rerere: explain MERGE_RR management helpers

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

This one covers the "$GIT_DIR/MERGE_RR" file and in-core merge_rr
that are used to keep track of the status of "rerere" session in
progress.

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

rerere.c

@@ -33,6 +33,13 @@ static int has_rerere_resolution(const char *hex)
 	return !stat(rerere_path(hex, "postimage"), &st);
 }
 
+/*
+ * $GIT_DIR/MERGE_RR file is a collection of records, each of which is
+ * "conflict ID", a HT and pathname, terminated with a NUL, and is
+ * used to keep track of the set of paths that "rerere" may need to
+ * work on (i.e. what is left by the previous invocation of "git
+ * rerere" during the current conflict resolution session).
+ */
 static void read_rr(struct string_list *rr)
 {
 	struct strbuf buf = STRBUF_INIT;
@@ -403,6 +410,14 @@ static int handle_cache(const char *path, unsigned char *sha1, const char *outpu
 	return hunk_no;
 }
 
+/*
+ * Look at a cache entry at "i" and see if it is not conflicting,
+ * conflicting and we are willing to handle, or conflicting and
+ * we are unable to handle, and return the determination in *type.
+ * Return the cache index to be looked at next, by skipping the
+ * stages we have already looked at in this invocation of this
+ * function.
+ */
 static int check_one_conflict(int i, int *type)
 {
 	const struct cache_entry *e = active_cache[i];
@@ -434,6 +449,17 @@ static int check_one_conflict(int i, int *type)
 	return i;
 }
 
+/*
+ * Scan the index and find paths that have conflicts that rerere can
+ * handle, i.e. the ones that has both stages #2 and #3.
+ *
+ * NEEDSWORK: we do not record or replay a previous "resolve by
+ * deletion" for a delete-modify conflict, as that is inherently risky
+ * without knowing what modification is being discarded.  The only
+ * safe case, i.e. both side doing the deletion and modification that
+ * are identical to the previous round, might want to be handled,
+ * though.
+ */
 static int find_conflict(struct string_list *conflict)
 {
 	int i;
@@ -450,6 +476,21 @@ static int find_conflict(struct string_list *conflict)
 	return 0;
 }
 
+/*
+ * The merge_rr list is meant to hold outstanding conflicted paths
+ * that rerere could handle.  Abuse the list by adding other types of
+ * entries to allow the caller to show "rerere remaining".
+ *
+ * - Conflicted paths that rerere does not handle are added
+ * - Conflicted paths that have been resolved are marked as such
+ *   by storing RERERE_RESOLVED to .util field (where conflict ID
+ *   is expected to be stored).
+ *
+ * Do *not* write MERGE_RR file out after calling this function.
+ *
+ * NEEDSWORK: we may want to fix the caller that implements "rerere
+ * remaining" to do this without abusing merge_rr.
+ */
 int rerere_remaining(struct string_list *merge_rr)
 {
 	int i;