Watcher: flush evict_list in remove() when full to prevent overflow

src/Watcher.zig

@@ -758,8 +758,19 @@
 pub fn remove(this: *Watcher, hash: HashType) void {
     this.mutex.lock();
     defer this.mutex.unlock();
     if (this.indexOf(hash)) |index| {
+        // `removeAtIndex` only queues the index into `evict_list`; the real
+        // removal happens in `flushEvictions()`, which is normally driven by
+        // `onFileUpdate()` on the watcher thread. `remove()` can be called
+        // many times without any fs events firing (e.g. `fs.watch()`
+        // followed by `.close()` in a tight loop), so the fixed-size
+        // `evict_list` can fill up and overflow. Drain it here when full —
+        // we already hold `this.mutex`, matching how `flushEvictions()` is
+        // invoked from the platform watch loops.
+        if (this.evict_list_i >= max_eviction_count) {
+            this.flushEvictions();
+        }
         this.removeAtIndex(@truncate(index), hash, &[_]HashType{}, .file);
     }
 }
 

test/js/node/watch/fs.watch.evict-list-overflow.test.ts

@@ -0,0 +1,78 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, isMacOS, isWindows, tempDir } from "harness";
+
+// Regression test for `Watcher.evict_list` overflow.
+//
+// `Watcher.remove()` appends the watchlist index to a fixed-size
+// `evict_list[max_eviction_count = 8096]` buffer; the actual removal
+// happens in `flushEvictions()`, which was only driven by `onFileUpdate()`
+// on the watcher thread — i.e. only when a filesystem event arrives.
+//
+// `fs.watch(path).close()` reaches `_decrementPathRefNoLock()` →
+// `main_watcher.remove(hash)` for the watched path. Repeating that
+// without ever modifying the filesystem means `flushEvictions()` never
+// runs, and once the cumulative remove count passes 8096,
+// `removeAtIndex()` writes past the end of `evict_list`:
+//
+//   panic(main thread): index out of bounds: index 8096, len 8096
+//   Watcher.removeAtIndex  src/Watcher.zig
+//   Watcher.remove
+//   PathWatcherManager._decrementPathRefNoLock
+//
+// The fix drains `evict_list` inside `remove()` when it's full (the
+// mutex is already held there, matching how `flushEvictions()` is
+// invoked from the platform watch loops).
+//
+// Watching a single file (not a directory) keeps the test deterministic:
+// file watches don't schedule a `DirectoryRegisterTask`, so `deinit()`
+// runs to completion on every `close()` and exactly one `remove()` is
+// issued per iteration; and the inotify file mask doesn't include
+// IN_OPEN / IN_CLOSE so re-opening the fd each cycle doesn't generate
+// events that would opportunistically flush.
+//
+// Windows uses win_watcher.zig (no evict_list); macOS directory watches
+// use FSEvents. The overflow is reachable on Linux/FreeBSD only.
+test.skipIf(isWindows || isMacOS)(
+  "Watcher.remove() does not overflow evict_list when no fs events fire",
+  async () => {
+    using dir = tempDir("fswatch-evict-overflow", { "f.txt": "x" });
+
+    const fixture = /* js */ `
+      const fs = require("fs");
+      const path = require("path");
+      const target = path.join(process.argv[1], "f.txt");
+      // > max_eviction_count (8096): one remove() per cycle, no fs events,
+      // so without the fix evict_list_i hits 8096 and removeAtIndex panics.
+      const ITERS = 8200;
+      for (let i = 0; i < ITERS; i++) {
+        const w = fs.watch(target, { persistent: false }, () => {});
+        w.close();
+      }
+      console.log("ok " + ITERS);
+    `;
+
+    await using proc = Bun.spawn({
+      cmd: [bunExe(), "-e", fixture, String(dir)],
+      env: bunEnv,
+      stdout: "pipe",
+      stderr: "pipe",
+    });
+    const [stdout, stderr, exitCode] = await Promise.all([
+      proc.stdout.text(),
+      proc.stderr.text(),
+      proc.exited,
+    ]);
+
+    const filteredStderr = stderr
+      .split("\n")
+      .filter(l => l && !l.startsWith("WARNING: ASAN interferes"))
+      .join("\n");
+
+    expect({ stdout: stdout.trim(), stderr: filteredStderr, exitCode }).toEqual({
+      stdout: "ok 8200",
+      stderr: "",
+      exitCode: 0,
+    });
+  },
+  30000,
+);