help: implement multi-valued "man.viewer" config option

This allows multiple viewer candidates to be listed in the configuration
file, like this:

        [man]
                viewer = woman
                viewer = konqueror
                viewer = man

The candidates are tried in the order listed in the configuration file,
and the first suitable one (e.g. konqueror cannot be used outside windowed
environment) is used.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Tested-by: Xavier Maillard <xma@gnu.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

help.c

@@ -10,7 +10,10 @@
 #include "parse-options.h"
 #include "run-command.h"
 
-static const char *man_viewer;
+static struct man_viewer_list {
+	void (*exec)(const char *);
+	struct man_viewer_list *next;
+} *man_viewer_list;
 
 enum help_format {
 	HELP_FORMAT_MAN,
@@ -45,6 +48,102 @@ static enum help_format parse_help_format(const char *format)
 	die("unrecognized help format '%s'", format);
 }
 
+static int check_emacsclient_version(void)
+{
+	struct strbuf buffer = STRBUF_INIT;
+	struct child_process ec_process;
+	const char *argv_ec[] = { "emacsclient", "--version", NULL };
+	int version;
+
+	/* emacsclient prints its version number on stderr */
+	memset(&ec_process, 0, sizeof(ec_process));
+	ec_process.argv = argv_ec;
+	ec_process.err = -1;
+	ec_process.stdout_to_stderr = 1;
+	if (start_command(&ec_process)) {
+		fprintf(stderr, "Failed to start emacsclient.\n");
+		return -1;
+	}
+	strbuf_read(&buffer, ec_process.err, 20);
+	close(ec_process.err);
+
+	/*
+	 * Don't bother checking return value, because "emacsclient --version"
+	 * seems to always exits with code 1.
+	 */
+	finish_command(&ec_process);
+
+	if (prefixcmp(buffer.buf, "emacsclient")) {
+		fprintf(stderr, "Failed to parse emacsclient version.\n");
+		strbuf_release(&buffer);
+		return -1;
+	}
+
+	strbuf_remove(&buffer, 0, strlen("emacsclient"));
+	version = atoi(buffer.buf);
+
+	if (version < 22) {
+		fprintf(stderr,
+			"emacsclient version '%d' too old (< 22).\n",
+			version);
+		strbuf_release(&buffer);
+		return -1;
+	}
+
+	strbuf_release(&buffer);
+	return 0;
+}
+
+static void exec_woman_emacs(const char *page)
+{
+	if (!check_emacsclient_version()) {
+		/* This works only with emacsclient version >= 22. */
+		struct strbuf man_page = STRBUF_INIT;
+		strbuf_addf(&man_page, "(woman \"%s\")", page);
+		execlp("emacsclient", "emacsclient", "-e", man_page.buf, NULL);
+	}
+}
+
+static void exec_man_konqueror(const char *page)
+{
+	const char *display = getenv("DISPLAY");
+	if (display && *display) {
+		struct strbuf man_page = STRBUF_INIT;
+		strbuf_addf(&man_page, "man:%s(1)", page);
+		execlp("kfmclient", "kfmclient", "newTab", man_page.buf, NULL);
+	}
+}
+
+static void exec_man_man(const char *page)
+{
+	execlp("man", "man", page, NULL);
+}
+
+static void do_add_man_viewer(void (*exec)(const char *))
+{
+	struct man_viewer_list **p = &man_viewer_list;
+
+	while (*p)
+		p = &((*p)->next);
+	*p = xmalloc(sizeof(**p));
+	(*p)->next = NULL;
+	(*p)->exec = exec;
+}
+
+static int add_man_viewer(const char *value)
+{
+	if (!strcasecmp(value, "man"))
+		do_add_man_viewer(exec_man_man);
+	else if (!strcasecmp(value, "woman"))
+		do_add_man_viewer(exec_woman_emacs);
+	else if (!strcasecmp(value, "konqueror"))
+		do_add_man_viewer(exec_man_konqueror);
+	else
+		return error("'%s': unsupported man viewer.", value);
+
+	return 0;
+}
+
 static int git_help_config(const char *var, const char *value)
 {
 	if (!strcmp(var, "help.format")) {
@@ -53,8 +152,11 @@ static int git_help_config(const char *var, const char *value)
 		help_format = parse_help_format(value);
 		return 0;
 	}
-	if (!strcmp(var, "man.viewer"))
-		return git_config_string(&man_viewer, var, value);
+	if (!strcmp(var, "man.viewer")) {
+		if (!value)
+			return config_error_nonbool(var);
+		return add_man_viewer(value);
+	}
 	return git_default_config(var, value);
 }
 
@@ -350,85 +452,18 @@ static void setup_man_path(void)
 	strbuf_release(&new_path);
 }
 
-static int check_emacsclient_version(void)
-{
-	struct strbuf buffer = STRBUF_INIT;
-	struct child_process ec_process;
-	const char *argv_ec[] = { "emacsclient", "--version", NULL };
-	int version;
-
-	/* emacsclient prints its version number on stderr */
-	memset(&ec_process, 0, sizeof(ec_process));
-	ec_process.argv = argv_ec;
-	ec_process.err = -1;
-	ec_process.stdout_to_stderr = 1;
-	if (start_command(&ec_process)) {
-		fprintf(stderr, "Failed to start emacsclient.\n");
-		return -1;
-	}
-	strbuf_read(&buffer, ec_process.err, 20);
-	close(ec_process.err);
-
-	/*
-	 * Don't bother checking return value, because "emacsclient --version"
-	 * seems to always exits with code 1.
-	 */
-	finish_command(&ec_process);
-
-	if (prefixcmp(buffer.buf, "emacsclient")) {
-		fprintf(stderr, "Failed to parse emacsclient version.\n");
-		strbuf_release(&buffer);
-		return -1;
-	}
-
-	strbuf_remove(&buffer, 0, strlen("emacsclient"));
-	version = atoi(buffer.buf);
-
-	if (version < 22) {
-		fprintf(stderr,
-			"emacsclient version '%d' too old (< 22).\n",
-			version);
-		strbuf_release(&buffer);
-		return -1;
-	}
-
-	strbuf_release(&buffer);
-	return 0;
-}
-
-static void exec_woman_emacs(const char *page)
-{
-	if (!check_emacsclient_version()) {
-		/* This works only with emacsclient version >= 22. */
-		struct strbuf man_page = STRBUF_INIT;
-		strbuf_addf(&man_page, "(woman \"%s\")", page);
-		execlp("emacsclient", "emacsclient", "-e", man_page.buf, NULL);
-	} else
-		execlp("man", "man", page, NULL);
-}
-
-static void exec_man_konqueror(const char *page)
-{
-	const char *display = getenv("DISPLAY");
-	if (display && *display) {
-		struct strbuf man_page = STRBUF_INIT;
-		strbuf_addf(&man_page, "man:%s(1)", page);
-		execlp("kfmclient", "kfmclient", "newTab", man_page.buf, NULL);
-	} else
-		execlp("man", "man", page, NULL);
-}
-
 static void show_man_page(const char *git_cmd)
 {
+	struct man_viewer_list *viewer;
 	const char *page = cmd_to_page(git_cmd);
+
 	setup_man_path();
-	if (!man_viewer || !strcmp(man_viewer, "man"))
-		execlp("man", "man", page, NULL);
-	if (!strcmp(man_viewer, "woman"))
-		exec_woman_emacs(page);
-	if (!strcmp(man_viewer, "konqueror"))
-		exec_man_konqueror(page);
-	die("'%s': unsupported man viewer.", man_viewer);
+	for (viewer = man_viewer_list; viewer; viewer = viewer->next)
+	{
+		viewer->exec(page); /* will return when unable */
+	}
+	exec_man_man(page);
+	die("no man viewer handled the request");
 }
 
 static void show_info_page(const char *git_cmd)