checkout: notice when the switched branch is behind or forked

When you are switching to a branch that is marked to merge from
somewhere else, e.g. when you have:

    [branch "next"]
            remote = upstream
            merge = refs/heads/next
    [remote "upstream"]
            url = ...
            fetch = refs/heads/*:refs/remotes/linus/*

and you say "git checkout next", the branch you checked out
may be behind, and you may want to update from the upstream
before continuing to work.

This patch makes the command to check the upstream (in this
example, "refs/remotes/linus/next") and our branch "next", and:

    (1) if they match, nothing happens;

    (2) if you are ahead (i.e. the upstream is a strict ancestor
        of you), one line message tells you so;

    (3) otherwise, you are either behind or you and the upstream
        have forked.  One line message will tell you which and
        then you will see a "log --pretty=oneline --left-right".

We could enhance this with an option that tells the command to
check if there is no local change, and automatically fast
forward when you are truly behind.  But I ripped out that change
because I was unsure what the right way should be to allow users
to control it (issues include that checkout should not become
automatically interactive).

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-checkout.c

@@ -12,6 +12,7 @@
 #include "branch.h"
 #include "diff.h"
 #include "revision.h"
+#include "remote.h"
 
 static const char * const checkout_usage[] = {
 	"git checkout [options] <branch>",
@@ -290,6 +291,139 @@ static int merge_working_tree(struct checkout_opts *opts,
 	return 0;
 }
 
+/*
+ * We really should allow cb_data... Yuck
+ */
+static const char *branch_name;
+static int branch_name_len;
+static char *found_remote;
+static char *found_merge;
+static int read_branch_config(const char *var, const char *value)
+{
+	const char *name;
+	if (prefixcmp(var, "branch."))
+		return 0; /* not ours */
+	name = var + strlen("branch.");
+	if (strncmp(name, branch_name, branch_name_len) ||
+	    name[branch_name_len] != '.')
+		return 0; /* not ours either */
+	if (!strcmp(name + branch_name_len, ".remote")) {
+		/*
+		 * Yeah, I know Christian's clean-up should
+		 * be used here, but the topic is based on an
+		 * older fork point.
+		 */
+		if (!value)
+			return error("'%s' not string", var);
+		found_remote = xstrdup(value);
+		return 0;
+	}
+	if (!strcmp(name + branch_name_len, ".merge")) {
+		if (!value)
+			return error("'%s' not string", var);
+		found_merge = xstrdup(value);
+		return 0;
+	}
+	return 0; /* not ours */
+}
+
+static int find_build_base(const char *ours, char **base)
+{
+	struct remote *remote;
+	struct refspec spec;
+
+	*base = NULL;
+
+	branch_name = ours + strlen("refs/heads/");
+	branch_name_len = strlen(branch_name);
+	found_remote = NULL;
+	found_merge = NULL;
+	git_config(read_branch_config);
+
+	if (!found_remote || !found_merge) {
+	cleanup:
+		free(found_remote);
+		free(found_merge);
+		return 0;
+	}
+
+	remote = remote_get(found_remote);
+	memset(&spec, 0, sizeof(spec));
+	spec.src = found_merge;
+	if (remote_find_tracking(remote, &spec))
+		goto cleanup;
+	*base = spec.dst;
+	return 1;
+}
+
+static void adjust_to_tracking(struct branch_info *new, struct checkout_opts *opts)
+{
+	/*
+	 * We have switched to a new branch; is it building on
+	 * top of another branch, and if so does that other branch
+	 * have changes we do not have yet?
+	 */
+	char *base;
+	unsigned char sha1[20];
+	struct commit *ours, *theirs;
+	const char *msgfmt;
+	char symmetric[84];
+	int show_log;
+
+	if (!resolve_ref(new->path, sha1, 1, NULL))
+		return;
+	ours = lookup_commit(sha1);
+
+	if (!find_build_base(new->path, &base))
+		return;
+
+	sprintf(symmetric, "%s", sha1_to_hex(sha1));
+
+	/*
+	 * Ok, it is tracking base; is it ahead of us?
+	 */
+	if (!resolve_ref(base, sha1, 1, NULL))
+		return;
+	theirs = lookup_commit(sha1);
+
+	sprintf(symmetric + 40, "...%s", sha1_to_hex(sha1));
+
+	if (!hashcmp(sha1, ours->object.sha1))
+		return; /* we are the same */
+
+	show_log = 1;
+	if (in_merge_bases(theirs, &ours, 1)) {
+		msgfmt = "You are ahead of the tracked branch '%s'\n";
+		show_log = 0;
+	}
+	else if (in_merge_bases(ours, &theirs, 1))
+		msgfmt = "Your branch can be fast-forwarded to the tracked branch '%s'\n";
+	else
+		msgfmt = "Both your branch and the tracked branch '%s' have own changes, you would eventually need to merge\n";
+
+	if (!prefixcmp(base, "refs/remotes/"))
+		base += strlen("refs/remotes/");
+	fprintf(stderr, msgfmt, base);
+
+	if (show_log) {
+		const char *args[32];
+		int ac;
+
+		ac = 0;
+		args[ac++] = "log";
+		args[ac++] = "--pretty=oneline";
+		args[ac++] = "--abbrev-commit";
+		args[ac++] = "--left-right";
+		args[ac++] = "--boundary";
+		args[ac++] = symmetric;
+		args[ac++] = "--";
+		args[ac] = NULL;
+
+		run_command_v_opt(args, RUN_GIT_CMD);
+	}
+}
+
+
 static void update_refs_for_switch(struct checkout_opts *opts,
 				   struct branch_info *old,
 				   struct branch_info *new)
@@ -332,6 +466,8 @@ static void update_refs_for_switch(struct checkout_opts *opts,
 	}
 	remove_branch_state();
 	strbuf_release(&msg);
+	if (new->path)
+		adjust_to_tracking(new, opts);
 }
 
 static int switch_branches(struct checkout_opts *opts,