::Ok(JSValue::js_number(self.wrote as f64))
     }
 
     pub fn to_js(&mut self, global_this: &JSGlobalObject) -> JSValue {

test/js/bun/s3/s3-write-return-bytes.test.ts

@@ -0,0 +1,155 @@
+import { S3Client, type S3Options } from "bun";
+import { describe, expect, it } from "bun:test";
+import { isASAN, tempDir } from "harness";
+import path from "node:path";
+
+// The S3 client routes through HTTP_PROXY without consulting NO_PROXY, which
+// breaks the localhost mock origin on proxied machines.
+process.env.HTTP_PROXY = "";
+process.env.HTTPS_PROXY = "";
+process.env.http_proxy = "";
+process.env.https_proxy = "";
+
+// writer() leaks its NetworkSink (pre-existing, fix in #34999), which trips
+// LeakSanitizer; skip those tests under ASAN until that PR lands.
+const itWriter = it.skipIf(isASAN);
+
+// Every write entry point is typed/documented as "Promise resolving to number of
+// bytes written". Buffered sources already returned the true count; streamed
+// sources (ReadableStream body, Bun.file, writer().end(), download-to-file)
+// resolved a hardcoded 0.
+
+describe("s3 write() resolves with bytes transferred", () => {
+  const PAYLOAD = 300_000;
+
+  function mockOrigin() {
+    let received = 0;
+    const server = Bun.serve({
+      port: 0,
+      async fetch(req) {
+        received = (await req.arrayBuffer()).byteLength;
+        const url = new URL(req.url);
+        if (url.search === "?uploads=") {
+          // InitiateMultipartUpload
+          return new Response(
+            `<?xml version="1.0"?><InitiateMultipartUploadResult><UploadId>abc123</UploadId></InitiateMultipartUploadResult>`,
+            { status: 200 },
+          );
+        }
+        if (req.method === "GET") {
+          return new Response(Buffer.alloc(PAYLOAD, "x"));
+        }
+        return new Response("", { status: 200, headers: { etag: '"e"' } });
+      },
+    });
+    const options: S3Options = {
+      endpoint: server.url.href,
+      accessKeyId: "a",
+      secretAccessKey: "b",
+      bucket: "bk",
+      region: "us-east-1",
+    };
+    return {
+      server,
+      options,
+      client: new S3Client(options),
+      received: () => received,
+      [Symbol.dispose]() {
+        server.stop(true);
+      },
+    };
+  }
+
+  it("buffered: Uint8Array source returns byte count", async () => {
+    using m = mockOrigin();
+    const n = await m.client.write("k", new Uint8Array(PAYLOAD));
+    expect({ returned: n, received: m.received() }).toEqual({ returned: PAYLOAD, received: PAYLOAD });
+  });
+
+  it("streamed: Response with ReadableStream body returns byte count", async () => {
+    using m = mockOrigin();
+    const stream = new ReadableStream({
+      start(c) {
+        c.enqueue(new Uint8Array(PAYLOAD));
+        c.close();
+      },
+    });
+    const n = await m.client.write("k", new Response(stream));
+    expect({ returned: n, received: m.received() }).toEqual({ returned: PAYLOAD, received: PAYLOAD });
+  });
+
+  it("streamed: Bun.file source returns byte count", async () => {
+    using m = mockOrigin();
+    using dir = tempDir("s3-write-ret", {
+      "src.bin": Buffer.alloc(PAYLOAD, "A"),
+    });
+    const n = await Bun.write(m.client.file("k"), Bun.file(path.join(String(dir), "src.bin")));
+    expect({ returned: n, received: m.received() }).toEqual({ returned: PAYLOAD, received: PAYLOAD });
+  });
+
+  it("streamed: S3Client.write(key, Bun.file) returns byte count", async () => {
+    using m = mockOrigin();
+    using dir = tempDir("s3-write-ret-direct", {
+      "src.bin": Buffer.alloc(PAYLOAD, "B"),
+    });
+    const n = await m.client.write("k", Bun.file(path.join(String(dir), "src.bin")));
+    expect({ returned: n, received: m.received() }).toEqual({ returned: PAYLOAD, received: PAYLOAD });
+  });
+
+  itWriter("writer(): end() returns total bytes written", async () => {
+    using m = mockOrigin();
+    const w = m.client.file("k").writer();
+    w.write(new Uint8Array(PAYLOAD));
+    w.write(new Uint8Array(PAYLOAD));
+    w.write(new Uint8Array(PAYLOAD));
+    const n = await w.end();
+    expect({ returned: n, received: m.received() }).toEqual({
+      returned: PAYLOAD * 3,
+      received: PAYLOAD * 3,
+    });
+  });
+
+  itWriter("writer(): end() returns total bytes for a multipart upload", async () => {
+    let partsReceived = 0;
+    using server = Bun.serve({
+      port: 0,
+      async fetch(req) {
+        const body = await req.arrayBuffer();
+        if (req.method === "POST" && req.url.includes("?uploads=")) {
+          return new Response(
+            "<InitiateMultipartUploadResult><UploadId>abc123</UploadId></InitiateMultipartUploadResult>",
+            { status: 200 },
+          );
+        }
+        if (req.method === "POST" && req.url.includes("uploadId=")) {
+          return new Response('<CompleteMultipartUploadResult><ETag>"etag"</ETag></CompleteMultipartUploadResult>', {
+            status: 200,
+          });
+        }
+        partsReceived += body.byteLength;
+        return new Response("", { status: 200, headers: { etag: '"e"' } });
+      },
+    });
+    const client = new S3Client({
+      endpoint: server.url.href,
+      accessKeyId: "a",
+      secretAccessKey: "b",
+      bucket: "bk",
+      region: "us-east-1",
+    });
+    const partSize = 5 * 1024 * 1024;
+    const total = partSize + 1024 * 1024;
+    const w = client.file("k").writer({ partSize });
+    w.write(Buffer.alloc(total, "a"));
+    const n = await w.end();
+    expect({ returned: n, received: partsReceived }).toEqual({ returned: total, received: total });
+  });
+
+  it("download: Bun.write(path, s3file) returns bytes written to disk", async () => {
+    using m = mockOrigin();
+    using dir = tempDir("s3-dl-ret", {});
+    const dest = path.join(String(dir), "out.bin");
+    const n = await Bun.write(dest, m.client.file("k"));
+    expect({ returned: n, onDisk: Bun.file(dest).size }).toEqual({ returned: PAYLOAD, onDisk: PAYLOAD });
+  });
+});