runtime: use the immortal weak handle map for sbrk mode

mknyszek · gopherbot · commit 53b9eae38751 · 2025-05-21T21:20:52.000-07:00
Currently weak pointers break in sbrk mode. We can just use the immortal weak handle map for weak pointers in this case, since nothing is ever freed. Fixes #69729. Change-Id: Ie9fa7e203c22776dc9eb3601c6480107d9ad0c99 Reviewed-on: https://go-review.googlesource.com/c/go/+/674656 Reviewed-by: Carlos Amedee <carlos@golang.org> Auto-Submit: Michael Knyszek <mknyszek@google.com> TryBot-Bypass: Michael Knyszek <mknyszek@google.com>
diff --git a/src/runtime/mheap.go b/src/runtime/mheap.go
@@ -2545,6 +2545,16 @@ func gcWakeAllStrongFromWeak() {
 
 // Retrieves or creates a weak pointer handle for the object p.
 func getOrAddWeakHandle(p unsafe.Pointer) *atomic.Uintptr {
+	if debug.sbrk != 0 {
+		// debug.sbrk never frees memory, so it'll never go nil. However, we do still
+		// need a weak handle that's specific to p. Use the immortal weak handle map.
+		// Keep p alive across the call to getOrAdd defensively, though it doesn't
+		// really matter in this particular case.
+		handle := mheap_.immortalWeakHandles.getOrAdd(uintptr(p))
+		KeepAlive(p)
+		return handle
+	}
+
 	// First try to retrieve without allocating.
 	if handle := getWeakHandle(p); handle != nil {
 		// Keep p alive for the duration of the function to ensure
