daemon/exec: don't overwrite exit code if set

If we fail to start an exec, the deferred error-handling block in [L181-L193](c7e42d855e/daemon/exec.go (L181-L193))
would set the exit code to `126` (`EACCES`). However, if we get far enough along
attempting to start the exec, we set the exit code according to the error returned
from starting the task [L288-L291](c7e42d855e/daemon/exec.go (L288-L291)).

For some situations (such as `docker exec [some-container]
missing-binary`), the 2nd block returns the correct exit code (`127`)
but that then gets overwritten by the 1st block.

This commit changes that logic to only set the default exit code `126`
if the exit code has not been set yet.

Signed-off-by: Laura Brehm <laurabrehm@hey.com>

daemon/exec.go

@@ -183,8 +183,12 @@ func (daemon *Daemon) ContainerExecStart(ctx context.Context, name string, optio
 			ec.Lock()
 			ec.Container.ExecCommands.Delete(ec.ID)
 			ec.Running = false
-			exitCode := 126
-			ec.ExitCode = &exitCode
+			if ec.ExitCode == nil {
+				// default to `126` (`EACCES`) if we fail to start
+				// the exec without setting an exit code.
+				exitCode := exitEaccess
+				ec.ExitCode = &exitCode
+			}
 			if err := ec.CloseStreams(); err != nil {
 				log.G(ctx).Errorf("failed to cleanup exec %s streams: %s", ec.Container.ID, err)
 			}

integration/container/exec_linux_test.go

@@ -30,3 +30,36 @@ func TestExecConsoleSize(t *testing.T) {
 	assert.NilError(t, err)
 	assert.Equal(t, strings.TrimSpace(result.Stdout()), "57 123")
 }
+
+func TestFailedExecExitCode(t *testing.T) {
+	testCases := []struct {
+		doc              string
+		command          []string
+		expectedExitCode int
+	}{
+		{
+			doc:              "executable not found",
+			command:          []string{"nonexistent"},
+			expectedExitCode: 127,
+		},
+		{
+			doc:              "executable cannot be invoked",
+			command:          []string{"/etc"},
+			expectedExitCode: 126,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.doc, func(t *testing.T) {
+			ctx := setupTest(t)
+			apiClient := testEnv.APIClient()
+
+			cID := container.Run(ctx, t, apiClient)
+
+			result, err := container.Exec(ctx, apiClient, cID, tc.command)
+			assert.NilError(t, err)
+
+			assert.Equal(t, result.ExitCode, tc.expectedExitCode)
+		})
+	}
+}