internal/synctest: new package for testing concurrent code

Add an internal (for now) implementation of testing/synctest.

The synctest.Run function executes a tree of goroutines in an
isolated environment using a fake clock. The synctest.Wait function
allows a test to wait for all other goroutines within the test
to reach a blocking point.

For #67434
For #69687

Change-Id: Icb39e54c54cece96517e58ef9cfb18bf68506cfc
Reviewed-on: https://go-review.googlesource.com/c/go/+/591997
Reviewed-by: Michael Pratt <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

src/go/build/deps_test.go

@@ -643,6 +643,9 @@ var depsRules = `
 	FMT, DEBUG, flag, runtime/trace, internal/sysinfo, math/rand
 	< testing;
 
+	RUNTIME
+	< internal/synctest;
+
 	log/slog, testing
 	< testing/slogtest;
 

src/internal/synctest/synctest.go

@@ -0,0 +1,62 @@
+// Copyright 2024 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 synctest provides support for testing concurrent code.
+package synctest
+
+import (
+	_ "unsafe" // for go:linkname
+)
+
+// Run executes f in a new goroutine.
+//
+// The new goroutine and any goroutines transitively started by it form
+// an isolated "bubble".
+// Run waits for all goroutines in the bubble to exit before returning.
+//
+// Goroutines in the bubble use a synthetic time implementation.
+// The initial time is midnight UTC 2000-01-01.
+//
+// Time advances when every goroutine in the bubble is blocked.
+// For example, a call to time.Sleep will block until all other
+// goroutines are blocked and return after the bubble's clock has
+// advanced. See [Wait] for the specific definition of blocked.
+//
+// If every goroutine is blocked and there are no timers scheduled,
+// Run panics.
+//
+// Channels, time.Timers, and time.Tickers created within the bubble
+// are associated with it. Operating on a bubbled channel, timer, or ticker
+// from outside the bubble panics.
+//
+//go:linkname Run
+func Run(f func())
+
+// Wait blocks until every goroutine within the current bubble,
+// other than the current goroutine, is durably blocked.
+// It panics if called from a non-bubbled goroutine,
+// or if two goroutines in the same bubble call Wait at the same time.
+//
+// A goroutine is durably blocked if can only be unblocked by another
+// goroutine in its bubble. The following operations durably block
+// a goroutine:
+//   - a send or receive on a channel from within the bubble
+//   - a select statement where every case is a channel within the bubble
+//   - sync.Cond.Wait
+//   - time.Sleep
+//
+// A goroutine executing a system call or waiting for an external event
+// such as a network operation is not durably blocked.
+// For example, a goroutine blocked reading from an network connection
+// is not durably blocked even if no data is currently available on the
+// connection, because it may be unblocked by data written from outside
+// the bubble or may be in the process of receiving data from a kernel
+// network buffer.
+//
+// A goroutine is not durably blocked when blocked on a send or receive
+// on a channel that was not created within its bubble, because it may
+// be unblocked by a channel receive or send from outside its bubble.
+//
+//go:linkname Wait
+func Wait()