runtime: disable use of template thread on Plan 9

CL 46033 added a "template thread" mechanism to
allow creation of thread with a known-good state
from a thread of unknown state.

However, we are experiencing issues on Plan 9
with programs using the os/exec and net package.
These package are relying on runtime.LockOSThread.

Updates #22227.

Change-Id: I85b71580a41df9fe8b24bd8623c064b6773288b0
Reviewed-on: https://go-review.googlesource.com/70231
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/proc.go

@@ -1813,7 +1813,7 @@ func newm(fn func(), _p_ *p) {
 	mp := allocm(_p_, fn)
 	mp.nextp.set(_p_)
 	mp.sigmask = initSigmask
-	if gp := getg(); gp != nil && gp.m != nil && (gp.m.lockedExt != 0 || gp.m.incgo) {
+	if gp := getg(); gp != nil && gp.m != nil && (gp.m.lockedExt != 0 || gp.m.incgo) && GOOS != "plan9" {
 		// We're on a locked M or a thread that may have been
 		// started by C. The kernel state of this thread may
 		// be strange (the user may have locked it for that
@@ -1821,6 +1821,8 @@ func newm(fn func(), _p_ *p) {
 		// thread. Instead, ask a known-good thread to create
 		// the thread for us.
 		//
+		// This is disabled on Plan 9. See golang.org/issue/22227.
+		//
 		// TODO: This may be unnecessary on Windows, which
 		// doesn't model thread creation off fork.
 		lock(&newmHandoff.lock)
@@ -3443,7 +3445,7 @@ func dolockOSThread() {
 // A goroutine should call LockOSThread before calling OS services or
 // non-Go library functions that depend on per-thread state.
 func LockOSThread() {
-	if atomic.Load(&newmHandoff.haveTemplateThread) == 0 {
+	if atomic.Load(&newmHandoff.haveTemplateThread) == 0 && GOOS != "plan9" {
 		// If we need to start a new thread from the locked
 		// thread, we need the template thread. Start it now
 		// while we're in a known-good state.