filter: allow returning exit code from filter

Filters can now indicate a status back to cgit by means of the exit code
for exec, or the return value from close for Lua.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

cgitrc.5.txt

@@ -590,7 +590,8 @@ specification with the relevant string; available values are:
 		This is called whenever cgit writes data to the webpage.
 	'filter_close()'::
 		This is called when the current filtering operation is
-		completed.
+		completed. It must return an integer value. Usually 0
+		indicates success.
 	
 	Additionally, cgit exposes to the Lua the following built-in functions:
 

filter.c

@@ -106,7 +106,7 @@ static int open_exec_filter(struct cgit_filter *base, va_list ap)
 static int close_exec_filter(struct cgit_filter *base)
 {
 	struct cgit_exec_filter *filter = (struct cgit_exec_filter *)base;
-	int i, exit_status;
+	int i, exit_status = 0;
 
 	chk_non_negative(dup2(filter->old_stdout, STDOUT_FILENO),
 		"Unable to restore STDOUT");
@@ -114,14 +114,14 @@ 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) && !WEXITSTATUS(exit_status))
+	if (WIFEXITED(exit_status))
 		goto done;
 	die("Subprocess %s exited abnormally", filter->cmd);
 
 done:
 	for (i = 0; i < filter->base.argument_count; i++)
 		filter->argv[i + 1] = NULL;
-	return 0;
+	return WEXITSTATUS(exit_status);
 
 }
 
@@ -315,10 +315,14 @@ static int close_lua_filter(struct cgit_filter *base)
 	int ret = 0;
 
 	lua_getglobal(filter->lua_state, "filter_close");
-	if (lua_pcall(filter->lua_state, 0, 0, 0)) {
+	if (lua_pcall(filter->lua_state, 0, 1, 0)) {
 		error_lua_filter(filter);
-		ret = 1;
+		ret = -1;
+	} else {
+		ret = lua_tonumber(filter->lua_state, -1);
+		lua_pop(filter->lua_state, 1);
 	}
+
 	unhook_write();
 	return ret;
 }

filters/email-gravatar.lua

@@ -16,6 +16,7 @@ end
 
 function filter_close()
 	html("<img src='//www.gravatar.com/avatar/" .. md5 .. "?s=13&amp;d=retro' style='height:10pt;width:10pt' alt='Gravatar' /> " .. buffer)
+	return 0
 end
 
 function filter_write(str)