runtime, syscall, time: add and use resettimer

As a small step toward speeding up timers, restrict modification
of the timer.when field to the timer code itself. Other code that
wants to change the when field of an existing timer must now call
resettimer rather than changing the when field and calling addtimer.
The new resettimer function also works for a new timer.

This is just a refactoring in preparation for later code.

Updates #27707

Change-Id: Iccd5dcad415ffbeac4c2a3cf015e91f82692acf8
Reviewed-on: https://go-review.googlesource.com/c/go/+/171825
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/runtime/mgcscavenge.go

diff --git a/src/runtime/mgcscavenge.go b/src/runtime/mgcscavenge.go
index 0273c8d..0a67f74 100644
--- a/src/runtime/mgcscavenge.go
+++ b/src/runtime/mgcscavenge.go
@@ -268,8 +268,7 @@
 	// because we can't close over any variables without
 	// failing escape analysis.
 	now := nanotime()
-	scavenge.timer.when = now + ns
-	startTimer(scavenge.timer)
+	resetTimer(scavenge.timer, now+ns)
 
 	// Mark ourself as asleep and go to sleep.
 	scavenge.parked = true

src/runtime/netpoll.go

diff --git a/src/runtime/netpoll.go b/src/runtime/netpoll.go
index 7d18dca..536dae3 100644
--- a/src/runtime/netpoll.go
+++ b/src/runtime/netpoll.go
@@ -239,13 +239,12 @@
 	if pd.rt.f == nil {
 		if pd.rd > 0 {
 			pd.rt.f = rtf
-			pd.rt.when = pd.rd
 			// Copy current seq into the timer arg.
 			// Timer func will check the seq against current descriptor seq,
 			// if they differ the descriptor was reused or timers were reset.
 			pd.rt.arg = pd
 			pd.rt.seq = pd.rseq
-			addtimer(&pd.rt)
+			resettimer(&pd.rt, pd.rd)
 		}
 	} else if pd.rd != rd0 || combo != combo0 {
 		pd.rseq++ // invalidate current timers
@@ -259,10 +258,9 @@
 	if pd.wt.f == nil {
 		if pd.wd > 0 && !combo {
 			pd.wt.f = netpollWriteDeadline
-			pd.wt.when = pd.wd
 			pd.wt.arg = pd
 			pd.wt.seq = pd.wseq
-			addtimer(&pd.wt)
+			resettimer(&pd.wt, pd.wd)
 		}
 	} else if pd.wd != wd0 || combo != combo0 {
 		pd.wseq++ // invalidate current timers

src/runtime/time.go

diff --git a/src/runtime/time.go b/src/runtime/time.go
index d667d11..5521b8a 100644
--- a/src/runtime/time.go
+++ b/src/runtime/time.go
@@ -116,6 +116,15 @@
 	return deltimer(t)
 }
 
+// resetTimer resets an inactive timer, adding it to the heap.
+//go:linkname resetTimer time.resetTimer
+func resetTimer(t *timer, when int64) {
+	if raceenabled {
+		racerelease(unsafe.Pointer(t))
+	}
+	resettimer(t, when)
+}
+
 // Go runtime.
 
 // Ready the goroutine arg.
@@ -236,6 +245,15 @@
 	}
 }
 
+// resettimer resets an existing inactive timer to turn it into an active timer,
+// with a new time for when the timer should fire.
+// This should be called instead of addtimer if the timer value has been,
+// or may have been, used previously.
+func resettimer(t *timer, when int64) {
+	t.when = when
+	addtimer(t)
+}
+
 // Timerproc runs the time-driven events.
 // It sleeps until the next event in the tb heap.
 // If addtimer inserts a new earlier event, it wakes timerproc early.

src/time/internal_test.go

diff --git a/src/time/internal_test.go b/src/time/internal_test.go
index 336deb9..3bca88e 100644
--- a/src/time/internal_test.go
+++ b/src/time/internal_test.go
@@ -64,8 +64,7 @@
 		// once more.
 		stopTimer(r)
 		t.Stop()
-		r.when = 0
-		startTimer(r)
+		resetTimer(r, 0)
 	}()
 
 	// If the test fails, we will hang here until the timeout in the testing package

src/time/sleep.go

diff --git a/src/time/sleep.go b/src/time/sleep.go
index 2cc908d..b381a1f 100644
--- a/src/time/sleep.go
+++ b/src/time/sleep.go
@@ -38,6 +38,7 @@
 
 func startTimer(*runtimeTimer)
 func stopTimer(*runtimeTimer) bool
+func resetTimer(*runtimeTimer, int64)
 
 // The Timer type represents a single event.
 // When the Timer expires, the current time will be sent on C,
@@ -122,8 +123,7 @@
 	}
 	w := when(d)
 	active := stopTimer(&t.r)
-	t.r.when = w
-	startTimer(&t.r)
+	resetTimer(&t.r, w)
 	return active
 }