proc: add test for attach/detach, fix detach (#773)

Detach did not work for processes we attach to via PID.

Linux: we were only detaching from the main thread, all threads are
detached independently

Windows: we must resume all threads before detaching.

macOS: still broken.

Updates #772

_fixtures/testnextnethttp.go

@@ -12,5 +12,13 @@ func main() {
 		header := w.Header().Get("Content-Type")
 		w.Write([]byte(msg + header))
 	})
-	http.ListenAndServe(":9191", nil)
+	http.HandleFunc("/nobp", func(w http.ResponseWriter, req *http.Request) {
+		msg := "hello, world!"
+		header := w.Header().Get("Content-Type")
+		w.Write([]byte(msg + header))
+	})
+	err := http.ListenAndServe(":9191", nil)
+	if err != nil {
+		panic(err)
+	}
 }

pkg/proc/proc.go

@@ -132,7 +132,7 @@ func (dbp *Process) Detach(kill bool) (err error) {
 		}
 	}
 	dbp.execPtraceFunc(func() {
-		err = PtraceDetach(dbp.pid, 0)
+		err = dbp.detach()
 		if err != nil {
 			return
 		}

pkg/proc/proc_darwin.go

@@ -509,3 +509,7 @@ func (dbp *Process) resume() error {
 	}
 	return nil
 }
+
+func (dbp *Process) detach() error {
+	return PtraceDetach(dbp.pid, 0)
+}

pkg/proc/proc_linux.go

@@ -484,6 +484,24 @@ func (dbp *Process) resume() error {
 	return nil
 }
 
+func (dbp *Process) detach() error {
+	for threadID := range dbp.threads {
+		err := PtraceDetach(threadID, 0)
+		if err != nil {
+			return err
+		}
+	}
+	// For some reason the process will sometimes enter stopped state after a
+	// detach, this doesn't happen immediately either.
+	// We have to wait a bit here, then check if the main thread is stopped and
+	// SIGCONT it if it is.
+	time.Sleep(50 * time.Millisecond)
+	if s := status(dbp.pid, dbp.os.comm); s == 'T' {
+		sys.Kill(dbp.pid, sys.SIGCONT)
+	}
+	return nil
+}
+
 func killProcess(pid int) error {
 	return sys.Kill(pid, sys.SIGINT)
 }

pkg/proc/proc_test.go

@@ -6,6 +6,7 @@ import (
 	"go/ast"
 	"go/constant"
 	"go/token"
+	"io/ioutil"
 	"net"
 	"net/http"
 	"os"
@@ -2581,3 +2582,55 @@ func TestStacktraceWithBarriers(t *testing.T) {
 		}
 	})
 }
+
+func TestAttachDetach(t *testing.T) {
+	if runtime.GOOS == "darwin" {
+		// does not work on darwin
+		return
+	}
+	fixture := protest.BuildFixture("testnextnethttp")
+	cmd := exec.Command(fixture.Path)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	assertNoError(cmd.Start(), t, "starting fixture")
+
+	// wait for testnextnethttp to start listening
+	t0 := time.Now()
+	for {
+		conn, err := net.Dial("tcp", "localhost:9191")
+		if err == nil {
+			conn.Close()
+			break
+		}
+		time.Sleep(50 * time.Millisecond)
+		if time.Since(t0) > 10*time.Second {
+			t.Fatal("fixture did not start")
+		}
+	}
+
+	p, err := Attach(cmd.Process.Pid)
+	assertNoError(err, t, "Attach")
+	go func() {
+		time.Sleep(1 * time.Second)
+		http.Get("http://localhost:9191")
+	}()
+
+	assertNoError(p.Continue(), t, "Continue")
+
+	f, ln := currentLineNumber(p, t)
+	if ln != 11 {
+		t.Fatalf("Expected line :11 got %s:%d", f, ln)
+	}
+
+	assertNoError(p.Detach(false), t, "Detach")
+
+	resp, err := http.Get("http://localhost:9191/nobp")
+	assertNoError(err, t, "Page request after detach")
+	bs, err := ioutil.ReadAll(resp.Body)
+	assertNoError(err, t, "Reading /nobp page")
+	if out := string(bs); strings.Index(out, "hello, world!") < 0 {
+		t.Fatalf("/nobp page does not contain \"hello, world!\": %q", out)
+	}
+
+	cmd.Process.Kill()
+}

pkg/proc/proc_windows.go

@@ -663,6 +663,16 @@ func (dbp *Process) resume() error {
 	return nil
 }
 
+func (dbp *Process) detach() error {
+	for _, thread := range dbp.threads {
+		_, err := _ResumeThread(thread.os.hThread)
+		if err != nil {
+			return err
+		}
+	}
+	return PtraceDetach(dbp.pid, 0)
+}
+
 func killProcess(pid int) error {
 	p, err := os.FindProcess(pid)
 	if err != nil {