Allow running exec with no attach streams requested

This is a small Docker compat fix with the API exec endpoints.
With Docker, sending a bare Exec Create (command only, nothing
else set) and then an Exec Start without Detach set performs
a detached exec. With Podman, we threw an error that at least one
stream must be attached to. Fix is trivial; look up the session
before start, check the config for attach streams, and force
detach on if none are set.

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

pkg/api/handlers/compat/exec.go

@@ -175,6 +175,19 @@ func ExecStartHandler(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// If no streams were requested for attach, detach is implied
+	if !bodyParams.Detach {
+		execSession, err := sessionCtr.ExecSession(sessionID)
+		if err != nil {
+			utils.InternalServerError(w, err)
+			return
+		}
+
+		if !execSession.Config.AttachStderr && !execSession.Config.AttachStdout && !execSession.Config.AttachStdin {
+			bodyParams.Detach = true
+		}
+	}
+
 	if bodyParams.Detach {
 		// If we are detaching, we do NOT want to hijack.
 		// Instead, we perform a detached start, and return 200 if

pkg/api/server/register_exec.go

@@ -273,7 +273,8 @@ func (s *APIServer) registerExecHandlers(r *mux.Router) error {
 	//      properties:
 	//        Detach:
 	//          type: boolean
-	//          description: Detach from the command.
+	//          description: |
+	//           "Detach from the command. Detach defaults to true if no attach streams were specified when the session was created (IE AttachStdin, AttachStdout, AttachStderr are all false)."
 	//        Tty:
 	//          type: boolean
 	//          description: Allocate a pseudo-TTY.

test/apiv2/20-containers.at

@@ -988,6 +988,29 @@ t POST libpod/containers/$cid/update \
 
 t DELETE containers/$cid 204
 
+# Verify that exec session with no attach streams specified can be started
+CTRNAME=exectest
+podman run -d --name $CTRNAME $IMAGE top
+echo '{ "Cmd":["sh", "-c", "echo hello > /tmp/testfile"]}' >${TMPD}/exec_empty.json
+t POST "containers/$CTRNAME/exec" ${TMPD}/exec_empty.json 201 .Id~[0-9a-f]\\{64\\}
+eid=$(jq -r '.Id' <<<"$output")
+t POST exec/$eid/start 200
+# Need to wait for the exec to actually run, given it's detached.
+# Use $PODMAN_BIN --root directly instead of the podman wrapper, since we
+# need to capture the command output.
+TRIES=5
+while [[ $TRIES -gt 0 ]]; do
+    exec_out=$($PODMAN_BIN --root $WORKDIR/server_root exec $CTRNAME ls /tmp/testfile 2>/dev/null)
+    if [[ "$exec_out" = "/tmp/testfile" ]]; then
+	break
+    fi
+    sleep 1
+    TRIES=$((TRIES - 1))
+done
+out=$($PODMAN_BIN --root $WORKDIR/server_root exec $CTRNAME cat /tmp/testfile 2>/dev/null)
+is "$out" "hello"
+podman rm -f $CTRNAME
+
 rm -rf $TMPD
 
 podman container rm -fa