runtime: make faketime more robust against GC

When using faketime, only run the scavenger for one loop.
It tries to run for 1 ms, but with faketime that calculation fails.

Prohibit write barriers in the faketime write function, in case
the GC wants to print something (e.g., with GODEBUG=gctrace=1).

Fixes #49614

Change-Id: Iab5097fe78b6e0032ea8b493088264dfb25013c6
Reviewed-on: https://go-review.googlesource.com/c/go/+/364757
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/runtime/mgcscavenge.go

@@ -344,6 +344,11 @@ func bgscavenge(c chan int) {
 				crit += float64(end - start)
 			}
 			released += r
+
+			// When using fake time just do one loop.
+			if faketime != 0 {
+				break
+			}
 		}
 
 		if released == 0 {

src/runtime/time_fake.go

@@ -41,6 +41,10 @@ func time_now() (sec int64, nsec int32, mono int64) {
 	return faketime / 1e9, int32(faketime % 1e9), faketime
 }
 
+// write is like the Unix write system call.
+// We have to avoid write barriers to avoid potential deadlock
+// on write calls.
+//go:nowritebarrierrec
 func write(fd uintptr, p unsafe.Pointer, n int32) int32 {
 	if !(fd == 1 || fd == 2) {
 		// Do an ordinary write.