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[release-branch.go1.9] runtime: avoid monotonic time zero on systems …

…with low-res timers

Otherwise low-res timers cause problems at call sites that expect to
be able to use 0 as meaning "no time set" and therefore expect that
nanotime never returns 0 itself. For example, sched.lastpoll == 0
means no last poll.

Fixes #22394.

Change-Id: Iea28acfddfff6f46bc90f041ec173e0fea591285
Reviewed-on: https://go-review.googlesource.com/73410
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-on: https://go-review.googlesource.com/73491
TryBot-Result: Russ Cox <rsc@golang.org>
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rsc committed Oct 25, 2017
1 parent 8bb333a commit 9be38a15e486f09663cf324539e2cb5045d54d80
Showing with 10 additions and 1 deletion.
  1. +3 −0 src/runtime/proc.go
  2. +7 −1 src/runtime/time.go
@@ -142,6 +142,9 @@ func main() {
}

runtime_init() // must be before defer
if nanotime() == 0 {
throw("nanotime returning zero")
}

// Defer unlock so that runtime.Goexit during init does the unlock too.
needUnlock := true
@@ -309,4 +309,10 @@ func time_runtimeNano() int64 {
return nanotime()
}

var startNano int64 = nanotime()
// Monotonic times are reported as offsets from startNano.
// We initialize startNano to nanotime() - 1 so that on systems where
// monotonic time resolution is fairly low (e.g. Windows 2008
// which appears to have a default resolution of 15ms),
// we avoid ever reporting a nanotime of 0.
// (Callers may want to use 0 as "time not set".)
var startNano int64 = nanotime() - 1

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