time: fix tick accuracy when using multiple Tickers

* correctly ignore obsolete ticks
* fix update of next alarm time
* tighten alarm channel buffers
Fixes #1379.

R=r, adg, r2
CC=golang-dev, soul9
https://golang.org/cl/3789045

diff --git a/src/pkg/time/tick.go b/src/pkg/time/tick.go
index 8f7d4226fedcb3d75675fdd87f5396b2a442b16f..ddd72727027d5938a0bcbb6fa22a1ca2bc3acd23 100644 (file)
--- a/src/pkg/time/tick.go
+++ b/src/pkg/time/tick.go
@@ -47,11 +47,12 @@ func (a *alarmer) set(ns int64) {
        case a.wakeTime > ns:
                // Next tick we expect is too late; shut down the late runner
                // and (after fallthrough) start a new wakeLoop.
-               a.wakeMeAt <- -1
+               close(a.wakeMeAt)
                fallthrough
        case a.wakeMeAt == nil:
                // There's no wakeLoop, start one.
-               a.wakeMeAt = make(chan int64, 10)
+               a.wakeMeAt = make(chan int64)
+               a.wakeUp = make(chan bool, 1)
                go wakeLoop(a.wakeMeAt, a.wakeUp)
                fallthrough
        case a.wakeTime == 0:
@@ -73,19 +74,10 @@ func startTickerLoop() {
 
 // wakeLoop delivers ticks at scheduled times, sleeping until the right moment.
 // If another, earlier Ticker is created while it sleeps, tickerLoop() will start a new
-// wakeLoop but they will share the wakeUp channel and signal that this one
-// is done by giving it a negative time request.
+// wakeLoop and signal that this one is done by closing the wakeMeAt channel.
 func wakeLoop(wakeMeAt chan int64, wakeUp chan bool) {
-       for {
-               wakeAt := <-wakeMeAt
-               if wakeAt < 0 { // tickerLoop has started another wakeLoop
-                       return
-               }
-               now := Nanoseconds()
-               if wakeAt > now {
-                       Sleep(wakeAt - now)
-                       now = Nanoseconds()
-               }
+       for wakeAt := range wakeMeAt {
+               Sleep(wakeAt - Nanoseconds())
                wakeUp <- true
        }
 }
@@ -96,9 +88,7 @@ func wakeLoop(wakeMeAt chan int64, wakeUp chan bool) {
 func tickerLoop() {
        // Represents the next alarm to be delivered.
        var alarm alarmer
-       // All wakeLoops deliver wakeups to this channel.
-       alarm.wakeUp = make(chan bool, 10)
-       var now, prevTime, wakeTime int64
+       var now, wakeTime int64
        var tickers *Ticker
        for {
                select {
@@ -110,10 +100,6 @@ func tickerLoop() {
                        alarm.set(t.nextTick)
                case <-alarm.wakeUp:
                        now = Nanoseconds()
-                       // Ignore an old time due to a dying wakeLoop
-                       if now < prevTime {
-                               continue
-                       }
                        wakeTime = now + 1e15 // very long in the future
                        var prev *Ticker = nil
                        // Scan list of tickers, delivering updates to those
@@ -151,12 +137,12 @@ func tickerLoop() {
                        if tickers != nil {
                                // Please send wakeup at earliest required time.
                                // If there are no tickers, don't bother.
+                               alarm.wakeTime = wakeTime
                                alarm.wakeMeAt <- wakeTime
                        } else {
                                alarm.wakeTime = 0
                        }
                }
-               prevTime = now
        }
 }