runtime: overwrite existing keys for mapassign_faststr variant

Fixes #45045

Change-Id: Ifcc7bd31591870446ce3e5127489a0b887d413f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/305089
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/map_faststr.go

@@ -255,6 +255,9 @@ bucketloop:
 			// already have a mapping for key. Update it.
 			inserti = i
 			insertb = b
+			// Overwrite existing key, so it can be garbage collected.
+			// The size is already guaranteed to be set correctly.
+			k.str = key.str
 			goto done
 		}
 		ovf := b.overflow(t)

test/fixedbugs/issue45045.go

@@ -0,0 +1,51 @@
+// run
+
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"reflect"
+	"runtime"
+	"unsafe"
+)
+
+func k(c chan string, val string) string {
+	b := make([]byte, 1000)
+	runtime.SetFinalizer(&b[0], func(*byte) {
+		c <- val
+	})
+	var s string
+	h := (*reflect.StringHeader)(unsafe.Pointer(&s))
+	h.Data = uintptr(unsafe.Pointer(&b[0]))
+	h.Len = len(b)
+	return s
+}
+
+func main() {
+	{
+		c := make(chan string, 2)
+		m := make(map[string]int)
+		m[k(c, "first")] = 0
+		m[k(c, "second")] = 0
+		runtime.GC()
+		if s := <-c; s != "first" {
+			panic("map[string], second key did not retain.")
+		}
+		runtime.KeepAlive(m)
+	}
+
+	{
+		c := make(chan string, 2)
+		m := make(map[[2]string]int)
+		m[[2]string{k(c, "first")}] = 0
+		m[[2]string{k(c, "second")}] = 0
+		runtime.GC()
+		if s := <-c; s != "first" {
+			panic("map[[2]string], second key did not retain.")
+		}
+		runtime.KeepAlive(m)
+	}
+}