[PATCH 7/8] reftable/stack: fix corruption on concurrent compaction

From

Patrick Steinhardt <ps@pks.im>

Date

Jul 31, 2024, 14:15 UTC

Message-ID

<b746565bf0ff80fff60821bdeb79867ad646c142.1722435214.git.ps@pks.im>

In-Reply-To

<cover.1722435214.git.ps@pks.im>

The locking employed by compaction uses the following schema:

  1. Lock "tables.list" and verify that it matches the version we have
     loaded in core.

  2. Lock each of the tables in the user-supplied range of tables that
     we are supposed to compact. These locks prohibit any concurrent
     process to compact those tables while we are doing that.

  3. Unlock "tables.list". This enables concurrent processes to add new
     tables to the stack, but also allows them to compact tables outside
     of the range of tables that we have locked.

  4. Perform the compaction.

  5. Lock "tables.list" again.

  6. Move the compacted table into place.

  7. Write the new order of tables, including the compacted table, into
     the lockfile.

  8. Commit the lockfile into place.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 reftable/stack.c | 101 ++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 96 insertions(+), 5 deletions(-)

diff --git a/reftable/stack.c b/reftable/stack.c
index 9cc91a262c..2b1ac58120 100644
--- a/reftable/stack.c
+++ b/reftable/stack.c
@@ -1021,7 +1021,9 @@ static int stack_compact_range(struct reftable_stack *st,
 	struct lock_file *table_locks = NULL;
 	struct tempfile *new_table = NULL;
 	int is_empty_table = 0, err = 0;
+	size_t first_to_replace, last_to_replace;
 	size_t i, nlocks = 0;
+	char **names = NULL;
 
 	if (first > last || (!expiry && first == last)) {
 		err = 0;
@@ -1124,6 +1126,94 @@ static int stack_compact_range(struct reftable_stack *st,
 		}
 	}
 
+	/*
+	 * As we have unlocked the stack while compacting our slice of tables
+	 * it may have happened that a concurrently running process has updated
+	 * the stack while we were compacting. In that case, we need to check
+	 * whether the tables that we have just compacted still exist in the
+	 * stack in the exact same order as we have compacted them.
+	 *
+	 * If they do exist, then it is fine to continue and replace those
+	 * tables with our compacted version. If they don't, then we need to
+	 * abort.
+	 */
+	err = stack_uptodate(st);
+	if (err < 0)
+		goto done;
+	if (err > 0) {
+		ssize_t new_offset = -1;
+		int fd;
+
+		fd = open(st->list_file, O_RDONLY);
+		if (fd < 0) {
+			err = REFTABLE_IO_ERROR;
+			goto done;
+		}
+
+		err = fd_read_lines(fd, &names);
+		close(fd);
+		if (err < 0)
+			goto done;
+
+		/*
+		 * Search for the offset of the first table that we have
+		 * compacted in the updated "tables.list" file.
+		 */
+		for (size_t i = 0; names[i]; i++) {
+			if (strcmp(names[i], st->readers[first]->name))
+				continue;
+
+			/*
+			 * We have found the first entry. Verify that all the
+			 * subsequent tables we have compacted still exist in
+			 * the modified stack in the exact same order as we
+			 * have compacted them.
+			 */
+			for (size_t j = 1; j < last - first + 1; j++) {
+				const char *old = first + j < st->merged->stack_len ?
+					st->readers[first + j]->name : NULL;
+				const char *new = names[i + j];
+
+				/*
+				 * If some entries are missing or in case the tables
+				 * have changed then we need to bail out. Again, this
+				 * shouldn't ever happen because we have locked the
+				 * tables we are compacting.
+				 */
+				if (!old || !new || strcmp(old, new)) {
+					err = REFTABLE_OUTDATED_ERROR;
+					goto done;
+				}
+			}
+
+			new_offset = i;
+			break;
+		}
+
+		/*
+		 * In case we didn't find our compacted tables in the stack we
+		 * need to bail out. In theory, this should have never happened
+		 * because we locked the tables we are compacting.
+		 */
+		if (new_offset < 0) {
+			err = REFTABLE_OUTDATED_ERROR;
+			goto done;
+		}
+
+		/*
+		 * We have found the new range that we want to replace, so
+		 * let's update the range of tables that we want to replace.
+		 */
+		last_to_replace = last + (new_offset - first);
+		first_to_replace = new_offset;
+	} else {
+		REFTABLE_CALLOC_ARRAY(names, st->merged->stack_len + 1);
+		for (size_t i = 0; i < st->merged->stack_len; i++)
+			names[i] = xstrdup(st->readers[i]->name);
+		last_to_replace = last;
+		first_to_replace = first;
+	}
+
 	/*
 	 * If the resulting compacted table is not empty, then we need to move
 	 * it into place now.
@@ -1146,12 +1236,12 @@ static int stack_compact_range(struct reftable_stack *st,
 	 * have just written. In case the compacted table became empty we
 	 * simply skip writing it.
 	 */
-	for (i = 0; i < first; i++)
-		strbuf_addf(&tables_list_buf, "%s\n", st->readers[i]->name);
+	for (i = 0; i < first_to_replace; i++)
+		strbuf_addf(&tables_list_buf, "%s\n", names[i]);
 	if (!is_empty_table)
 		strbuf_addf(&tables_list_buf, "%s\n", new_table_name.buf);
-	for (i = last + 1; i < st->merged->stack_len; i++)
-		strbuf_addf(&tables_list_buf, "%s\n", st->readers[i]->name);
+	for (i = last_to_replace + 1; names[i]; i++)
+		strbuf_addf(&tables_list_buf, "%s\n", names[i]);
 
 	err = write_in_full(get_lock_file_fd(&tables_list_lock),
 			    tables_list_buf.buf, tables_list_buf.len);
@@ -1204,9 +1294,10 @@ static int stack_compact_range(struct reftable_stack *st,
 	delete_tempfile(&new_table);
 	strbuf_release(&new_table_name);
 	strbuf_release(&new_table_path);
-
 	strbuf_release(&tables_list_buf);
 	strbuf_release(&table_name);
+	free_names(names);
+
 	return err;
 }
 

-- 
2.46.0.dirty