Honor Bun.file().slice() bounds for unread file streams and in file routes

src/runtime/webcore/ReadableStream.rs

@@ -226,6 +226,15 @@ impl ReadableStream {
                 let blobby = self.ptr.file().expect("matched File");
                 if let webcore::file_reader::Lazy::Blob(store) = blobby.lazy.get() {
                     let blob = Blob::init_with_store(store.clone(), global_this);
+                    // `init_with_store` spans the whole store; the window of the
+                    // Blob this stream was made from lives on the reader
+                    // (see `from_blob_copy_ref`).
+                    if let Some(offset) = blobby.start_offset {
+                        blob.offset.set(offset as webcore::blob::SizeType);
+                    }
+                    if let Some(size) = blobby.max_size {
+                        blob.size.set(size as webcore::blob::SizeType);
+                    }
                     // it should be lazy, file shouldn't have opened yet.
                     debug_assert!(!blobby.started.get());
                     self.done(global_this);

test/js/bun/http/bun-serve-file.test.ts

@@ -72,6 +72,15 @@ describe("Bun.file in serve routes", () => {
       }),
       "/partial.txt": new Response(Bun.file(join(tempDir, "partial.txt"))),
       "/partial-slice.txt": new Response(Bun.file(join(tempDir, "partial.txt")).slice(5, 10)),
+      // An unread Bun.file() stream is turned back into the file Blob when a
+      // route or handler response is built from it; the slice must survive.
+      "/partial-slice-stream.txt": new Response(Bun.file(join(tempDir, "partial.txt")).slice(5, 10).stream()),
+      "/partial-slice-stream-handler": () => new Response(Bun.file(join(tempDir, "partial.txt")).slice(5, 10).stream()),
+      "/partial-open-slice-stream-handler": () =>
+        new Response(Bun.file(join(tempDir, "partial.txt")).slice(10).stream()),
+      "/partial-empty-slice-stream-handler": () =>
+        new Response(Bun.file(join(tempDir, "partial.txt")).slice(5, 5).stream()),
+      "/partial-stream-handler": () => new Response(Bun.file(join(tempDir, "partial.txt")).stream()),
       "/fd-not-supported.txt": (() => {
         // This would test file descriptors, but they're not supported yet
         return new Response(Bun.file(join(tempDir, "hello.txt")));
@@ -713,6 +722,36 @@ describe("Bun.file in serve routes", () => {
       expect(res.headers.get("Content-Length")).toBe("5");
     });
 
+    it("serves the slice behind a sliced file's stream as a route", async () => {
+      const res = await fetch(new URL(`/partial-slice-stream.txt`, server.url));
+      expect(res.status).toBe(200);
+      expect(await res.text()).toBe("56789");
+      expect(res.headers.get("Content-Length")).toBe("5");
+    });
+
+    it("serves the slice behind a sliced file's stream from a handler", async () => {
+      const serve = async (pathname: string) => {
+        const get = await fetch(new URL(pathname, server.url));
+        const head = await fetch(new URL(pathname, server.url), { method: "HEAD" });
+        return {
+          body: await get.text(),
+          contentLength: get.headers.get("Content-Length"),
+          headContentLength: head.headers.get("Content-Length"),
+        };
+      };
+      expect({
+        "slice(5, 10)": await serve("/partial-slice-stream-handler"),
+        "slice(10)": await serve("/partial-open-slice-stream-handler"),
+        "slice(5, 5)": await serve("/partial-empty-slice-stream-handler"),
+        "whole file": await serve("/partial-stream-handler"),
+      }).toEqual({
+        "slice(5, 10)": { body: "56789", contentLength: "5", headContentLength: "5" },
+        "slice(10)": { body: "ABCDEF", contentLength: "6", headContentLength: "6" },
+        "slice(5, 5)": { body: "", contentLength: "0", headContentLength: "0" },
+        "whole file": { body: "0123456789ABCDEF", contentLength: "16", headContentLength: "16" },
+      });
+    });
+
     // The slice is shorter than the file, so the byte budget runs out before
     // the reader reports EOF: the response completes inline while a deferred
     // completion still hops through the event loop. Repeated requests must

test/js/web/fetch/body.test.ts

@@ -1,6 +1,6 @@
 import { file, spawn, version, type Socket } from "bun";
 import { describe, expect, test } from "bun:test";
-import { bunEnv, bunExe, exampleSite } from "harness";
+import { bunEnv, bunExe, exampleSite, tempDir } from "harness";
 import net from "net";
 
 const exampleServer = exampleSite("http");
@@ -287,6 +287,59 @@ for (const { body, fn } of bodyTypes) {
         expect(await subject.text()).toBe("bye");
         expect(subject.bodyUsed).toBe(true);
       });
+
+      // The readers move an unread Bun.file() stream back into a Blob the same
+      // way. That Blob has to cover the slice the stream was made from, not
+      // the whole file. (text() is not in here: it pumps the stream instead.)
+      describe("made from a sliced Bun.file()", () => {
+        const alphabet = "abcdefghijklmnopqrstuvwxyz";
+
+        test("bytes() returns the window of the slice the stream was made from", async () => {
+          using dir = tempDir("body-file-slice-stream", { "data.txt": alphabet });
+          const file = () => Bun.file(`${dir}/data.txt`);
+          const bytesOf = async (blob: Blob) => Buffer.from(await fn(blob.stream()).bytes()).toString();
+          expect({
+            "slice(3, 8)": await bytesOf(file().slice(3, 8)),
+            "slice(21)": await bytesOf(file().slice(21)),
+            "slice(3, 1000)": await bytesOf(file().slice(3, 1000)),
+            "slice(4, 4)": await bytesOf(file().slice(4, 4)),
+            "slice(3, 20).slice(2, 6)": await bytesOf(file().slice(3, 20).slice(2, 6)),
+            "whole file": await bytesOf(file()),
+          }).toEqual({
+            "slice(3, 8)": "defgh",
+            "slice(21)": "vwxyz",
+            "slice(3, 1000)": "defghijklmnopqrstuvwxyz",
+            "slice(4, 4)": "",
+            "slice(3, 20).slice(2, 6)": "fghi",
+            "whole file": alphabet,
+          });
+        });
+
+        test("arrayBuffer() and blob() return the slice too", async () => {
+          using dir = tempDir("body-file-slice-stream-readers", { "data.txt": alphabet });
+          const slice = () => Bun.file(`${dir}/data.txt`).slice(3, 8);
+          const blob = await fn(slice().stream()).blob();
+          expect({
+            arrayBuffer: Buffer.from(await fn(slice().stream()).arrayBuffer()).toString(),
+            blob: [blob.size, await blob.text()],
+          }).toEqual({
+            arrayBuffer: "defgh",
+            blob: [5, "defgh"],
+          });
+        });
+
+        test("json() parses only the slice", async () => {
+          using dir = tempDir("body-file-slice-stream-json", { "data.json": `--{"ok":true}--` });
+          expect(await fn(Bun.file(`${dir}/data.json`).slice(2, 13).stream()).json()).toEqual({ ok: true });
+        });
+
+        test("bytes() after the body getter turned a sliced Bun.file() body into a stream", async () => {
+          using dir = tempDir("body-file-slice-body-getter", { "data.txt": alphabet });
+          const subject = fn(Bun.file(`${dir}/data.txt`).slice(3, 8));
+          expect(subject.body).toBeInstanceOf(ReadableStream);
+          expect([Buffer.from(await subject.bytes()).toString(), subject.bodyUsed]).toEqual(["defgh", true]);
+        });
+      });
     });
     for (const { string, buffer } of utf8) {
       describe("arrayBuffer()", () => {