runc exec: fail with exit code of 255

Currently there's no way to distinguish between the two cases:
 - runc exec failed;
 - the command executed returned 1.

This was possible before commit 8477638aab, as runc exec exited with
the code of 255 if exec itself has failed. The code of 255 is the same
convention as used by e.g. ssh.

Re-introduce the feature, document it, and add some tests so it won't be
broken again.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

exec.go

@@ -101,7 +101,8 @@ following will output a list of processes running in the container:
 		if err == nil {
 			os.Exit(status)
 		}
-		return fmt.Errorf("exec failed: %w", err)
+		fatalWithCode(fmt.Errorf("exec failed: %w", err), 255)
+		return nil // to satisfy the linter
 	},
 	SkipArgReorder: true,
 }

man/runc-exec.8.md

@@ -59,6 +59,11 @@ multiple times.
 : Pass _N_ additional file descriptors to the container (**stdio** +
 **$LISTEN_FDS** + _N_ in total). Default is **0**.
 
+# EXIT STATUS
+
+Exits with a status of _command_ (unless **-d** is used), or **255** if
+an error occurred.
+
 # EXAMPLES
 If the container can run **ps**(1) command, the following
 will output a list of processes running in the container:

tests/integration/exec.bats

@@ -21,6 +21,26 @@ function teardown() {
 	[[ "${output}" == *"Hello from exec"* ]]
 }
 
+@test "runc exec [exit codes]" {
+	runc run -d --console-socket "$CONSOLE_SOCKET" test_busybox
+	[ "$status" -eq 0 ]
+
+	runc exec test_busybox false
+	[ "$status" -eq 1 ]
+
+	runc exec test_busybox sh -c "exit 42"
+	[ "$status" -eq 42 ]
+
+	runc exec --pid-file /non-existent/directory test_busybox true
+	[ "$status" -eq 255 ]
+
+	runc exec test_busybox no-such-binary
+	[ "$status" -eq 255 ]
+
+	runc exec no_such_container true
+	[ "$status" -eq 255 ]
+}
+
 @test "runc exec --pid-file" {
 	# run busybox detached
 	runc run -d --console-socket "$CONSOLE_SOCKET" test_busybox

utils.go

@@ -53,13 +53,17 @@ func logrusToStderr() bool {
 // fatal prints the error's details if it is a libcontainer specific error type
 // then exits the program with an exit status of 1.
 func fatal(err error) {
-	// make sure the error is written to the logger
+	fatalWithCode(err, 1)
+}
+
+func fatalWithCode(err error, ret int) {
+	// Make sure the error is written to the logger.
 	logrus.Error(err)
 	if !logrusToStderr() {
 		fmt.Fprintln(os.Stderr, err)
 	}
 
-	os.Exit(1)
+	os.Exit(ret)
 }
 
 // setupSpec performs initial setup based on the cli.Context for the container