[dev.link] all: merge branch 'master' into dev.link

Weekly merge.

Change-Id: I98c6121f04f347df2788ac5eaf99afad5da4a039

4 files changed, 23 insertions, 6 deletions

diff --git a/src/os/exec/exec.go b/src/os/exec/exec.go
index 17ef003eca..19c7e2406a 100644
--- a/src/os/exec/exec.go
+++ b/src/os/exec/exec.go
@@ -606,8 +606,8 @@ func (c *closeOnce) close() {
 // standard output when the command starts.
 //
 // Wait will close the pipe after seeing the command exit, so most callers
-// need not close the pipe themselves; however, an implication is that
-// it is incorrect to call Wait before all reads from the pipe have completed.
+// need not close the pipe themselves. It is thus incorrect to call Wait
+// before all reads from the pipe have completed.
 // For the same reason, it is incorrect to call Run when using StdoutPipe.
 // See the example for idiomatic usage.
 func (c *Cmd) StdoutPipe() (io.ReadCloser, error) {
@@ -631,8 +631,8 @@ func (c *Cmd) StdoutPipe() (io.ReadCloser, error) {
 // standard error when the command starts.
 //
 // Wait will close the pipe after seeing the command exit, so most callers
-// need not close the pipe themselves; however, an implication is that
-// it is incorrect to call Wait before all reads from the pipe have completed.
+// need not close the pipe themselves. It is thus incorrect to call Wait
+// before all reads from the pipe have completed.
 // For the same reason, it is incorrect to use Run when using StderrPipe.
 // See the StdoutPipe example for idiomatic usage.
 func (c *Cmd) StderrPipe() (io.ReadCloser, error) {
diff --git a/src/os/signal/signal.go b/src/os/signal/signal.go
index a0eba0d50f..136dd9cc97 100644
--- a/src/os/signal/signal.go
+++ b/src/os/signal/signal.go
@@ -92,6 +92,15 @@ func Ignored(sig os.Signal) bool {
 	return sn >= 0 && signalIgnored(sn)
 }
 
+var (
+	// watchSignalLoopOnce guards calling the conditionally
+	// initialized watchSignalLoop. If watchSignalLoop is non-nil,
+	// it will be run in a goroutine lazily once Notify is invoked.
+	// See Issue 21576.
+	watchSignalLoopOnce sync.Once
+	watchSignalLoop     func()
+)
+
 // Notify causes package signal to relay incoming signals to c.
 // If no signals are provided, all incoming signals will be relayed to c.
 // Otherwise, just the provided signals will.
@@ -113,6 +122,12 @@ func Notify(c chan<- os.Signal, sig ...os.Signal) {
 		panic("os/signal: Notify using nil channel")
 	}
 
+	watchSignalLoopOnce.Do(func() {
+		if watchSignalLoop != nil {
+			go watchSignalLoop()
+		}
+	})
+
 	handlers.Lock()
 	defer handlers.Unlock()
 
diff --git a/src/os/signal/signal_plan9.go b/src/os/signal/signal_plan9.go
index a1eb68855e..8408607c7f 100644
--- a/src/os/signal/signal_plan9.go
+++ b/src/os/signal/signal_plan9.go
@@ -20,7 +20,8 @@ func signal_recv() string
 
 func init() {
 	signal_enable(0) // first call - initialize
-	go loop()
+
+	watchSignalLoop = loop
 }
 
 func loop() {
diff --git a/src/os/signal/signal_unix.go b/src/os/signal/signal_unix.go
index 7fa634f15a..0bbf41bfde 100644
--- a/src/os/signal/signal_unix.go
+++ b/src/os/signal/signal_unix.go
@@ -26,7 +26,8 @@ func loop() {
 
 func init() {
 	signal_enable(0) // first call - initialize
-	go loop()
+
+	watchSignalLoop = loop
 }
 
 const (