sync: move Mutex to internal/task

The mutex implementation needs a different implementation once support
for threading lands. This implementation just moves code to the
internal/task package to centralize these algorithms.

src/internal/task/mutex-cooperative.go

@@ -0,0 +1,43 @@
+package task
+
+type Mutex struct {
+	locked  bool
+	blocked Stack
+}
+
+func (m *Mutex) Lock() {
+	if m.locked {
+		// Push self onto stack of blocked tasks, and wait to be resumed.
+		m.blocked.Push(Current())
+		Pause()
+		return
+	}
+
+	m.locked = true
+}
+
+func (m *Mutex) Unlock() {
+	if !m.locked {
+		panic("sync: unlock of unlocked Mutex")
+	}
+
+	// Wake up a blocked task, if applicable.
+	if t := m.blocked.Pop(); t != nil {
+		scheduleTask(t)
+	} else {
+		m.locked = false
+	}
+}
+
+// TryLock tries to lock m and reports whether it succeeded.
+//
+// Note that while correct uses of TryLock do exist, they are rare,
+// and use of TryLock is often a sign of a deeper problem
+// in a particular use of mutexes.
+func (m *Mutex) TryLock() bool {
+	if m.locked {
+		return false
+	}
+	m.Lock()
+	return true
+}

src/sync/cond.go

@@ -89,3 +89,6 @@ func (c *Cond) Wait() {
 	// signal.
 	task.Pause()
 }
+
+//go:linkname scheduleTask runtime.scheduleTask
+func scheduleTask(*task.Task)

src/sync/mutex.go

@@ -2,53 +2,9 @@ package sync
 
 import (
 	"internal/task"
-	_ "unsafe"
 )
 
-type Mutex struct {
-	locked  bool
-	blocked task.Stack
-}
-
-//go:linkname scheduleTask runtime.scheduleTask
-func scheduleTask(*task.Task)
-
-func (m *Mutex) Lock() {
-	if m.locked {
-		// Push self onto stack of blocked tasks, and wait to be resumed.
-		m.blocked.Push(task.Current())
-		task.Pause()
-		return
-	}
-
-	m.locked = true
-}
-
-func (m *Mutex) Unlock() {
-	if !m.locked {
-		panic("sync: unlock of unlocked Mutex")
-	}
-
-	// Wake up a blocked task, if applicable.
-	if t := m.blocked.Pop(); t != nil {
-		scheduleTask(t)
-	} else {
-		m.locked = false
-	}
-}
-
-// TryLock tries to lock m and reports whether it succeeded.
-//
-// Note that while correct uses of TryLock do exist, they are rare,
-// and use of TryLock is often a sign of a deeper problem
-// in a particular use of mutexes.
-func (m *Mutex) TryLock() bool {
-	if m.locked {
-		return false
-	}
-	m.Lock()
-	return true
-}
+type Mutex = task.Mutex
 
 type RWMutex struct {
 	// waitingWriters are all of the tasks waiting for write locks.