runtime: use the immortal weak handle map for sbrk mode

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
index 0729184e22e6f92c206ab01ca7d8d7063b285838..3612d71e666a0f9b713be654e11049079e29c5a6 100644 (file)
--- 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