cmd/go, misc: switch from breakpoint to SIGUSR2

The iOS test harness has set a breakpoint early in the life of Go
programs so that it can change the current working directory using
information only available from the host debugger. Somewhere in the
upgrade to iOS 10 / XCode 8.2, breakpoints stopped working. This
may be an LLDB bug, or a bug in the ios-deploy LLDB scripts, it's
not clear.

Work around the problem by giving up on breakpoints. Instead, early
in the life of every test binary built for iOS, send (and ignore) a
SIGUSR2 signal. The debugger will catch this, giving the script
go_darwin_arm_exec a chance to change the working directory.

For the iOS builders.

Change-Id: I7476531985217d0c76bc176904c48379210576c2
Reviewed-on: https://go-review.googlesource.com/34926Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/ios/go_darwin_arm_exec.go

@@ -99,7 +99,7 @@ func main() {
 	// Approximately 1 in a 100 binaries fail to start. If it happens,
 	// try again. These failures happen for several reasons beyond
 	// our control, but all of them are safe to retry as they happen
-	// before lldb encounters the initial getwd breakpoint. As we
+	// before lldb encounters the initial SIGUSR2 stop. As we
 	// know the tests haven't started, we are not hiding flaky tests
 	// with this retry.
 	for i := 0; i < 5; i++ {
@@ -224,6 +224,7 @@ func run(bin string, args []string) (err error) {
 	s.do(`process handle SIGHUP  --stop false --pass true --notify false`)
 	s.do(`process handle SIGPIPE --stop false --pass true --notify false`)
 	s.do(`process handle SIGUSR1 --stop false --pass true --notify false`)
+	s.do(`process handle SIGUSR2 --stop true --pass false --notify true`) // sent by test harness
 	s.do(`process handle SIGCONT --stop false --pass true --notify false`)
 	s.do(`process handle SIGSEGV --stop false --pass true --notify false`) // does not work
 	s.do(`process handle SIGBUS  --stop false --pass true --notify false`) // does not work
@@ -236,11 +237,9 @@ func run(bin string, args []string) (err error) {
 		return nil
 	}
 
-	s.do(`breakpoint set -n getwd`) // in runtime/cgo/gcc_darwin_arm.go
-
 	started = true
 
-	s.doCmd("run", "stop reason = breakpoint", 20*time.Second)
+	s.doCmd("run", "stop reason = signal SIGUSR2", 20*time.Second)
 
 	// Move the current working directory into the faux gopath.
 	if pkgpath != "src" {

src/cmd/go/test.go

@@ -894,9 +894,13 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 
 	if buildContext.GOOS == "darwin" {
 		if buildContext.GOARCH == "arm" || buildContext.GOARCH == "arm64" {
-			t.NeedCgo = true
+			t.IsIOS = true
+			t.NeedOS = true
 		}
 	}
+	if t.TestMain == nil {
+		t.NeedOS = true
+	}
 
 	for _, cp := range pmain.imports {
 		if len(cp.coverVars) > 0 {
@@ -1343,7 +1347,8 @@ type testFuncs struct {
 	NeedTest    bool
 	ImportXtest bool
 	NeedXtest   bool
-	NeedCgo     bool
+	NeedOS      bool
+	IsIOS       bool
 	Cover       []coverInfo
 }
 
@@ -1444,7 +1449,7 @@ var testmainTmpl = template.Must(template.New("main").Parse(`
 package main
 
 import (
-{{if not .TestMain}}
+{{if .NeedOS}}
 	"os"
 {{end}}
 	"testing"
@@ -1460,8 +1465,10 @@ import (
 	_cover{{$i}} {{$p.Package.ImportPath | printf "%q"}}
 {{end}}
 
-{{if .NeedCgo}}
+{{if .IsIOS}}
+	"os/signal"
 	_ "runtime/cgo"
+	"syscall"
 {{end}}
 )
 
@@ -1523,6 +1530,12 @@ func coverRegisterFile(fileName string, counter []uint32, pos []uint32, numStmts
 {{end}}
 
 func main() {
+{{if .IsIOS}}
+	signal.Notify(make(chan os.Signal), syscall.SIGUSR2)
+	syscall.Kill(0, syscall.SIGUSR2)
+	signal.Reset(syscall.SIGUSR2)
+{{end}}
+
 {{if .CoverEnabled}}
 	testing.RegisterCover(testing.Cover{
 		Mode: {{printf "%q" .CoverMode}},