rerere: explain the remainder

Explain the internals of rerere as in-code comments, while
sprinkling "NEEDSWORK" comment to highlight iffy bits and
questionable assumptions.

This covers the codepath that implements "rerere gc" and "rerere
clear".

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

rerere.c

@@ -853,6 +853,9 @@ int rerere_forget(struct pathspec *pathspec)
 	return write_rr(&merge_rr, fd);
 }
 
+/*
+ * Garbage collection support
+ */
 static time_t rerere_created_at(const char *name)
 {
 	struct stat st;
@@ -865,11 +868,19 @@ static time_t rerere_last_used_at(const char *name)
 	return stat(rerere_path(name, "postimage"), &st) ? (time_t) 0 : st.st_mtime;
 }
 
+/*
+ * Remove the recorded resolution for a given conflict ID
+ */
 static void unlink_rr_item(const char *name)
 {
 	unlink(rerere_path(name, "thisimage"));
 	unlink(rerere_path(name, "preimage"));
 	unlink(rerere_path(name, "postimage"));
+	/*
+	 * NEEDSWORK: what if this rmdir() fails?  Wouldn't we then
+	 * assume that we already have preimage recorded in
+	 * do_plain_rerere()?
+	 */
 	rmdir(git_path("rr-cache/%s", name));
 }
 
@@ -889,6 +900,7 @@ void rerere_gc(struct string_list *rr)
 	dir = opendir(git_path("rr-cache"));
 	if (!dir)
 		die_errno("unable to open rr-cache directory");
+	/* Collect stale conflict IDs ... */
 	while ((e = readdir(dir))) {
 		if (is_dot_or_dotdot(e->d_name))
 			continue;
@@ -906,11 +918,19 @@ void rerere_gc(struct string_list *rr)
 			string_list_append(&to_remove, e->d_name);
 	}
 	closedir(dir);
+	/* ... and then remove them one-by-one */
 	for (i = 0; i < to_remove.nr; i++)
 		unlink_rr_item(to_remove.items[i].string);
 	string_list_clear(&to_remove, 0);
 }
 
+/*
+ * During a conflict resolution, after "rerere" recorded the
+ * preimages, abandon them if the user did not resolve them or
+ * record their resolutions.  And drop $GIT_DIR/MERGE_RR.
+ *
+ * NEEDSWORK: shouldn't we be calling this from "reset --hard"?
+ */
 void rerere_clear(struct string_list *merge_rr)
 {
 	int i;