reflog --expire-unreachable: special case entries in "HEAD" reflog

"git reflog expire" (and "git gc") examines the reflog entries and
discards old/stale ones using two criteria.  The entries that are older
than "reflogexpire" (defaults to 90 days) are unconditionally removed, and
the entries that are older than "reflogexpireunreachable" (defaults to 30
days) are removed if the entry point at commits that are not reachable
from the value of the ref.

This is reasonable for local branches, remote tracking branches and tags.
You (or other people) may have failed experiments that have been made and
then later discarded by resetting the tip of the branch back, and setting
the value of "reflogexpireunreachable" shorter than that of "reflogexpire"
will prune the entries that describe these failed experiments earlier than
the entries that describe the steps that led to the current history.

It however doesn't make much sense for "HEAD" reflog.  When you switch
between branches, it is normal that the tip of the branch you were on is
not an ancestor of the branch you have switched to.  The moral equivalent
of expiring failed experiments in per-branch reflog for "HEAD" reflog is
to expire entries that talk about commits that cannot be reached from any
ref.

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

builtin-reflog.c

@@ -34,8 +34,11 @@ struct cmd_reflog_expire_cb {
 
 struct expire_reflog_cb {
 	FILE *newlog;
-	const char *ref;
-	struct commit *ref_commit;
+	enum {
+		UE_NORMAL,
+		UE_ALWAYS,
+		UE_HEAD
+	} unreachable_expire_kind;
 	struct commit_list *mark_list;
 	unsigned long mark_limit;
 	struct cmd_reflog_expire_cb *cmd;
@@ -305,7 +308,7 @@ static int expire_reflog_ent(unsigned char *osha1, unsigned char *nsha1,
 		goto prune;
 
 	if (timestamp < cb->cmd->expire_unreachable) {
-		if (!cb->ref_commit)
+		if (cb->unreachable_expire_kind == UE_ALWAYS)
 			goto prune;
 		if (unreachable(cb, old, osha1) || unreachable(cb, new, nsha1))
 			goto prune;
@@ -332,12 +335,27 @@ static int expire_reflog_ent(unsigned char *osha1, unsigned char *nsha1,
 	return 0;
 }
 
+static int push_tip_to_list(const char *refname, const unsigned char *sha1, int flags, void *cb_data)
+{
+	struct commit_list **list = cb_data;
+	struct commit *tip_commit;
+	if (flags & REF_ISSYMREF)
+		return 0;
+	tip_commit = lookup_commit_reference_gently(sha1, 1);
+	if (!tip_commit)
+		return 0;
+	commit_list_insert(tip_commit, list);
+	return 0;
+}
+
 static int expire_reflog(const char *ref, const unsigned char *sha1, int unused, void *cb_data)
 {
 	struct cmd_reflog_expire_cb *cmd = cb_data;
 	struct expire_reflog_cb cb;
 	struct ref_lock *lock;
 	char *log_file, *newlog_path = NULL;
+	struct commit *tip_commit;
+	struct commit_list *tips;
 	int status = 0;
 
 	memset(&cb, 0, sizeof(cb));
@@ -357,18 +375,49 @@ static int expire_reflog(const char *ref, const unsigned char *sha1, int unused,
 		cb.newlog = fopen(newlog_path, "w");
 	}
 
-	cb.ref_commit = lookup_commit_reference_gently(sha1, 1);
-	cb.ref = ref;
 	cb.cmd = cmd;
-	if (cb.ref_commit) {
-		cb.mark_list = NULL;
-		commit_list_insert(cb.ref_commit, &cb.mark_list);
+
+	if (!cmd->expire_unreachable || !strcmp(ref, "HEAD")) {
+		tip_commit = NULL;
+		cb.unreachable_expire_kind = UE_HEAD;
+	} else {
+		tip_commit = lookup_commit_reference_gently(sha1, 1);
+		if (!tip_commit)
+			cb.unreachable_expire_kind = UE_ALWAYS;
+		else
+			cb.unreachable_expire_kind = UE_NORMAL;
+	}
+
+	if (cmd->expire_unreachable <= cmd->expire_total)
+		cb.unreachable_expire_kind = UE_ALWAYS;
+
+	cb.mark_list = NULL;
+	tips = NULL;
+	if (cb.unreachable_expire_kind != UE_ALWAYS) {
+		if (cb.unreachable_expire_kind == UE_HEAD) {
+			struct commit_list *elem;
+			for_each_ref(push_tip_to_list, &tips);
+			for (elem = tips; elem; elem = elem->next)
+				commit_list_insert(elem->item, &cb.mark_list);
+		} else {
+			commit_list_insert(tip_commit, &cb.mark_list);
+		}
 		cb.mark_limit = cmd->expire_total;
 		mark_reachable(&cb);
 	}
+
 	for_each_reflog_ent(ref, expire_reflog_ent, &cb);
-	if (cb.ref_commit)
-		clear_commit_marks(cb.ref_commit, REACHABLE);
+
+	if (cb.unreachable_expire_kind != UE_ALWAYS) {
+		if (cb.unreachable_expire_kind == UE_HEAD) {
+			struct commit_list *elem;
+			for (elem = tips; elem; elem = elem->next)
+				clear_commit_marks(tip_commit, REACHABLE);
+			free_commit_list(tips);
+		} else {
+			clear_commit_marks(tip_commit, REACHABLE);
+		}
+	}
  finish:
 	if (cb.newlog) {
 		if (fclose(cb.newlog)) {