fix the panic caused by resizing a starting exec

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

integration-cli/docker_api_exec_resize_test.go

@@ -1,6 +1,10 @@
 package main
 
 import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
 	"net/http"
 	"strings"
 
@@ -16,3 +20,59 @@ func (s *DockerSuite) TestExecResizeApiHeightWidthNoInt(c *check.C) {
 	c.Assert(status, check.Equals, http.StatusInternalServerError)
 	c.Assert(err, check.IsNil)
 }
+
+// Part of #14845
+func (s *DockerSuite) TestExecResizeImmediatelyAfterExecStart(c *check.C) {
+	name := "exec_resize_test"
+	dockerCmd(c, "run", "-d", "-i", "-t", "--name", name, "--restart", "always", "busybox", "/bin/sh")
+
+	// The panic happens when daemon.ContainerExecStart is called but the
+	// container.Exec is not called.
+	// Because the panic is not 100% reproducible, we send the requests concurrently
+	// to increase the probability that the problem is triggered.
+	n := 10
+	ch := make(chan struct{})
+	for i := 0; i < n; i++ {
+		go func() {
+			defer func() {
+				ch <- struct{}{}
+			}()
+
+			data := map[string]interface{}{
+				"AttachStdin": true,
+				"Cmd":         []string{"/bin/sh"},
+			}
+			status, body, err := sockRequest("POST", fmt.Sprintf("/containers/%s/exec", name), data)
+			c.Assert(err, check.IsNil)
+			c.Assert(status, check.Equals, http.StatusCreated)
+
+			out := map[string]string{}
+			err = json.Unmarshal(body, &out)
+			c.Assert(err, check.IsNil)
+
+			execID := out["Id"]
+			if len(execID) < 1 {
+				c.Fatal("ExecCreate got invalid execID")
+			}
+
+			payload := bytes.NewBufferString(`{"Tty":true}`)
+			conn, _, err := sockRequestHijack("POST", fmt.Sprintf("/exec/%s/start", execID), payload, "application/json")
+			c.Assert(err, check.IsNil)
+			defer conn.Close()
+
+			_, rc, err := sockRequestRaw("POST", fmt.Sprintf("/exec/%s/resize?h=24&w=80", execID), nil, "text/plain")
+			// It's probably a panic of the daemon if io.ErrUnexpectedEOF is returned.
+			if err == io.ErrUnexpectedEOF {
+				c.Fatal("The daemon might have crashed.")
+			}
+
+			if err == nil {
+				rc.Close()
+			}
+		}()
+	}
+
+	for i := 0; i < n; i++ {
+		<-ch
+	}
+}