Fail cleanly when a filter cannot run

A filter program that could not be run answered with two responses,
and a filter that exited without reading its input ended cgit with
the page half written.

1 file changed, 17 insertions, 5 deletions

diff --git a/source/filter.c b/source/filter.c
index 81c4681..1cfdde2 100644
--- a/source/filter.c
+++ b/source/filter.c
@@ -18,6 +18,10 @@
 #include <lauxlib.h>
 #endif
 
+// Exit status of a child that could not run the filter, the shell's own value
+// for a command not found.
+#define EXEC_FAILED 127
+
 static int open_exec_filter(struct cgit_filter *base, va_list ap)
 {
 	struct cgit_exec_filter *filter = (struct cgit_exec_filter *)base;
@@ -32,11 +36,18 @@ static int open_exec_filter(struct cgit_filter *base, va_list ap)
 	filter->pid = cgit_die_unless_non_negative(fork(), "Unable to create subprocess");
 	if (filter->pid == 0) {
 		close(pipefd[1]);
-		cgit_die_unless_non_negative(dup2(pipefd[0], STDIN_FILENO), "Unable to use pipe as STDIN");
+		if (dup2(pipefd[0], STDIN_FILENO) < 0)
+			_exit(EXEC_FAILED);
 		execvp(filter->cmd, filter->argv);
-		die_errno("Unable to exec subprocess %s", filter->cmd);
+		// The child shares the parent's page buffer, cache lock and exit
+		// handlers, so it must not die through them.
+		fprintf(stderr, "[cgit] Unable to exec filter %s: %s\n", filter->cmd, strerror(errno));
+		_exit(EXEC_FAILED);
 	}
 	close(pipefd[0]);
+	// The child keeps the page's stdout, but not the descriptor the parent
+	// needs it back from.
+	fcntl(filter->old_stdout, F_SETFD, FD_CLOEXEC);
 	cgit_die_unless_non_negative(dup2(pipefd[1], STDOUT_FILENO), "Unable to use pipe as STDOUT");
 	close(pipefd[1]);
 	return 0;
@@ -52,9 +63,10 @@ static int close_exec_filter(struct cgit_filter *base)
 	if (filter->pid < 0)
 		goto done;
 	waitpid(filter->pid, &exit_status, 0);
-	if (WIFEXITED(exit_status))
-		goto done;
-	die("Subprocess %s exited abnormally", filter->cmd);
+	if (!WIFEXITED(exit_status))
+		die("Subprocess %s exited abnormally", filter->cmd);
+	if (WEXITSTATUS(exit_status) == EXEC_FAILED)
+		die("Unable to run filter %s", filter->cmd);
 
 done:
 	for (i = 0; i < filter->base.argument_count; i++)