pkg/proc: tolerate absence of stack barriers in Go 1.9 (#762)

Stack barriers were removed in Go 1.9, and thus code that
expected various stack-barrier-related symbols to exist
does not find them.  Check for their absence and do not
crash when they are missing.  Disable stack-barrier-handling
test for 1.9 and beyond.

Fixes #754.

pkg/proc/proc_test.go

@@ -2489,14 +2489,16 @@ func TestStacktraceWithBarriers(t *testing.T) {
 	// The original return address is saved into the stkbar slice inside the G
 	// struct.
 
+	// In Go 1.9 stack barriers have been removed and this test must be disabled.
+	if ver, _ := ParseVersionString(runtime.Version()); ver.Major < 0 || ver.AfterOrEqual(GoVersion{1, 9, -1, 0, 0}) {
+		return
+	}
+
 	// In Go 1.8 stack barriers are not inserted by default, this enables them.
 	godebugOld := os.Getenv("GODEBUG")
 	defer os.Setenv("GODEBUG", godebugOld)
 	os.Setenv("GODEBUG", "gcrescanstacks=1")
 
-	// TODO(aarzilli): in Go 1.9 stack barriers will be removed completely, therefore
-	// this test will have to be disabled
-
 	withTestProcess("binarytrees", t, func(p *Process, fixture protest.Fixture) {
 		// We want to get a user goroutine with a stack barrier, to get that we execute the program until runtime.gcInstallStackBarrier is executed AND the goroutine it was executed onto contains a call to main.bottomUpTree
 		_, err := setFunctionBreakpoint(p, "runtime.gcInstallStackBarrier")

pkg/proc/stack.go

@@ -130,8 +130,10 @@ type savedLR struct {
 }
 
 func newStackIterator(dbp *Process, pc, sp, bp uint64, stkbar []savedLR, stkbarPos int) *stackIterator {
-	stackBarrierPC := dbp.goSymTable.LookupFunc(runtimeStackBarrier).Entry
+	stackBarrierFunc := dbp.goSymTable.LookupFunc(runtimeStackBarrier) // stack barriers were removed in Go 1.9
-	if stkbar != nil {
+	var stackBarrierPC uint64
+	if stackBarrierFunc != nil && stkbar != nil {
+		stackBarrierPC = stackBarrierFunc.Entry
 		fn := dbp.goSymTable.PCToFunc(pc)
 		if fn != nil && fn.Name == runtimeStackBarrier {
 			// We caught the goroutine as it's executing the stack barrier, we must

pkg/proc/variables.go

@@ -396,8 +396,14 @@ func (gvar *Variable) parseG() (*G, error) {
 	id, _ := constant.Int64Val(gvar.fieldVariable("goid").Value)
 	gopc, _ := constant.Int64Val(gvar.fieldVariable("gopc").Value)
 	waitReason := constant.StringVal(gvar.fieldVariable("waitreason").Value)
+
 	stkbarVar, _ := gvar.structMember("stkbar")
-	stkbarPos, _ := constant.Int64Val(gvar.fieldVariable("stkbarPos").Value)
+	stkbarVarPosFld := gvar.fieldVariable("stkbarPos")
+	var stkbarPos int64
+	if stkbarVarPosFld != nil { // stack barriers were removed in Go 1.9
+		stkbarPos, _ = constant.Int64Val(stkbarVarPosFld.Value)
+	}
+
 	status, _ := constant.Int64Val(gvar.fieldVariable("atomicstatus").Value)
 	f, l, fn := gvar.dbp.goSymTable.PCToLine(uint64(pc))
 	g := &G{
@@ -498,6 +504,9 @@ func (g *G) Go() Location {
 
 // Returns the list of saved return addresses used by stack barriers
 func (g *G) stkbar() ([]savedLR, error) {
+	if g.stkbarVar == nil { // stack barriers were removed in Go 1.9
+		return nil, nil
+	}
 	g.stkbarVar.loadValue(LoadConfig{false, 1, 0, int(g.stkbarVar.Len), 3})
 	if g.stkbarVar.Unreadable != nil {
 		return nil, fmt.Errorf("unreadable stkbar: %v\n", g.stkbarVar.Unreadable)