sync: panic rather than throw on nil *Pool

Fixes #61651

Change-Id: I27d581719e6bf38910f9d47dcf023bbff74ddf72
Reviewed-on: https://go-review.googlesource.com/c/go/+/514037
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>

src/sync/pool.go

@@ -196,6 +196,13 @@ func (p *Pool) getSlow(pid int) any {
 // returns poolLocal pool for the P and the P's id.
 // Caller must call runtime_procUnpin() when done with the pool.
 func (p *Pool) pin() (*poolLocal, int) {
+	// Check whether p is nil to get a panic.
+	// Otherwise the nil dereference happens while the m is pinned,
+	// causing a fatal error rather than a panic.
+	if p == nil {
+		panic("nil Pool")
+	}
+
 	pid := runtime_procPin()
 	// In pinSlow we store to local and then to localSize, here we load in opposite order.
 	// Since we've disabled preemption, GC cannot happen in between.

src/sync/pool_test.go

@@ -247,6 +247,28 @@ func testPoolDequeue(t *testing.T, d PoolDequeue) {
 	}
 }
 
+func TestNilPool(t *testing.T) {
+	catch := func() {
+		if recover() == nil {
+			t.Error("expected panic")
+		}
+	}
+
+	var p *Pool
+	t.Run("Get", func(t *testing.T) {
+		defer catch()
+		if p.Get() != nil {
+			t.Error("expected empty")
+		}
+		t.Error("should have panicked already")
+	})
+	t.Run("Put", func(t *testing.T) {
+		defer catch()
+		p.Put("a")
+		t.Error("should have panicked already")
+	})
+}
+
 func BenchmarkPool(b *testing.B) {
 	var p Pool
 	b.RunParallel(func(pb *testing.PB) {