safe_create_leading_directories(): add new error value SCLD_VANISHED

Add a new possible error result that can be returned by
safe_create_leading_directories() and
safe_create_leading_directories_const(): SCLD_VANISHED.  This value
indicates that a file or directory on the path existed at one point
(either it already existed or the function created it), but then it
disappeared.  This probably indicates that another process deleted the
directory while we were working.  If SCLD_VANISHED is returned, the
caller might want to retry the function call, as there is a chance
that a new attempt will succeed.

Why doesn't safe_create_leading_directories() do the retrying
internally?  Because an empty directory isn't really ever safe until
it holds a file.  So even if safe_create_leading_directories() were
absolutely sure that the directory existed before it returned, there
would be no guarantee that the directory still existed when the caller
tried to write something in it.
Signed-off-by: NMichael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

cache.h

@@ -741,12 +741,20 @@ int adjust_shared_perm(const char *path);
  * Create the directory containing the named path, using care to be
  * somewhat safe against races.  Return one of the scld_error values
  * to indicate success/failure.
+ *
+ * SCLD_VANISHED indicates that one of the ancestor directories of the
+ * path existed at one point during the function call and then
+ * suddenly vanished, probably because another process pruned the
+ * directory while we were working.  To be robust against this kind of
+ * race, callers might want to try invoking the function again when it
+ * returns SCLD_VANISHED.
  */
 enum scld_error {
 	SCLD_OK = 0,
 	SCLD_FAILED = -1,
 	SCLD_PERMS = -2,
-	SCLD_EXISTS = -3
+	SCLD_EXISTS = -3,
+	SCLD_VANISHED = -4
 };
 enum scld_error safe_create_leading_directories(char *path);
 enum scld_error safe_create_leading_directories_const(const char *path);

sha1_file.c

@@ -132,6 +132,17 @@ enum scld_error safe_create_leading_directories(char *path)
 			if (errno == EEXIST &&
 			    !stat(path, &st) && S_ISDIR(st.st_mode))
 				; /* somebody created it since we checked */
+			else if (errno == ENOENT)
+				/*
+				 * Either mkdir() failed because
+				 * somebody just pruned the containing
+				 * directory, or stat() failed because
+				 * the file that was in our way was
+				 * just removed.  Either way, inform
+				 * the caller that it might be worth
+				 * trying again:
+				 */
+				ret = SCLD_VANISHED;
 			else
 				ret = SCLD_FAILED;
 		} else if (adjust_shared_perm(path)) {