3 files changed, 119 insertions, 0 deletions
diff --git a/syncs/tsync/tsync.go b/syncs/tsync/tsync.go
new file mode 100644
index 000000000..0cda18be8
--- /dev/null
+++ b/syncs/tsync/tsync.go
@@ -0,0 +1 @@
+package tsync
diff --git a/syncs/tsync/tsync_noop.go b/syncs/tsync/tsync_noop.go
new file mode 100644
index 000000000..a536a9064
--- /dev/null
+++ b/syncs/tsync/tsync_noop.go
@@ -0,0 +1,10 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_tsync_test
+
+package tsync
+
+import "sync"
+
+type Mutex = sync.Mutex
diff --git a/syncs/tsync/tsync_real.go b/syncs/tsync/tsync_real.go
new file mode 100644
index 000000000..b81346fd2
--- /dev/null
+++ b/syncs/tsync/tsync_real.go
@@ -0,0 +1,108 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ts_tsync_test
+
+package tsync
+
+import (
+	"fmt"
+	"sync"
+	"weak"
+)
+
+type Mutex struct {
+	mu sync.Mutex
+}
+
+func (m *Mutex) Lock() {
+	noteLock(m)
+	m.mu.Lock()
+}
+
+func (m *Mutex) Unlock() {
+	noteUnlock(m)
+	m.mu.Unlock()
+}
+
+func (m *Mutex) TryLock() bool {
+	locked := m.mu.TryLock()
+	if locked {
+		noteLock(m)
+	}
+	return locked
+}
+
+type lockInfo struct {
+	locked bool
+}
+
+var (
+	// TODO: this should be a per-G datastructure
+	locksMu sync.Mutex
+	locks   map[weak.Pointer[Mutex]]*lockInfo
+)
+
+func panicLocked(format string, args ...any) {
+	msg := fmt.Sprintf(format, args...)
+
+	// First, gather all current (non-GCed) locks.
+	type lockEntry struct {
+		wp weak.Pointer[Mutex]
+		li *lockInfo
+	}
+	var currLocks []lockEntry
+	for wp, li := range locks {
+		if wp.Value() != nil {
+			currLocks = append(currLocks, lockEntry{wp, li})
+		}
+	}
+
+	msg += fmt.Sprintf("\ncurrent locks (%d):", len(currLocks))
+	for _, cl := range currLocks {
+		if cl.li.locked {
+			msg += fmt.Sprintf("\n\tlocked: %p", cl.wp.Value())
+		} else {
+			msg += fmt.Sprintf("\n\tunlocked: %p", cl.wp.Value())
+		}
+	}
+	panic(msg)
+}
+
+func noteLock(m *Mutex) {
+	locksMu.Lock()
+	defer locksMu.Unlock()
+	if locks == nil {
+		locks = make(map[weak.Pointer[Mutex]]*lockInfo)
+	}
+
+	wp := weak.Make(m)
+	li, ok := locks[wp]
+	if !ok {
+		locks[wp] = &lockInfo{
+			locked: true,
+		}
+		return
+	}
+
+	li.locked = true
+
+	// TODO: additional checks here
+	// TODO: clear things out of the locks map when GCed
+}
+
+func noteUnlock(m *Mutex) {
+	locksMu.Lock()
+	defer locksMu.Unlock()
+
+	wp := weak.Make(m)
+	li, ok := locks[wp]
+	if !ok {
+		panicLocked("unknown Unlock on mutex %p", m)
+	}
+
+	li.locked = false
+
+	// TODO: additional checks here
+	// TODO: clear things out of the locks map when GCed
+}