runtime: disable GC during debug call tests

Currently the debug call protocol implementation we use for testing is
riddled with write barriers, and called from a signal handler. This is
not safe, as write barriers need a P to execute.

Ideally this implementation would be rewritten to avoid the write
barriers, but it's not straightforward, and needs some thought. As a
temporary measure, disable GC during the debug call tests to avoid a
write barrier.

Note that this does not indicate a problem with real use of the debug
call protocol. Only our test implementation has this issue, because it
needs to get executed in a signal handler, normally a separate process
is interfacing with the protocol via process signals and ptrace (and the
like).

Fixes #49370.

Change-Id: Ic0fde5d0f4c64f9ecc9789b7dabb3954538fe0a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/361896
Trust: Michael Knyszek <[email protected]>
Run-TryBot: Michael Knyszek <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: mknyszek

src/runtime/debug_test.go

@@ -114,6 +114,13 @@ func skipUnderDebugger(t *testing.T) {
 }
 
 func TestDebugCall(t *testing.T) {
+	// InjectDebugCall cannot be executed while a GC is actively in
+	// progress. Wait until the current GC is done, and turn it off.
+	//
+	// See #49370.
+	runtime.GC()
+	defer debug.SetGCPercent(debug.SetGCPercent(-1))
+
 	g, after := startDebugCallWorker(t)
 	defer after()
 
@@ -143,6 +150,7 @@ func TestDebugCall(t *testing.T) {
 			x1: 42.0,
 		}
 	}
+
 	if _, err := runtime.InjectDebugCall(g, fn, &regs, args, debugCallTKill, false); err != nil {
 		t.Fatal(err)
 	}
@@ -164,6 +172,13 @@ func TestDebugCall(t *testing.T) {
 }
 
 func TestDebugCallLarge(t *testing.T) {
+	// InjectDebugCall cannot be executed while a GC is actively in
+	// progress. Wait until the current GC is done, and turn it off.
+	//
+	// See #49370.
+	runtime.GC()
+	defer debug.SetGCPercent(debug.SetGCPercent(-1))
+
 	g, after := startDebugCallWorker(t)
 	defer after()
 
@@ -193,6 +208,13 @@ func TestDebugCallLarge(t *testing.T) {
 }
 
 func TestDebugCallGC(t *testing.T) {
+	// InjectDebugCall cannot be executed while a GC is actively in
+	// progress. Wait until the current GC is done, and turn it off.
+	//
+	// See #49370.
+	runtime.GC()
+	defer debug.SetGCPercent(debug.SetGCPercent(-1))
+
 	g, after := startDebugCallWorker(t)
 	defer after()
 
@@ -203,6 +225,13 @@ func TestDebugCallGC(t *testing.T) {
 }
 
 func TestDebugCallGrowStack(t *testing.T) {
+	// InjectDebugCall cannot be executed while a GC is actively in
+	// progress. Wait until the current GC is done, and turn it off.
+	//
+	// See #49370.
+	runtime.GC()
+	defer debug.SetGCPercent(debug.SetGCPercent(-1))
+
 	g, after := startDebugCallWorker(t)
 	defer after()
 
@@ -233,6 +262,12 @@ func TestDebugCallUnsafePoint(t *testing.T) {
 	// This can deadlock if there aren't enough threads or if a GC
 	// tries to interrupt an atomic loop (see issue #10958).
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(8))
+
+	// InjectDebugCall cannot be executed while a GC is actively in
+	// progress. Wait until the current GC is done, and turn it off.
+	//
+	// See #49370.
+	runtime.GC()
 	defer debug.SetGCPercent(debug.SetGCPercent(-1))
 
 	// Test that the runtime refuses call injection at unsafe points.
@@ -256,6 +291,13 @@ func TestDebugCallPanic(t *testing.T) {
 	// This can deadlock if there aren't enough threads.
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(8))
 
+	// InjectDebugCall cannot be executed while a GC is actively in
+	// progress. Wait until the current GC is done, and turn it off.
+	//
+	// See #49370.
+	runtime.GC()
+	defer debug.SetGCPercent(debug.SetGCPercent(-1))
+
 	ready := make(chan *runtime.G)
 	var stop uint32
 	defer atomic.StoreUint32(&stop, 1)

src/runtime/export_debug_test.go

@@ -107,6 +107,10 @@ type debugCallHandler struct {
 }
 
 func (h *debugCallHandler) inject(info *siginfo, ctxt *sigctxt, gp2 *g) bool {
+	// TODO(49370): This code is riddled with write barriers, but called from
+	// a signal handler. Add the go:nowritebarrierrec annotation and restructure
+	// this to avoid write barriers.
+
 	switch h.gp.atomicstatus {
 	case _Grunning:
 		if getg().m != h.mp {
@@ -141,7 +145,11 @@ func (h *debugCallHandler) inject(info *siginfo, ctxt *sigctxt, gp2 *g) bool {
 }
 
 func (h *debugCallHandler) handle(info *siginfo, ctxt *sigctxt, gp2 *g) bool {
-	// Sanity check.
+	// TODO(49370): This code is riddled with write barriers, but called from
+	// a signal handler. Add the go:nowritebarrierrec annotation and restructure
+	// this to avoid write barriers.
+
+	// Double-check m.
 	if getg().m != h.mp {
 		println("trap on wrong M", getg().m, h.mp)
 		return false