shell: fail the command when a > ${buf} redirect overflows the target Buffer

src/runtime/shell/Builtin.rs

@@ -29,6 +29,10 @@
     /// Set by `done()` and stashed by `write_failing_error` so the async
     /// `on_io_writer_chunk` path can recover the intended exit code.
     pub exit_code: Option<ExitCode>,
+    /// First error latched by [`Builtin::write_no_io`]; [`Builtin::done`]
+    /// folds it into the exit code so a builtin whose only failure is a write
+    /// error (`> ${buf}` too small) does not exit 0.
+    pub write_err: Option<bun_sys::Error>,
     /// Scratch for `fmt_error_arena`. One outstanding error string at a time.
     pub err_buf: Vec<u8>,
     pub impl_: Impl,
@@ -393,16 +397,18 @@
                 // stored cursor is u32.
                 let idx = *i as usize;
                 let total = arraybuf.array_buffer.byte_len as usize;
-                if idx >= total {
+                let write_len = total.saturating_sub(idx).min(buf.len());
+                if write_len > 0 {
+                    let dst = &mut arraybuf.slice_mut()[idx..idx + write_len];
+                    dst.copy_from_slice(&buf[..write_len]);
+                    *i = i.saturating_add(write_len as u32);
+                }
+                if write_len < buf.len() {
                     return Err(bun_sys::Error::from_code(
                         bun_sys::E::ENOSPC,
                         bun_sys::Tag::write,
                     ));
                 }
-                let write_len = (total - idx).min(buf.len());
-                let dst = &mut arraybuf.slice_mut()[idx..idx + write_len];
-                dst.copy_from_slice(&buf[..write_len]);
-                *i = i.saturating_add(write_len as u32);
                 Ok(write_len)
             }
             BuiltinIO::Blob(_) | BuiltinIO::Ignore => Ok(buf.len()),
@@ -535,6 +541,7 @@
             stdout,
             stderr,
             exit_code: None,
+            write_err: None,
             err_buf: Vec::new(),
             impl_: Self::make_impl(kind),
         }));
@@ -863,6 +870,21 @@
 
     /// Finish the builtin with `exit_code` and signal the owning Cmd.
     pub fn done(interp: &Interpreter, cmd: NodeId, exit_code: ExitCode) -> Yield {
+        // A `write_no_io` error that the builtin itself did not handle
+        // (discarded `let _ = write_no_io(...)`) fails the command here,
+        // mirroring the async `on_io_writer_chunk` error path.
+        let exit_code = match (exit_code, Self::of_mut(interp, cmd).write_err.take()) {
+            (0, Some(e)) => {
+                if Self::of(interp, cmd).stderr.needs_io().is_none() {
+                    let msg =
+                        Self::task_error_to_string(interp, cmd, Self::kind_of(interp, cmd), &e)
+                            .to_vec();
+                    let _ = Self::write_no_io(interp, cmd, IoKind::Stderr, &msg);
+                }
+                1
+            }
+            (code, _) => code,
+        };
         Self::of_mut(interp, cmd).exit_code = Some(exit_code);
         // Output is written through immediately in `write_no_io`, so there
         // is nothing to flush here.
@@ -931,7 +953,13 @@
             IoKind::Stdin => return Ok(0),
         };
         // SAFETY: `shell` is `cmd_node.base.shell`, live for the Cmd's lifetime.
-        unsafe { out.write_no_io_to(shell, buf) }
+        let r = unsafe { out.write_no_io_to(shell, buf) };
+        if let Err(e) = &r {
+            if me.write_err.is_none() {
+                me.write_err = Some(e.clone());
+            }
+        }
+        r
     }
 
     /// Shell exec env of the owning Cmd.

src/runtime/shell/subproc.rs

@@ -1324,6 +1324,7 @@
                         .buffered_output = BufferedOutput::ArrayBuffer {
                         buf: core::mem::take(array_buffer),
                         i: 0,
+                        overflow: false,
                     };
                     Readable::Pipe(pipe)
                 }
@@ -1369,6 +1370,7 @@
                         .buffered_output = BufferedOutput::ArrayBuffer {
                         buf: core::mem::take(array_buffer),
                         i: 0,
+                        overflow: false,
                     };
                     Readable::Pipe(pipe)
                 }
@@ -1648,6 +1650,10 @@
     ArrayBuffer {
         buf: jsc::array_buffer::ArrayBufferStrong,
         i: u32,
+        /// Set by [`Self::append`] when the target buffer cannot hold the
+        /// full chunk; read by [`PipeReader::try_signal_done_to_cmd`] to fail
+        /// the command instead of exiting 0 with truncated output.
+        overflow: bool,
     },
 }
 
@@ -1678,19 +1684,25 @@
             BufferedOutput::Bytelist(b) => {
                 let _ = b.append_slice(bytes); // OOM/capacity: fire-and-forget
             }
-            BufferedOutput::ArrayBuffer { buf, i } => {
+            BufferedOutput::ArrayBuffer { buf, i, overflow } => {
                 let array_buf_slice = buf.slice_mut();
                 let idx = *i as usize;
-                // TODO: We should probably throw error here?
-                if idx >= array_buf_slice.len() {
-                    return;
+                let length = array_buf_slice.len().saturating_sub(idx).min(bytes.len());
+                if length > 0 {
+                    array_buf_slice[idx..idx + length].copy_from_slice(&bytes[..length]);
+                    *i += u32::try_from(length).expect("int cast");
+                }
+                if length < bytes.len() {
+                    *overflow = true;
                 }
-                let length = (array_buf_slice.len() - idx).min(bytes.len());
-                array_buf_slice[idx..idx + length].copy_from_slice(&bytes[..length]);
-                *i += u32::try_from(length).expect("int cast");
             }
         }
     }
+
+    #[inline]
+    pub fn overflowed(&self) -> bool {
+        matches!(self, BufferedOutput::ArrayBuffer { overflow: true, .. })
+    }
 }
 
 impl Drop for BufferedOutput {
@@ -2211,6 +2223,11 @@
                 // `state.Err` after this point is `Drop`, which tolerates `None`).
                 if let PipeReaderState::Err(slot) = &mut me.state {
                     slot.take().map(|b| *b)
+                } else if me.buffered_output.overflowed() {
+                    Some(
+                        bun_sys::Error::from_code(bun_sys::E::ENOSPC, bun_sys::Tag::write)
+                            .to_system_error(),
+                    )
                 } else {
                     None
                 }

test/js/bun/shell/bunshell.test.ts

@@ -463,6 +463,61 @@ describe("bunshell", () => {
     expect(new TextDecoder().decode(buffer.slice(0, sentinel))).toEqual(await thisFile.text());
   });
 
+  describe("redirect into a too-small Buffer fails the command", () => {
+    // A `> ${buf}` target that cannot hold the full output must surface as a
+    // nonzero exit code (like `> /dev/full`), not exit 0 with the data
+    // silently cut short.
+    test("builtin", async () => {
+      const buf = Buffer.alloc(4);
+      const r = await $`echo hello world > ${buf}`.quiet();
+      expect({
+        written: buf.toString("latin1"),
+        stderr: r.stderr.toString(),
+        exitCode: r.exitCode,
+      }).toEqual({
+        written: "hell",
+        stderr: "echo: No space left on device\n",
+        exitCode: 1,
+      });
+    });
+
+    test("builtin: `|| rhs` runs", async () => {
+      const buf = Buffer.alloc(4);
+      const r = await $`echo hello world > ${buf} || echo write_failed`.quiet();
+      expect({ stdout: r.stdout.toString(), exitCode: r.exitCode }).toEqual({
+        stdout: "write_failed\n",
+        exitCode: 0,
+      });
+    });
+
+    test("builtin: exact fit succeeds", async () => {
+      const buf = Buffer.alloc(12);
+      const r = await $`echo hello world > ${buf}`.quiet();
+      expect({
+        written: buf.toString("latin1"),
+        stderr: r.stderr.toString(),
+        exitCode: r.exitCode,
+      }).toEqual({ written: "hello world\n", stderr: "", exitCode: 0 });
+    });
+
+    test("subprocess", async () => {
+      const buf = Buffer.alloc(4);
+      const r = await $`${BUN} -e ${'process.stdout.write("hello world")'} > ${buf}`.quiet();
+      expect(buf.toString("latin1")).toBe("hell");
+      expect(r.exitCode).not.toBe(0);
+    });
+
+    test("subprocess: `|| rhs` runs", async () => {
+      const buf = Buffer.alloc(4);
+      const r =
+        await $`${BUN} -e ${'process.stdout.write("hello world")'} > ${buf} || echo write_failed`.quiet();
+      expect({ stdout: r.stdout.toString(), exitCode: r.exitCode }).toEqual({
+        stdout: "write_failed\n",
+        exitCode: 0,
+      });
+    });
+  });
+
   test("redirect Buffer", async () => {
     const buffer = Buffer.alloc(1 << 20);
     const result = await $`cat ${import.meta.path} > ${buffer}`;