runtime: add new dodeltimer and dodeltimer0 functions

The dodeltimer function removes a timer from a heap. The dodeltimer0
function removes the first timer from a heap; in the old timer code
this common special case was inlined in the timerproc function.

Updates #27707

Change-Id: I1b7c0af46866abb4bffa8aa4d8e7143f9ae8f402
Reviewed-on: https://go-review.googlesource.com/c/go/+/171834
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/time.go

@@ -417,6 +417,59 @@ func deltimer(t *timer) bool {
 	}
 }
 
+// dodeltimer removes timer i from the current P's heap.
+// We are locked on the P when this is called.
+// It reports whether it saw no problems due to races.
+// The caller must have locked the timers for pp.
+func dodeltimer(pp *p, i int) bool {
+	if t := pp.timers[i]; t.pp.ptr() != pp {
+		throw("dodeltimer: wrong P")
+	} else {
+		t.pp = 0
+	}
+	last := len(pp.timers) - 1
+	if i != last {
+		pp.timers[i] = pp.timers[last]
+	}
+	pp.timers[last] = nil
+	pp.timers = pp.timers[:last]
+	ok := true
+	if i != last {
+		// Moving to i may have moved the last timer to a new parent,
+		// so sift up to preserve the heap guarantee.
+		if !siftupTimer(pp.timers, i) {
+			ok = false
+		}
+		if !siftdownTimer(pp.timers, i) {
+			ok = false
+		}
+	}
+	return ok
+}
+
+// dodeltimer0 removes timer 0 from the current P's heap.
+// We are locked on the P when this is called.
+// It reports whether it saw no problems due to races.
+// The caller must have locked the timers for pp.
+func dodeltimer0(pp *p) bool {
+	if t := pp.timers[0]; t.pp.ptr() != pp {
+		throw("dodeltimer0: wrong P")
+	} else {
+		t.pp = 0
+	}
+	last := len(pp.timers) - 1
+	if last > 0 {
+		pp.timers[0] = pp.timers[last]
+	}
+	pp.timers[last] = nil
+	pp.timers = pp.timers[:last]
+	ok := true
+	if last > 0 {
+		ok = siftdownTimer(pp.timers, 0)
+	}
+	return ok
+}
+
 func deltimerOld(t *timer) bool {
 	if t.tb == nil {
 		// t.tb can be nil if the user created a timer