patch, 3 partsIntroduce a global level warn() function.

git-compat-util.h, usage.c

diff --git a/git-compat-util.h b/git-compat-util.h
index 41fa7f6..304cdbf 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -71,10 +71,12 @@
 extern void usage(const char *err) NORETURN;
 extern void die(const char *err, ...) NORETURN __attribute__((format (printf, 1, 2)));
 extern int error(const char *err, ...) __attribute__((format (printf, 1, 2)));
+extern void warn(const char *err, ...) __attribute__((format (printf, 1, 2)));
 
 extern void set_usage_routine(void (*routine)(const char *err) NORETURN);
 extern void set_die_routine(void (*routine)(const char *err, va_list params) NORETURN);
 extern void set_error_routine(void (*routine)(const char *err, va_list params));
+extern void set_warn_routine(void (*routine)(const char *warn, va_list params));
 
 #ifdef NO_MMAP
 
diff --git a/usage.c b/usage.c
index 52c2e96..4dc5c77 100644
--- a/usage.c
+++ b/usage.c
@@ -29,12 +29,17 @@ static void error_builtin(const char *err, va_list params)
 	report("error: ", err, params);
 }
 
+static void warn_builtin(const char *warn, va_list params)
+{
+	report("warning: ", warn, params);
+}
 
 /* If we are in a dlopen()ed .so write to a global variable would segfault
  * (ugh), so keep things static. */
 static void (*usage_routine)(const char *err) NORETURN = usage_builtin;
 static void (*die_routine)(const char *err, va_list params) NORETURN = die_builtin;
 static void (*error_routine)(const char *err, va_list params) = error_builtin;
+static void (*warn_routine)(const char *err, va_list params) = warn_builtin;
 
 void set_usage_routine(void (*routine)(const char *err) NORETURN)
 {
@@ -51,6 +56,11 @@ void set_error_routine(void (*routine)(const char *err, va_list params))
 	error_routine = routine;
 }
 
+void set_warn_routine(void (*routine)(const char *warn, va_list params))
+{
+	warn_routine = routine;
+}
+
 
 void usage(const char *err)
 {
@@ -75,3 +85,12 @@ int error(const char *err, ...)
 	va_end(params);
 	return -1;
 }
+
+void warn(const char *warn, ...)
+{
+	va_list params;
+
+	va_start(params, warn);
+	warn_routine(warn, params);
+	va_end(params);
+}

diff --git a/revision.c b/revision.c
index de2cbd8..52330cc 100644
--- a/revision.c
+++ b/revision.c
@@ -466,6 +466,7 @@ static void limit_list(struct rev_info *revs)
 
 struct all_refs_cb {
 	int all_flags;
+	int warned_bad_reflog;
 	struct rev_info *all_revs;
 	const char *name_for_errormsg;
 };
@@ -487,25 +488,33 @@ static void handle_all(struct rev_info *revs, unsigned flags)
 	for_each_ref(handle_one_ref, &cb);
 }
 
-static int handle_one_reflog_ent(unsigned char *osha1, unsigned char *nsha1, char *detail, void *cb_data)
+static int handle_one_reflog_commit(unsigned char *sha1, void *cb_data)
 {
 	struct all_refs_cb *cb = cb_data;
-	struct object *object;
-
-	if (!is_null_sha1(osha1)) {
-		object = get_reference(cb->all_revs, cb->name_for_errormsg,
-				       osha1, cb->all_flags);
-		add_pending_object(cb->all_revs, object, "");
+	if (!is_null_sha1(sha1)) {
+		struct object *o = parse_object(sha1);
+		if (o) {
+			o->flags |= cb->all_flags;
+			add_pending_object(cb->all_revs, o, "");
+		} else if (!cb->warned_bad_reflog) {
+			warn("reflog of '%s' references pruned commits",
+				cb->name_for_errormsg);
+			cb->warned_bad_reflog = 1;
+		}
 	}
-	object = get_reference(cb->all_revs, cb->name_for_errormsg,
-			       nsha1, cb->all_flags);
-	add_pending_object(cb->all_revs, object, "");
+}
+
+static int handle_one_reflog_ent(unsigned char *osha1, unsigned char *nsha1, char *detail, void *cb_data)
+{
+	handle_one_reflog_commit(osha1, cb_data);
+	handle_one_reflog_commit(nsha1, cb_data);
 	return 0;
 }
 
 static int handle_one_reflog(const char *path, const unsigned char *sha1, int flag, void *cb_data)
 {
 	struct all_refs_cb *cb = cb_data;
+	cb->warned_bad_reflog = 0;
 	cb->name_for_errormsg = path;
 	for_each_reflog_ent(path, handle_one_reflog_ent, cb_data);
 	return 0;

diff --git a/builtin-reflog.c b/builtin-reflog.c
index d4f7353..4097c32 100644
--- a/builtin-reflog.c
+++ b/builtin-reflog.c
@@ -3,7 +3,7 @@
 #include "commit.h"
 #include "refs.h"
 #include "dir.h"
-#include <time.h>
+#include "tree-walk.h"
 
 struct expire_reflog_cb {
 	FILE *newlog;
@@ -13,13 +13,50 @@ struct expire_reflog_cb {
 	unsigned long expire_unreachable;
 };
 
+static int tree_is_complete(const unsigned char *sha1)
+{
+	struct tree_desc desc;
+	void *buf;
+	char type[20];
+
+	buf = read_sha1_file(sha1, type, &desc.size);
+	if (!buf)
+		return 0;
+	desc.buf = buf;
+	while (desc.size) {
+		const unsigned char *elem;
+		const char *name;
+		unsigned mode;
+
+		elem = tree_entry_extract(&desc, &name, &mode);
+		if (!has_sha1_file(elem) ||
+		    (S_ISDIR(mode) && !tree_is_complete(elem))) {
+			free(buf);
+			return 0;
+		}
+		update_tree_entry(&desc);
+	}
+	free(buf);
+	return 1;
+}
+
 static int keep_entry(struct commit **it, unsigned char *sha1)
 {
+	struct commit *commit;
+
 	*it = NULL;
 	if (is_null_sha1(sha1))
 		return 1;
-	*it = lookup_commit_reference_gently(sha1, 1);
-	return (*it != NULL);
+	commit = lookup_commit_reference_gently(sha1, 1);
+	if (!commit)
+		return 0;
+
+	/* Make sure everything in this commit exists. */
+	parse_object(commit->object.sha1);
+	if (!tree_is_complete(commit->tree->object.sha1))
+		return 0;
+	*it = commit;
+	return 1;
 }
 
 static int expire_reflog_ent(unsigned char *osha1, unsigned char *nsha1,