runtime: tweak js and plan9 to avoid/disable write barrier & gc problems

runtime code for js contains possible write barriers that fail
the nowritebarrierrec check when internal local package naming
conventions are changed.  The problem was there all already; this
allows the code to compile, and it seems to work anyway in the
(single-threaded) js/wasm environment.  The offending operations
are noted with TODO, which is an improvement.

runtime code for plan9 contained an apparent allocation that was
not really an allocation; rewrite to remove the potential allocation
to avoid nowritebarrierrec problems.

This CL is a prerequisite for a pending code cleanup,
https://go-review.googlesource.com/c/go/+/393715

Updates #51734.

Change-Id: I93f31831ff9b92632137dd7b0055eaa721c81556
Reviewed-on: https://go-review.googlesource.com/c/go/+/405901
Run-TryBot: David Chase <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>

Author: dr2chase

src/runtime/lock_js.go

@@ -144,8 +144,12 @@ func notetsleepg(n *note, ns int64) bool {
 }
 
 // checkTimeouts resumes goroutines that are waiting on a note which has reached its deadline.
+// TODO(drchase): need to understand if write barriers are really okay in this context.
+//
+//go:yeswritebarrierrec
 func checkTimeouts() {
 	now := nanotime()
+	// TODO: map iteration has the write barriers in it; is that okay?
 	for n, nt := range notesWithTimeout {
 		if n.key == note_cleared && now >= nt.deadline {
 			n.key = note_timeout
@@ -175,6 +179,9 @@ var idleID int32
 // If an event handler returned, we resume it and it will pause the execution.
 // beforeIdle either returns the specific goroutine to schedule next or
 // indicates with otherReady that some goroutine became ready.
+// TODO(drchase): need to understand if write barriers are really okay in this context.
+//
+//go:yeswritebarrierrec
 func beforeIdle(now, pollUntil int64) (gp *g, otherReady bool) {
 	delay := int64(-1)
 	if pollUntil != 0 {
@@ -196,6 +203,7 @@ func beforeIdle(now, pollUntil int64) (gp *g, otherReady bool) {
 	}
 
 	if len(events) == 0 {
+		// TODO: this is the line that requires the yeswritebarrierrec
 		go handleAsyncEvent()
 		return nil, true
 	}

src/runtime/os_js.go

@@ -129,7 +129,7 @@ func initsig(preinit bool) {
 //
 //go:nowritebarrier
 func newosproc(mp *m) {
-	panic("newosproc: not implemented")
+	throw("newosproc: not implemented")
 }
 
 func setProcessCPUProfiler(hz int32) {}

src/runtime/os_plan9.go

@@ -437,7 +437,9 @@ func exit(e int32) {
 	} else {
 		// build error string
 		var tmp [32]byte
-		status = append(itoa(tmp[:len(tmp)-1], uint64(e)), 0)
+		sl := itoa(tmp[:len(tmp)-1], uint64(e))
+		// Don't append, rely on the existing data being zero.
+		status = tmp[:len(sl)+1]
 	}
 	goexitsall(&status[0])
 	exits(&status[0])