add a stat_validity struct

It can sometimes be useful to know whether a path in the
filesystem has been updated without going to the work of
opening and re-reading its content. We trust the stat()
information on disk already to handle index updates, and we
can use the same trick here.

This patch introduces a "stat_validity" struct which
encapsulates the concept of checking the stat-freshness of a
file. It is implemented on top of "struct stat_data" to
reuse the logic about which stat entries to trust for a
particular platform, but hides the complexity behind two
simple functions: check and update.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1360,4 +1360,31 @@ int checkout_fast_forward(const unsigned char *from,
 
 int sane_execvp(const char *file, char *const argv[]);
 
+/*
+ * A struct to encapsulate the concept of whether a file has changed
+ * since we last checked it. This uses criteria similar to those used
+ * for the index.
+ */
+struct stat_validity {
+	struct stat_data *sd;
+};
+
+void stat_validity_clear(struct stat_validity *sv);
+
+/*
+ * Returns 1 if the path is a regular file (or a symlink to a regular
+ * file) and matches the saved stat_validity, 0 otherwise.  A missing
+ * or inaccessible file is considered a match if the struct was just
+ * initialized, or if the previous update found an inaccessible file.
+ */
+int stat_validity_check(struct stat_validity *sv, const char *path);
+
+/*
+ * Update the stat_validity from a file opened at descriptor fd. If
+ * the file is missing, inaccessible, or not a regular file, then
+ * future calls to stat_validity_check will match iff one of those
+ * conditions continues to be true.
+ */
+void stat_validity_update(struct stat_validity *sv, int fd);
+
 #endif /* CACHE_H */

read-cache.c

@@ -1950,3 +1950,33 @@ void *read_blob_data_from_index(struct index_state *istate, const char *path, un
 		*size = sz;
 	return data;
 }
+
+void stat_validity_clear(struct stat_validity *sv)
+{
+	free(sv->sd);
+	sv->sd = NULL;
+}
+
+int stat_validity_check(struct stat_validity *sv, const char *path)
+{
+	struct stat st;
+
+	if (stat(path, &st) < 0)
+		return sv->sd == NULL;
+	if (!sv->sd)
+		return 0;
+	return S_ISREG(st.st_mode) && !match_stat_data(sv->sd, &st);
+}
+
+void stat_validity_update(struct stat_validity *sv, int fd)
+{
+	struct stat st;
+
+	if (fstat(fd, &st) < 0 || !S_ISREG(st.st_mode))
+		stat_validity_clear(sv);
+	else {
+		if (!sv->sd)
+			sv->sd = xcalloc(1, sizeof(struct stat_data));
+		fill_stat_data(sv->sd, &st);
+	}
+}