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)
 			}