log: Add Exit, Exitf etc.

These are equivalent to Fatal etc. except that they do not dump stacks.
Addresses a common complaint.

glog.go

@@ -703,7 +703,16 @@ func (l *loggingT) output(s severity, buf *buffer, file string, line int, alsoTo
 		}
 	}
 	if s == fatalLog {
-		// Make sure we see the trace for the current goroutine on standard error.
+		// If we got here via Exit rather than Fatal, print no stacks.
+		if atomic.LoadUint32(&fatalNoStacks) > 0 {
+			l.mu.Unlock()
+			timeoutFlush(10 * time.Second)
+			os.Exit(1)
+		}
+		// Dump all goroutine stacks before exiting.
+		// First, make sure we see the trace for the current goroutine on standard error.
+		// If -logtostderr has been specified, the loop below will do that anyway
+		// as the first stack in the full dump.
 		if !l.toStderr {
 			os.Stderr.Write(stacks(false))
 		}
@@ -1135,3 +1144,34 @@ func Fatalln(args ...interface{}) {
 func Fatalf(format string, args ...interface{}) {
 	logging.printf(fatalLog, format, args...)
 }
+
+// fatalNoStacks is non-zero if we are to exit without dumping goroutine stacks.
+// It allows Exit and relatives to use the Fatal logs.
+var fatalNoStacks uint32
+
+// Exit logs to the FATAL, ERROR, WARNING, and INFO logs, then calls os.Exit(1).
+// Arguments are handled in the manner of fmt.Print; a newline is appended if missing.
+func Exit(args ...interface{}) {
+	atomic.StoreUint32(&fatalNoStacks, 1)
+	logging.print(fatalLog, args...)
+}
+
+// ExitDepth acts as Exit but uses depth to determine which call frame to log.
+// ExitDepth(0, "msg") is the same as Exit("msg").
+func ExitDepth(depth int, args ...interface{}) {
+	atomic.StoreUint32(&fatalNoStacks, 1)
+	logging.printDepth(fatalLog, depth, args...)
+}
+
+// Exitln logs to the FATAL, ERROR, WARNING, and INFO logs, then calls os.Exit(1).
+func Exitln(args ...interface{}) {
+	atomic.StoreUint32(&fatalNoStacks, 1)
+	logging.println(fatalLog, args...)
+}
+
+// Exitf logs to the FATAL, ERROR, WARNING, and INFO logs, then calls os.Exit(1).
+// Arguments are handled in the manner of fmt.Printf; a newline is appended if missing.
+func Exitf(format string, args ...interface{}) {
+	atomic.StoreUint32(&fatalNoStacks, 1)
+	logging.printf(fatalLog, format, args...)
+}