proc/native/linux: better handling of process death due to signals (#2477)

Handle the signaled status for the thread leader like we handle the
exited status, by returning ErrProcessExited and recording the killer
signal  in it.
Prior to this commit we would find out about the death of the thread
later in the loop, the condition would still be reported as
ErrProcessExited, but without recording the signal number anywhere.

Also fixes a bug in TestAttachStopOnEntry where the test would
inadvertently cause a SIGPIPE to be sent to the target process, making
it terminate early.

pkg/proc/proc_unix_test.go

@@ -4,11 +4,17 @@ package proc_test
 
 import (
 	"fmt"
+	"os"
+	"os/exec"
+	"runtime"
 	"syscall"
 	"testing"
 	"time"
 
+	"golang.org/x/sys/unix"
+
 	"github.com/go-delve/delve/pkg/proc"
+	"github.com/go-delve/delve/pkg/proc/native"
 	protest "github.com/go-delve/delve/pkg/proc/test"
 )
 
@@ -66,3 +72,31 @@ func TestIssue419(t *testing.T) {
 		}
 	}
 }
+
+func TestSignalDeath(t *testing.T) {
+	if testBackend != "native" || runtime.GOOS != "linux" {
+		t.Skip("skipped on non-linux non-native backends")
+	}
+	var buildFlags protest.BuildFlags
+	if buildMode == "pie" {
+		buildFlags |= protest.BuildModePIE
+	}
+	fixture := protest.BuildFixture("loopprog", buildFlags)
+	cmd := exec.Command(fixture.Path)
+	stdout, err := cmd.StdoutPipe()
+	assertNoError(err, t, "StdoutPipe")
+	cmd.Stderr = os.Stderr
+	assertNoError(cmd.Start(), t, "starting fixture")
+	p, err := native.Attach(cmd.Process.Pid, []string{})
+	assertNoError(err, t, "Attach")
+	stdout.Close() // target will receive SIGPIPE later on
+	err = p.Continue()
+	t.Logf("error is %v", err)
+	exitErr, isexited := err.(proc.ErrProcessExited)
+	if !isexited {
+		t.Fatal("did not exit")
+	}
+	if exitErr.Status != -int(unix.SIGPIPE) {
+		t.Fatalf("expected SIGPIPE got %d\n", exitErr.Status)
+	}
+}