bool rootdescend);
static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
BtreeLevel level);
+static void bt_recheck_sibling_links(BtreeCheckState *state,
+ BlockNumber btpo_prev_from_target,
+ BlockNumber leftcurrent);
static void bt_target_page_check(BtreeCheckState *state);
static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state);
static void bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
*/
}
- /*
- * readonly mode can only ever land on live pages and half-dead pages,
- * so sibling pointers should always be in mutual agreement
- */
- if (state->readonly && opaque->btpo_prev != leftcurrent)
- ereport(ERROR,
- (errcode(ERRCODE_INDEX_CORRUPTED),
- errmsg("left link/right link pair in index \"%s\" not in agreement",
- RelationGetRelationName(state->rel)),
- errdetail_internal("Block=%u left block=%u left link from block=%u.",
- current, leftcurrent, opaque->btpo_prev)));
+ /* Sibling links should be in mutual agreement */
+ if (opaque->btpo_prev != leftcurrent)
+ bt_recheck_sibling_links(state, opaque->btpo_prev, leftcurrent);
/* Check level, which must be valid for non-ignorable page */
if (level.level != opaque->btpo.level)
return nextleveldown;
}
+/*
+ * Raise an error when target page's left link does not point back to the
+ * previous target page, called leftcurrent here. The leftcurrent page's
+ * right link was followed to get to the current target page, and we expect
+ * mutual agreement among leftcurrent and the current target page. Make sure
+ * that this condition has definitely been violated in the !readonly case,
+ * where concurrent page splits are something that we need to deal with.
+ *
+ * Cross-page inconsistencies involving pages that don't agree about being
+ * siblings are known to be a particularly good indicator of corruption
+ * involving partial writes/lost updates. The bt_right_page_check_scankey
+ * check also provides a way of detecting cross-page inconsistencies for
+ * !readonly callers, but it can only detect sibling pages that have an
+ * out-of-order keyspace, which can't catch many of the problems that we
+ * expect to catch here.
+ *
+ * The classic example of the kind of inconsistency that we can only catch
+ * with this check (when in !readonly mode) involves three sibling pages that
+ * were affected by a faulty page split at some point in the past. The
+ * effects of the split are reflected in the original page and its new right
+ * sibling page, with a lack of any accompanying changes for the _original_
+ * right sibling page. The original right sibling page's left link fails to
+ * point to the new right sibling page (its left link still points to the
+ * original page), even though the first phase of a page split is supposed to
+ * work as a single atomic action. This subtle inconsistency will probably
+ * only break backwards scans in practice.
+ *
+ * Note that this is the only place where amcheck will "couple" buffer locks
+ * (and only for !readonly callers). In general we prefer to avoid more
+ * thorough cross-page checks in !readonly mode, but it seems worth the
+ * complexity here. Also, the performance overhead of performing lock
+ * coupling here is negligible in practice. Control only reaches here with a
+ * non-corrupt index when there is a concurrent page split at the instant
+ * caller crossed over to target page from leftcurrent page.
+ */
+static void
+bt_recheck_sibling_links(BtreeCheckState *state,
+ BlockNumber btpo_prev_from_target,
+ BlockNumber leftcurrent)
+{
+ if (!state->readonly)
+ {
+ Buffer lbuf;
+ Buffer newtargetbuf;
+ Page page;
+ BTPageOpaque opaque;
+ BlockNumber newtargetblock;
+
+ /* Couple locks in the usual order for nbtree: Left to right */
+ lbuf = ReadBufferExtended(state->rel, MAIN_FORKNUM, leftcurrent,
+ RBM_NORMAL, state->checkstrategy);
+ LockBuffer(lbuf, BT_READ);
+ _bt_checkpage(state->rel, lbuf);
+ page = BufferGetPage(lbuf);
+ opaque = (BTPageOpaque) PageGetSpecialPointer(page);
+ if (P_ISDELETED(opaque))
+ {
+ /*
+ * Cannot reason about concurrently deleted page -- the left link
+ * in the page to the right is expected to point to some other
+ * page to the left (not leftcurrent page).
+ *
+ * Note that we deliberately don't give up with a half-dead page.
+ */
+ UnlockReleaseBuffer(lbuf);
+ return;
+ }
+
+ newtargetblock = opaque->btpo_next;
+ /* Avoid self-deadlock when newtargetblock == leftcurrent */
+ if (newtargetblock != leftcurrent)
+ {
+ newtargetbuf = ReadBufferExtended(state->rel, MAIN_FORKNUM,
+ newtargetblock, RBM_NORMAL,
+ state->checkstrategy);
+ LockBuffer(newtargetbuf, BT_READ);
+ _bt_checkpage(state->rel, newtargetbuf);
+ page = BufferGetPage(newtargetbuf);
+ opaque = (BTPageOpaque) PageGetSpecialPointer(page);
+ /* btpo_prev_from_target may have changed; update it */
+ btpo_prev_from_target = opaque->btpo_prev;
+ }
+ else
+ {
+ /*
+ * leftcurrent right sibling points back to leftcurrent block.
+ * Index is corrupt. Easiest way to handle this is to pretend
+ * that we actually read from a distinct page that has an invalid
+ * block number in its btpo_prev.
+ */
+ newtargetbuf = InvalidBuffer;
+ btpo_prev_from_target = InvalidBlockNumber;
+ }
+
+ /*
+ * No need to check P_ISDELETED here, since new target block cannot be
+ * marked deleted as long as we hold a lock on lbuf
+ */
+ if (BufferIsValid(newtargetbuf))
+ UnlockReleaseBuffer(newtargetbuf);
+ UnlockReleaseBuffer(lbuf);
+
+ if (btpo_prev_from_target == leftcurrent)
+ {
+ /* Report split in left sibling, not target (or new target) */
+ ereport(DEBUG1,
+ (errcode(ERRCODE_INTERNAL_ERROR),
+ errmsg("harmless concurrent page split detected in index \"%s\"",
+ RelationGetRelationName(state->rel)),
+ errdetail_internal("Block=%u new right sibling=%u original right sibling=%u.",
+ leftcurrent, newtargetblock,
+ state->targetblock)));
+ return;
+ }
+
+ /*
+ * Index is corrupt. Make sure that we report correct target page.
+ *
+ * This could have changed in cases where there was a concurrent page
+ * split, as well as index corruption (at least in theory). Note that
+ * btpo_prev_from_target was already updated above.
+ */
+ state->targetblock = newtargetblock;
+ }
+
+ ereport(ERROR,
+ (errcode(ERRCODE_INDEX_CORRUPTED),
+ errmsg("left link/right link pair in index \"%s\" not in agreement",
+ RelationGetRelationName(state->rel)),
+ errdetail_internal("Block=%u left block=%u left link from block=%u.",
+ state->targetblock, leftcurrent,
+ btpo_prev_from_target)));
+}
+
/*
* Function performs the following checks on target page, or pages ancillary to
* target page:
* downlink, which was concurrently physically removed in target/parent as
* part of deletion's first phase.)
*
- * Note that while the cross-page-same-level last item check uses a trick
- * that allows it to perform verification for !readonly callers, a similar
- * trick seems difficult here. The trick that that other check uses is,
- * in essence, to lock down race conditions to those that occur due to
- * concurrent page deletion of the target; that's a race that can be
- * reliably detected before actually reporting corruption.
- *
- * On the other hand, we'd need to lock down race conditions involving
- * deletion of child's left page, for long enough to read the child page
- * into memory (in other words, a scheme with concurrently held buffer
- * locks on both child and left-of-child pages). That's unacceptable for
- * amcheck functions on general principle, though.
+ * While we use various techniques elsewhere to perform cross-page
+ * verification for !readonly callers, a similar trick seems difficult
+ * here. The tricks used by bt_recheck_sibling_links and by
+ * bt_right_page_check_scankey both involve verification of a same-level,
+ * cross-sibling invariant. Cross-level invariants are far more squishy,
+ * though. The nbtree REDO routines do not actually couple buffer locks
+ * across levels during page splits, so making any cross-level check work
+ * reliably in !readonly mode may be impossible.
*/
Assert(state->readonly);
* There is never an attempt to get a consistent view of multiple pages using
* multiple concurrent buffer locks; in general, we only acquire a single pin
* and buffer lock at a time, which is often all that the nbtree code requires.
+ * (Actually, bt_recheck_sibling_links couples buffer locks, which is the only
+ * exception to this general rule.)
*
* Operating on a copy of the page is useful because it prevents control
* getting stuck in an uninterruptible state when an underlying operator class
projects / users / andresfreund / postgres.git / commitdiff