[PATCH v2 04/19] read-cache: Re-read index if index file changed

[Thomas Gummerer <t.gummerer@gmail.com>]

Add the possibility of re-reading the index file, if it changed
while reading.

The index file might change during the read, causing outdated
information to be displayed. We check if the index file changed
by using its stat data as heuristic.

Helped-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
---
 read-cache.c | 91 +++++++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 57 insertions(+), 34 deletions(-)

diff --git a/read-cache.c b/read-cache.c
index 1e7ffc2..3e3a0e2 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1275,11 +1275,31 @@ int read_index(struct index_state *istate)
 	return read_index_from(istate, get_index_file());
 }
 
+static int index_changed(struct stat *st_old, struct stat *st_new)
+{
+	if (st_old->st_mtime != st_new->st_mtime ||
+#if !defined (__CYGWIN__)
+	    st_old->st_uid   != st_new->st_uid ||
+	    st_old->st_gid   != st_new->st_gid ||
+	    st_old->st_ino   != st_new->st_ino ||
+#endif
+#if USE_NSEC
+	    ST_MTIME_NSEC(*st_old) != ST_MTIME_NSEC(*st_new) ||
+#endif
+#if USE_STDEV
+	    st_old->st_dev != st_new->st_dev ||
+#endif
+	    st_old->st_size != st_new->st_size)
+		return 1;
+
+	return 0;
+}
+
 /* remember to discard_cache() before reading a different cache! */
 int read_index_from(struct index_state *istate, const char *path)
 {
-	int fd;
-	struct stat st;
+	int fd, err, i;
+	struct stat st_old, st_new;
 	struct cache_version_header *hdr;
 	void *mmap;
 	size_t mmap_size;
@@ -1291,41 +1311,44 @@ int read_index_from(struct index_state *istate, const char *path)
 	errno = ENOENT;
 	istate->timestamp.sec = 0;
 	istate->timestamp.nsec = 0;
+	for (i = 0; i < 50; i++) {
+		err = 0;
+		fd = open(path, O_RDONLY);
+		if (fd < 0) {
+			if (errno == ENOENT)
+				return 0;
+			die_errno("index file open failed");
+		}
 
-	fd = open(path, O_RDONLY);
-	if (fd < 0) {
-		if (errno == ENOENT)
-			return 0;
-		die_errno("index file open failed");
+		if (fstat(fd, &st_old))
+			die_errno("cannot stat the open index");
+
+		errno = EINVAL;
+		mmap_size = xsize_t(st_old.st_size);
+		mmap = xmmap(NULL, mmap_size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
+		close(fd);
+		if (mmap == MAP_FAILED)
+			die_errno("unable to map index file");
+
+		hdr = mmap;
+		if (verify_hdr_version(istate, hdr, mmap_size) < 0)
+			err = 1;
+
+		if (istate->ops->verify_hdr(mmap, mmap_size) < 0)
+			err = 1;
+
+		if (istate->ops->read_index(istate, mmap, mmap_size) < 0)
+			err = 1;
+		istate->timestamp.sec = st_old.st_mtime;
+		istate->timestamp.nsec = ST_MTIME_NSEC(st_old);
+		if (lstat(path, &st_new))
+			die_errno("cannot stat the open index");
+
+		munmap(mmap, mmap_size);
+		if (!index_changed(&st_old, &st_new) && !err)
+			return istate->cache_nr;
 	}
 
-	if (fstat(fd, &st))
-		die_errno("cannot stat the open index");
-
-	errno = EINVAL;
-	mmap_size = xsize_t(st.st_size);
-	mmap = xmmap(NULL, mmap_size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
-	close(fd);
-	if (mmap == MAP_FAILED)
-		die_errno("unable to map index file");
-
-	hdr = mmap;
-	if (verify_hdr_version(istate, hdr, mmap_size) < 0)
-		goto unmap;
-
-	if (istate->ops->verify_hdr(mmap, mmap_size) < 0)
-		goto unmap;
-
-	if (istate->ops->read_index(istate, mmap, mmap_size) < 0)
-		goto unmap;
-	istate->timestamp.sec = st.st_mtime;
-	istate->timestamp.nsec = ST_MTIME_NSEC(st);
-
-	munmap(mmap, mmap_size);
-	return istate->cache_nr;
-
-unmap:
-	munmap(mmap, mmap_size);
 	die("index file corrupt");
 }
 
-- 
1.8.3.453.g1dfc63d