fix a panic when the exec fails to start

Signed-off-by: Shijiang Wei <mountkin@gmail.com>

api/server/exec.go

@@ -65,7 +65,7 @@ func (s *Server) postContainerExecStart(version version.Version, w http.Response
 	}
 	var (
 		execName                  = vars["name"]
-		stdin                     io.ReadCloser
+		stdin, inStream           io.ReadCloser
 		stdout, stderr, outStream io.Writer
 	)
 
@@ -77,7 +77,7 @@ func (s *Server) postContainerExecStart(version version.Version, w http.Response
 	if !execStartCheck.Detach {
 		var err error
 		// Setting up the streaming http interface.
-		inStream, outStream, err := hijackServer(w)
+		inStream, outStream, err = hijackServer(w)
 		if err != nil {
 			return err
 		}
@@ -95,6 +95,8 @@ func (s *Server) postContainerExecStart(version version.Version, w http.Response
 			stderr = stdcopy.NewStdWriter(outStream, stdcopy.Stderr)
 			stdout = stdcopy.NewStdWriter(outStream, stdcopy.Stdout)
 		}
+	} else {
+		outStream = w
 	}
 
 	// Now run the user process in container.

integration-cli/docker_cli_exec_test.go

@@ -595,3 +595,14 @@ func (s *DockerSuite) TestExecOnReadonlyContainer(c *check.C) {
 		c.Fatalf("exec into a read-only container failed with exit status %d", status)
 	}
 }
+
+// #15750
+func (s *DockerSuite) TestExecStartFails(c *check.C) {
+	name := "exec-15750"
+	dockerCmd(c, "run", "-d", "--name", name, "busybox", "top")
+
+	_, errmsg, status := dockerCmdWithStdoutStderr(nil, "exec", name, "no-such-cmd")
+	if status == 255 && !strings.Contains(errmsg, "executable file not found") {
+		c.Fatal("exec error message not received. The daemon might had crashed")
+	}
+}

integration-cli/docker_utils.go

@@ -602,7 +602,9 @@ func dockerCmdWithError(args ...string) (string, int, error) {
 
 func dockerCmdWithStdoutStderr(c *check.C, args ...string) (string, string, int) {
 	stdout, stderr, status, err := runCommandWithStdoutStderr(exec.Command(dockerBinary, args...))
-	c.Assert(err, check.IsNil, check.Commentf("%q failed with errors: %s, %v", strings.Join(args, " "), stderr, err))
+	if c != nil {
+		c.Assert(err, check.IsNil, check.Commentf("%q failed with errors: %s, %v", strings.Join(args, " "), stderr, err))
+	}
 	return stdout, stderr, status
 }