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 "))
+}

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

@@ -1,4 +1,4 @@
-// Copyright (c) Tailscale Inc & AUTHORS
+// Copyright (c) Tailscale Inc & contributors
 // SPDX-License-Identifier: BSD-3-Clause
 
 // Package testenv provides utility functions for tests. It does not depend on
@@ -8,6 +8,7 @@ package testenv
 import (
 	"context"
 	"flag"
+	"io"
 
 	"tailscale.com/types/lazy"
 )
@@ -21,8 +22,23 @@ func InTest() bool {
 	})
 }
 
+// Verbose reports whether the test binary is running with verbose
+// output (the -test.v flag), like testing.Verbose but without
+// importing the testing package.
+func Verbose() bool {
+	f := flag.Lookup("test.v")
+	if f == nil {
+		return false
+	}
+	// The flag's String value is "false" when off, and "true" or
+	// "test2json" when on.
+	return f.Value.String() != "false"
+}
+
 // TB is testing.TB, to avoid importing "testing" in non-test code.
 type TB interface {
+	ArtifactDir() string
+	Attr(key, value string)
 	Cleanup(func())
 	Error(args ...any)
 	Errorf(format string, args ...any)
@@ -35,6 +51,7 @@ type TB interface {
 	Log(args ...any)
 	Logf(format string, args ...any)
 	Name() string
+	Output() io.Writer
 	Setenv(key, value string)
 	Chdir(dir string)
 	Skip(args ...any)