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[PATCH v3 17/25] lockfile: avoid transitory invalid states

From
Michael Haggerty <mhagger@alum.mit.edu>
Date
Apr 14, 2014, 13:54 UTC
Message-ID
<1397483695-10888-18-git-send-email-mhagger@alum.mit.edu>
In-Reply-To
<1397483695-10888-1-git-send-email-mhagger@alum.mit.edu>

Because remove_lock_file() can be called any time by the signal handler, it is important that any lock_file objects that are in the lock_file_list are always in a valid state. And since lock_file objects are often reused (but are never removed from lock_file_list), that means we have to be careful whenever mutating a lock_file object to always keep it in a well-defined state.

This was formerly not the case, because part of the state was recorded by whether lk->filename was the empty string or a valid filename. It is wrong to assume that this string can be updated atomically; for example, even

    strcpy(lk->filename, value)
is unsafe.  But the old code was even more reckless; for example,
    strcpy(lk->filename, path);
    if (!(flags & LOCK_NODEREF))
            resolve_symlink(lk->filename, max_path_len);
    strcat(lk->filename, ".lock");

During the call to resolve_symlink(), lk->filename contained the name of the file that was being locked, not the name of the lockfile. If a signal would have been raised during that interval, then the signal handler would have deleted the valuable file!

We could probably continue to use the filename field to encode the state by being careful to write characters 1..N-1 of the filename first, and then overwrite the NUL at filename[0] with the first character of the filename, but that would be awkward and error-prone.

So, instead of using the filename field to determine whether the lock_file object is active, add a new field "lock_file::active" for this purpose. Be careful to set this field only when filename really contains the name of a file that should be deleted on cleanup.

Helped-by: Johannes Sixt <j.sixt@viscovery.net>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
---
 cache.h    |  1 +
 lockfile.c | 45 +++++++++++++++++++++++++++++++++------------
 2 files changed, 34 insertions(+), 12 deletions(-)
diff --git a/cache.h b/cache.h
index 825cd0a..b7af173 100644
--- a/cache.h
+++ b/cache.h
@@ -539,6 +539,7 @@ extern int refresh_index(struct index_state *, unsigned int flags, const struct
 
 struct lock_file {
 	struct lock_file *next;
+	volatile sig_atomic_t active;
 	int fd;
 	pid_t owner;
 	char on_list;
diff --git a/lockfile.c b/lockfile.c
index 1453a7a..50a0541 100644
--- a/lockfile.c
+++ b/lockfile.c
@@ -27,11 +27,14 @@
  * Instead of (3), the change can be rolled back by deleting lockfile.
  *
  * This module keeps track of all locked files in lock_file_list.
- * When the first file is locked, it registers an atexit(3) handler;
- * when the program exits, the handler rolls back any files that have
- * been locked but were never committed or rolled back.
+ * When the first file is locked, it registers an atexit(3) handler
+ * and a signal handler; when the program exits, the handler rolls
+ * back any files that have been locked but were never committed or
+ * rolled back.
  *
- * A lock_file object can be in several states:
+ * Because the signal handler can be called at any time, a lock_file
+ * object must always be in a well-defined state.  The possible states
+ * are as follows:
  *
  * - Uninitialized.  In this state the object's on_list field must be
  *   zero but the rest of its contents need not be initialized.  As
@@ -39,18 +42,29 @@
  *   registered in the lock_file_list, and on_list is set.
  *
  * - Locked, lockfile open (after hold_lock_file_for_update() or
- *   hold_lock_file_for_append()).  In this state, the lockfile
- *   exists, filename holds the filename of the lockfile, fd holds a
- *   file descriptor open for writing to the lockfile, and owner holds
- *   the PID of the process that locked the file.
+ *   hold_lock_file_for_append()).  In this state:
+ *   - the lockfile exists
+ *   - active is set
+ *   - filename holds the filename of the lockfile
+ *   - fd holds a file descriptor open for writing to the lockfile
+ *   - owner holds the PID of the process that locked the file
  *
- * - Locked, lockfile closed (after close_lock_file()).  Same as the
- *   previous state, except that the lockfile is closed and fd is -1.
+ * - Locked, lockfile closed (after close_lock_file() or an
+ *   unsuccessful commit_lock_file()).  Same as the previous state,
+ *   except that the lockfile is closed and fd is -1.
  *
  * - Unlocked (after commit_lock_file(), rollback_lock_file(), or a
  *   failed attempt to lock).  In this state, filename[0] == '\0' and
  *   fd is -1.  The object is left registered in the lock_file_list,
  *   and on_list is set.
+ * - Unlocked (after rollback_lock_file(), a successful
+ *   commit_lock_file(), or a failed attempt to lock).  In this state:
+ *   - active is unset
+ *   - filename[0] == '\0' (usually, though there are transitory states
+ *     in which this condition doesn't hold)
+ *   - fd is -1
+ *   - the object is left registered in the lock_file_list, and
+ *     on_list is set.
  *
  * See Documentation/api-lockfile.txt for more information.
  */
@@ -183,9 +197,12 @@ static int lock_file(struct lock_file *lk, const char *path, int flags)
 		atexit(remove_lock_file);
 	}
 
+	assert(!lk->active);
+
 	if (!lk->on_list) {
 		/* Initialize *lk and add it to lock_file_list: */
 		lk->fd = -1;
+		lk->active = 0;
 		lk->owner = 0;
 		lk->on_list = 1;
 		lk->filename[0] = 0;
@@ -205,6 +222,7 @@ static int lock_file(struct lock_file *lk, const char *path, int flags)
 		return -1;
 	}
 	lk->owner = getpid();
+	lk->active = 1;
 	if (adjust_shared_perm(lk->filename)) {
 		error("cannot fix permission bits on %s", lk->filename);
 		rollback_lock_file(lk);
@@ -292,7 +310,7 @@ int commit_lock_file(struct lock_file *lk)
 	if (lk->fd >= 0 && close_lock_file(lk))
 		return -1;
 
-	if (!lk->filename[0])
+	if (!lk->active)
 		die("BUG: attempt to commit unlocked object");
 
 	strcpy(result_file, lk->filename);
@@ -301,6 +319,7 @@ int commit_lock_file(struct lock_file *lk)
 
 	if (rename(lk->filename, result_file))
 		return -1;
+	lk->active = 0;
 	lk->filename[0] = 0;
 	return 0;
 }
@@ -325,6 +344,7 @@ int commit_locked_index(struct lock_file *lk)
 			return -1;
 		if (rename(lk->filename, alternate_index_output))
 			return -1;
+		lk->active = 0;
 		lk->filename[0] = 0;
 		return 0;
 	}
@@ -334,10 +354,11 @@ int commit_locked_index(struct lock_file *lk)
 
 void rollback_lock_file(struct lock_file *lk)
 {
-	if (lk->filename[0]) {
+	if (lk->active) {
 		if (lk->fd >= 0)
 			close_lock_file(lk);
 		unlink_or_warn(lk->filename);
+		lk->active = 0;
 		lk->filename[0] = 0;
 	}
 }

-- 
1.9.1
Previous: Michael HaggertyNext: Michael Haggerty
Message 20 of 32 in “Lockfile correctness and refactoring”
  1. 00/25 Lockfile correctness and refactoringMichael Haggerty, Apr 14, 2014
  2. 01/25 unable_to_lock_die(): rename function from unable_to_lock_index_die()Michael Haggerty, Apr 14, 2014
  3. 02/25 api-lockfile: expand the documentationMichael Haggerty, Apr 14, 2014
  4. 03/25 rollback_lock_file(): do not clear filename redundantlyMichael Haggerty, Apr 14, 2014
  5. 04/25 rollback_lock_file(): set fd to -1Michael Haggerty, Apr 14, 2014
  6. 05/25 lockfile: unlock file if lockfile permissions cannot be adjustedMichael Haggerty, Apr 14, 2014
  7. 06/25 hold_lock_file_for_append(): release lock on errorsMichael Haggerty, Apr 14, 2014
  8. 07/25 lock_file(): always add lock_file object to lock_file_listMichael Haggerty, Apr 14, 2014
  9. 08/25 lockfile.c: document the various states of lock_file objectsMichael Haggerty, Apr 14, 2014
  10. 09/25 cache.h: define constants LOCK_SUFFIX and LOCK_SUFFIX_LENMichael Haggerty, Apr 14, 2014
  11. 10/25 delete_ref_loose(): don't muck around in the lock_file's filenameMichael Haggerty, Apr 14, 2014
  12. 11/25 prepare_index(): declare return value to be (const char *)Michael Haggerty, Apr 14, 2014
  13. 12/25 write_packed_entry_fn(): convert cb_data into a (const int *)Michael Haggerty, Apr 14, 2014
  14. 13/25 lock_file(): exit early if lockfile cannot be openedMichael Haggerty, Apr 14, 2014
  15. 14/25 remove_lock_file(): call rollback_lock_file()Michael Haggerty, Apr 14, 2014
  16. 15/25 commit_lock_file(): inline temporary variableMichael Haggerty, Apr 14, 2014
  17. 16/25 commit_lock_file(): die() if called for unlocked lockfile objectMichael Haggerty, Apr 14, 2014
  18. Johannes SixtApr 15, 2014
  19. Michael HaggertyApr 16, 2014
  20. 17/25 lockfile: avoid transitory invalid statesMichael Haggerty, Apr 14, 2014
  21. 18/25 struct lock_file: declare some fields volatileMichael Haggerty, Apr 14, 2014
  22. Johannes SixtApr 15, 2014
  23. Michael HaggertyApr 16, 2014
  24. 19/25 try_merge_strategy(): remove redundant lock_file allocationMichael Haggerty, Apr 14, 2014
  25. 20/25 try_merge_strategy(): use a statically-allocated lock_file objectMichael Haggerty, Apr 14, 2014
  26. 21/25 commit_lock_file(): use a strbuf to manage temporary spaceMichael Haggerty, Apr 14, 2014
  27. 22/25 Change lock_file::filename into a strbufMichael Haggerty, Apr 14, 2014
  28. 23/25 resolve_symlink(): use a strbuf for internal scratch spaceMichael Haggerty, Apr 14, 2014
  29. 24/25 resolve_symlink(): take a strbuf parameterMichael Haggerty, Apr 14, 2014
  30. 25/25 trim_last_path_elm(): replace last_path_elm()Michael Haggerty, Apr 14, 2014
  31. Torsten BögershausenApr 15, 2014
  32. Michael HaggertyApr 16, 2014

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