bundler: report an unresolvable relative import longer than the path buffer instead of panicking in watch mode

src/resolver/resolver.rs

@@ -102,18 +102,6 @@
             |P| ::bun_paths::resolve_path::join_abs_string_buf::<P>(cwd, buf, parts)
         )
     }
-    pub(super) fn join_abs(cwd: &[u8], platform: Platform, part: &[u8]) -> &'static [u8] {
-        // NOTE: `resolve_path::join_abs` ties the result lifetime to `cwd`, but the
-        // returned slice always points into the threadlocal `PARSER_JOIN_INPUT_BUFFER`
-        // (or is `cwd` itself when `parts.is_empty()`, which never happens here — we
-        // pass exactly one part). Re-erase to `'static` so the resolver can hold it
-        // across `&mut self` calls.
-        let s = dispatch_platform!(platform, |P| ::bun_paths::resolve_path::join_abs::<P>(
-            cwd, part
-        ));
-        // SAFETY: see NOTE — slice borrows threadlocal storage, valid 'static per-thread.
-        unsafe { bun_ptr::detach_lifetime(s) }
-    }
     pub(super) fn join(parts: &[&[u8]], platform: Platform) -> &'static [u8] {
         dispatch_platform!(platform, |P| ::bun_paths::resolve_path::join::<P>(parts))
     }
@@ -2482,11 +2470,15 @@
             return false;
         }
 
-        let joined = bun_paths::join_abs(
-            bun_paths::dirname_platform(import_source_file, bun_paths::Platform::AUTO),
-            bun_paths::Platform::AUTO,
-            specifier,
-        );
+        // `specifier` is arbitrary source text. A path that does not fit in a
+        // path buffer cannot be in either cache, so there is nothing to bust.
+        let source_dir = bun_paths::dirname_platform(import_source_file, bun_paths::Platform::AUTO);
+        let mut buf = bun_paths::path_buffer_pool::get();
+        let Some(joined) = bun_paths::resolve_path::join_abs_string_buf_checked::<
+            bun_paths::platform::Auto,
+        >(source_dir, &mut buf.0, &[specifier]) else {
+            return false;
+        };
         let dir = bun_paths::dirname_platform(joined, bun_paths::Platform::AUTO);
 
         let a = self.bust_dir_cache(dir);

src/runtime/bake/dev_server/mod.rs

@@ -1273,12 +1273,18 @@ impl DirectoryWatchStore {
             _ => debug_assert!(false),
         }
 
+        // `specifier` is arbitrary source text. A path that does not fit in a
+        // path buffer cannot exist, so there is no directory to watch for it.
         let mut buf = bun_paths::path_buffer_pool::get();
-        let joined = bun_paths::resolve_path::join_abs_string_buf::<bun_paths::platform::Auto>(
-            bun_paths::resolve_path::dirname::<bun_paths::platform::Auto>(import_source),
-            &mut buf.0,
-            &[specifier],
-        );
+        let Some(joined) =
+            bun_paths::resolve_path::join_abs_string_buf_checked::<bun_paths::platform::Auto>(
+                bun_paths::resolve_path::dirname::<bun_paths::platform::Auto>(import_source),
+                &mut buf.0,
+                &[specifier],
+            )
+        else {
+            return Ok(());
+        };
         let dir = bun_paths::resolve_path::dirname::<bun_paths::platform::Auto>(joined);
 
         // The `import_source` parameter is not a stable string. Since the

test/bake/dev/bundle.test.ts

@@ -1,5 +1,6 @@
 // Bundle tests are tests concerning bundling bugs that only occur in DevServer.
 import { expect } from "bun:test";
+import { isWindows } from "harness";
 import { devTest, emptyHtmlFile, minimalFramework } from "../bake-harness";
 
 devTest("import identifier doesnt get renamed", {
@@ -865,3 +866,46 @@ devTest("barrel optimization: namespace re-export cycle through a star-exported
     await c.expectMessage("result: object Y KEEP DEEP OTHER");
   },
 });
+
+// Resolution failures are tracked so the import is retried when its directory
+// changes (Resolver.bust_dir_cache_from_specifier, then
+// DirectoryWatchStore.track_resolution_failure). Both joined the importer's
+// directory with the specifier into a fixed-size path buffer; a specifier that
+// did not fit aborted the whole process with
+// "panic: range end index N out of range for slice of length M" instead of
+// reporting the unresolved import. The buffer is MAX_PATH_BYTES: 4 KiB on
+// Linux, 1 KiB on macOS and about 96 KiB on Windows.
+const specifierLongerThanPathBuffer = Buffer.alloc((isWindows ? 96 : 4) * 1024 + 1024, "a").toString();
+
+devTest("unresolvable relative import longer than the path buffer is a bundling error", {
+  files: {
+    "index.html": emptyHtmlFile({ scripts: ["index.ts"] }),
+    "index.ts": `
+      import './${specifierLongerThanPathBuffer}';
+      console.log('loaded');
+    `,
+  },
+  async test(dev) {
+    expect((await dev.fetch("/")).status).toBe(500);
+    await dev.write("index.ts", `console.log('fixed');`);
+    expect((await dev.fetch("/")).status).toBe(200);
+  },
+});
+
+// A CSS url() without "./" skips the resolver's directory cache busting, so
+// this only reaches DirectoryWatchStore.track_resolution_failure.
+devTest("unresolvable css url() longer than the path buffer is a bundling error", {
+  files: {
+    "index.html": emptyHtmlFile({ styles: ["styles.css"] }),
+    "styles.css": `
+      body {
+        background-image: url(${specifierLongerThanPathBuffer});
+      }
+    `,
+  },
+  async test(dev) {
+    expect((await dev.fetch("/")).status).toBe(500);
+    await dev.write("styles.css", `body { color: blue; }`);
+    expect((await dev.fetch("/")).status).toBe(200);
+  },
+});

test/bundler/cli.test.ts

@@ -466,6 +466,50 @@ test("multi-entry build writes each entry point into the output directory", asyn
   expect(b).toContain('"B"');
 });
 
+async function readUntil(stream: ReadableStream<Uint8Array>, needle: string): Promise<string> {
+  const reader = stream.getReader();
+  const decoder = new TextDecoder();
+  let output = "";
+  try {
+    while (!output.includes(needle)) {
+      const { value, done } = await reader.read();
+      if (done) throw new Error(`stream closed before ${JSON.stringify(needle)} appeared. Output:\n${output}`);
+      output += decoder.decode(value, { stream: true });
+    }
+  } finally {
+    reader.releaseLock();
+  }
+  return output;
+}
+
+test("--watch reports an unresolvable relative import longer than the path buffer and keeps watching", async () => {
+  // When watching, an unresolved relative import busts the resolver's directory
+  // cache for the path it would have resolved to. That path used to be joined
+  // into a 4 KiB buffer, so a longer specifier aborted the process with
+  // "panic: range end index N out of range for slice of length 4095".
+  const specifier = "./" + Buffer.alloc(5 * 1024, "a").toString();
+  using dir = tempDir("build-watch-long-specifier", {
+    "entry.ts": `import "${specifier}";\nconsole.log("entry");`,
+  });
+
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "build", "--watch", "entry.ts", "--outdir", "dist"],
+    env: bunEnv,
+    cwd: String(dir),
+    stdin: "ignore",
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  await readUntil(proc.stderr, `error: Could not resolve: "${specifier}"`);
+  expect(proc.exitCode).toBeNull();
+
+  // The failed build left the watcher running: fixing the file triggers a rebuild.
+  await Bun.write(path.join(String(dir), "entry.ts"), `console.log("fixed");`);
+  expect(await readUntil(proc.stdout, "entry.js")).toContain("Bundled 1 module");
+  expect(await Bun.file(path.join(String(dir), "dist", "entry.js")).text()).toContain("fixed");
+});
+
 describe("CLI argument error messages", () => {
   test("--format with an unrecognized value echoes the value back", async () => {
     using dir = tempDir("build-format-err", { "in.js": "console.log(1)" });