remote: separate the concept of push and fetch mirrors

git-remote currently has one option, "--mirror", which sets
up mirror configuration which can be used for either
fetching or pushing. It looks like this:

  [remote "mirror"]
    url = wherever
    fetch = +refs/*:refs/*
    mirror = true

However, a remote like this can be dangerous and confusing.
Specifically:

  1. If you issue the wrong command, it can be devastating.
     You are not likely to "push" when you meant to "fetch",
     but "git remote update" will try to fetch it, even if
     you intended the remote only for pushing. In either
     case, the results can be quite destructive. An
     unintended push will overwrite or delete remote refs,
     and an unintended fetch can overwrite local branches.

  2. The tracking setup code can produce confusing results.
     The fetch refspec above means that "git checkout -b new
     master" will consider refs/heads/master to come from
     the remote "mirror", even if you only ever intend to
     push to the mirror. It will set up the "new" branch to
     track mirror's refs/heads/master.

  3. The push code tries to opportunistically update
     tracking branches. If you "git push mirror foo:bar",
     it will see that we are updating mirror's
     refs/heads/bar, which corresponds to our local
     refs/heads/bar, and will update our local branch.

To solve this, we split the concept into "push mirrors" and
"fetch mirrors". Push mirrors set only remote.*.mirror,
solving (2) and (3), and making an accidental fetch write
only into FETCH_HEAD. Fetch mirrors set only the fetch
refspec, meaning an accidental push will not force-overwrite
or delete refs on the remote end.

The new syntax is "--mirror=<fetch|push>". For
compatibility, we keep "--mirror" as-is, setting up both
types simultaneously.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 38 insertions, 13 deletions

diff --git a/builtin/remote.c b/builtin/remote.c
index 952be2e27..f9522d5d8 100644
--- a/builtin/remote.c
+++ b/builtin/remote.c
@@ -9,7 +9,7 @@
 
 static const char * const builtin_remote_usage[] = {
 	"git remote [-v | --verbose]",
-	"git remote add [-t <branch>] [-m <master>] [-f] [--mirror] <name> <url>",
+	"git remote add [-t <branch>] [-m <master>] [-f] [--mirror=<fetch|push>] <name> <url>",
 	"git remote rename <old> <new>",
 	"git remote rm <name>",
 	"git remote set-head <name> (-a | -d | <branch>)",
@@ -117,6 +117,11 @@ enum {
 	TAGS_SET = 2
 };
 
+#define MIRROR_NONE 0
+#define MIRROR_FETCH 1
+#define MIRROR_PUSH 2
+#define MIRROR_BOTH (MIRROR_FETCH|MIRROR_PUSH)
+
 static int add_branch(const char *key, const char *branchname,
 		const char *remotename, int mirror, struct strbuf *tmp)
 {
@@ -131,9 +136,26 @@ static int add_branch(const char *key, const char *branchname,
 	return git_config_set_multivar(key, tmp->buf, "^$", 0);
 }
 
+static int parse_mirror_opt(const struct option *opt, const char *arg, int not)
+{
+	unsigned *mirror = opt->value;
+	if (not)
+		*mirror = MIRROR_NONE;
+	else if (!arg)
+		*mirror = MIRROR_BOTH;
+	else if (!strcmp(arg, "fetch"))
+		*mirror = MIRROR_FETCH;
+	else if (!strcmp(arg, "push"))
+		*mirror = MIRROR_PUSH;
+	else
+		return error("unknown mirror argument: %s", arg);
+	return 0;
+}
+
 static int add(int argc, const char **argv)
 {
-	int fetch = 0, mirror = 0, fetch_tags = TAGS_DEFAULT;
+	int fetch = 0, fetch_tags = TAGS_DEFAULT;
+	unsigned mirror = MIRROR_NONE;
 	struct string_list track = STRING_LIST_INIT_NODUP;
 	const char *master = NULL;
 	struct remote *remote;
@@ -151,7 +173,9 @@ static int add(int argc, const char **argv)
 		OPT_CALLBACK('t', "track", &track, "branch",
 			"branch(es) to track", opt_parse_track),
 		OPT_STRING('m', "master", &master, "branch", "master branch"),
-		OPT_BOOLEAN(0, "mirror", &mirror, "no separate remotes"),
+		{ OPTION_CALLBACK, 0, "mirror", &mirror, "push|fetch",
+			"set up remote as a mirror to push to or fetch from",
+			PARSE_OPT_OPTARG, parse_mirror_opt },
 		OPT_END()
 	};
 
@@ -182,18 +206,19 @@ static int add(int argc, const char **argv)
 	if (git_config_set(buf.buf, url))
 		return 1;
 
-	strbuf_reset(&buf);
-	strbuf_addf(&buf, "remote.%s.fetch", name);
-
-	if (track.nr == 0)
-		string_list_append(&track, "*");
-	for (i = 0; i < track.nr; i++) {
-		if (add_branch(buf.buf, track.items[i].string,
-				name, mirror, &buf2))
-			return 1;
+	if (!mirror || mirror & MIRROR_FETCH) {
+		strbuf_reset(&buf);
+		strbuf_addf(&buf, "remote.%s.fetch", name);
+		if (track.nr == 0)
+			string_list_append(&track, "*");
+		for (i = 0; i < track.nr; i++) {
+			if (add_branch(buf.buf, track.items[i].string,
+				       name, mirror, &buf2))
+				return 1;
+		}
 	}
 
-	if (mirror) {
+	if (mirror & MIRROR_PUSH) {
 		strbuf_reset(&buf);
 		strbuf_addf(&buf, "remote.%s.mirror", name);
 		if (git_config_set(buf.buf, "true"))