Handle execve exit status

This changes the execve exit status for the child process to be inline
with standard exit status codes for common execve failures.

I don't think this breaks any backwards compat with existing tini users
because it is still returning a non zero exit status but with correct
codes providing more information why it failed.

ci/run_build.sh

@@ -131,6 +131,20 @@ if [[ -n "${ARCH_NATIVE:=}" ]]; then
     echo "Testing ${tini} supports TINI_VERBOSITY"
     TINI_VERBOSITY=3 "$tini" true 2>&1 | grep -q 'Received SIGCHLD'
 
+    echo "Testing ${tini} exits with 127 if the command does not exist"
+    "$tini" foobar123 && rc="$?" || rc="$?"
+    if [[ "$rc" != 127 ]]; then
+      echo "Exit code was: ${rc}"
+      exit 1
+    fi
+
+    echo "Testing ${tini} exits with 126 if the command is not executable"
+    "$tini" /etc && rc="$?" || rc="$?"
+    if [[ "$rc" != 126 ]]; then
+      echo "Exit code was: ${rc}"
+      exit 1
+    fi
+
     # Test stdin / stdout are handed over to child
     echo "Testing ${tini} does not break pipes"
     echo "exit 0" | "${tini}" sh

src/tini.c

@@ -148,8 +148,21 @@ int spawn(const signal_configuration_t* const sigconf_ptr, char* const argv[], i
 		}
 
 		execvp(argv[0], argv);
-		PRINT_FATAL("Executing child process '%s' failed: '%s'", argv[0], strerror(errno));
-		return 1;
+
+		// execvp will only return on an error so make sure that we check the errno
+		// and exit with the correct return status for the error that we encountered
+		// See: http://www.tldp.org/LDP/abs/html/exitcodes.html#EXITCODESREF
+		int status = 1;
+		switch errno {
+			case ENOENT:
+				status = 127;
+				break;
+			case EACCES:
+				status = 126;
+				break;
+		}
+		PRINT_FATAL("exec %s failed: %s", argv[0], strerror(errno));
+		return status;
 	} else {
 		// Parent
 		PRINT_INFO("Spawned child process '%s' with pid '%i'", argv[0], pid);
@@ -509,8 +522,9 @@ int main(int argc, char *argv[]) {
 	reaper_check();
 
 	/* Go on */
-	if(spawn(&child_sigconf, *child_args_ptr, &child_pid)) {
-		return 1;
+	int spawn_ret = spawn(&child_sigconf, *child_args_ptr, &child_pid);
+	if (spawn_ret) {
+		return spawn_ret;
 	}
 	free(child_args_ptr);