Update

vendor/tailscale.com/util/testenv/synctest.go

@@ -0,0 +1,48 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package testenv
+
+import (
+	"bytes"
+	"flag"
+	"runtime"
+	"sync"
+)
+
+// stackBufPool holds buffers for [InSynctestBubble]'s runtime.Stack calls,
+// which would otherwise allocate on every call: the compiler cannot prove
+// that a stack-allocated buffer does not escape into runtime.Stack. Plain
+// byte arrays, unlike the zstd coders that motivated InSynctestBubble, are
+// safe to share across synctest bubble boundaries.
+var stackBufPool = sync.Pool{New: func() any { return new([128]byte) }}
+
+// InSynctestBubble reports whether the current goroutine is running within a
+// testing/synctest bubble.
+//
+// As of Go 1.26 there is no public API to query bubble membership
+// (internal/synctest.IsInBubble is runtime-internal), so this instead
+// looks for the "synctest bubble N" annotation that the runtime renders
+// in the current goroutine's [runtime.Stack] header. If a future Go
+// release changes that annotation, this reports false; tests that depend
+// on it for correctness (such as tailscale.com/util/zstdframe's) should
+// exercise the misdetection failure mode so the breakage is loud.
+func InSynctestBubble() bool {
+	// Bubbles only exist within tests, so skip the (relatively) expensive
+	// stack header check in non-test binaries. This deliberately does not
+	// use InTest: InSynctestBubble may be reached from package init
+	// functions (before the testing package has registered its flags),
+	// and InTest would permanently latch a false result there. Bubbles
+	// cannot exist during init, so returning false then is correct.
+	if flag.Lookup("test.v") == nil {
+		return false
+	}
+	// The current goroutine's header looks like:
+	//	goroutine 9 [running, synctest bubble 1]:
+	// A 128-byte buffer always fits the header, and runtime.Stack
+	// truncates the rest.
+	buf := stackBufPool.Get().(*[128]byte)
+	defer stackBufPool.Put(buf)
+	n := runtime.Stack(buf[:], false)
+	return bytes.Contains(buf[:n], []byte(" synctest bubble "))
+}