filter: introduce "filter type" prefix

This allows different filter implementations to be specified in the
configuration file.  Currently only "exec" is supported, but it may now
be specified either with or without the "exec:" prefix.

Signed-off-by: John Keeping <john@keeping.me.uk>

cgitrc.5.txt

@@ -557,6 +557,15 @@ config files, e.g. "repo.desc" becomes "desc".
 
 FILTER API
 ----------
+By default, filters are separate processes that are executed each time they
+are needed.  Alternative technologies may be used by prefixing the filter
+specification with the relevant string; available values are:
+
+'exec:'::
+	The default "one process per filter" mode.
+
+Parameters are provided to filters as follows.
+
 about filter::
 	This filter is given a single parameter: the filename of the source
 	file to filter. The filter can use the filename to determine (for

filter.c

@@ -64,7 +64,7 @@ done:
 static void fprintf_exec_filter(struct cgit_filter *base, FILE *f, const char *prefix)
 {
 	struct cgit_exec_filter *filter = (struct cgit_exec_filter *) base;
-	fprintf(f, "%s%s\n", prefix, filter->cmd);
+	fprintf(f, "%sexec:%s\n", prefix, filter->cmd);
 }
 
 int cgit_open_filter(struct cgit_filter *filter, ...)
@@ -125,10 +125,39 @@ static struct cgit_filter *new_exec_filter(const char *cmd, filter_type filterty
 	return &f->base;
 }
 
+static const struct {
+	const char *prefix;
+	struct cgit_filter *(*ctor)(const char *cmd, filter_type filtertype);
+} filter_specs[] = {
+	{ "exec", new_exec_filter },
+};
+
 struct cgit_filter *cgit_new_filter(const char *cmd, filter_type filtertype)
 {
+	char *colon;
+	int i;
+	size_t len;
 	if (!cmd || !cmd[0])
 		return NULL;
 
-	return new_exec_filter(cmd, filtertype);
+	colon = strchr(cmd, ':');
+	len = colon - cmd;
+	/*
+	 * In case we're running on Windows, don't allow a single letter before
+	 * the colon.
+	 */
+	if (len == 1)
+		colon = NULL;
+
+	/* If no prefix is given, exec filter is the default. */
+	if (!colon)
+		return new_exec_filter(cmd, filtertype);
+
+	for (i = 0; i < ARRAY_SIZE(filter_specs); i++) {
+		if (len == strlen(filter_specs[i].prefix) &&
+		    !strncmp(filter_specs[i].prefix, cmd, len))
+			return filter_specs[i].ctor(colon + 1, filtertype);
+	}
+
+	die("Invalid filter type: %.*s", (int) len, cmd);
 }