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amcheck: Distinguish interrupted page deletion from corruption.

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This prevents false-positive reports about "the first child of leftmost
target page is not leftmost of its level", "block %u is not leftmost"
and "left link/right link pair".  They appeared if amcheck ran before
VACUUM cleaned things, after a cluster exited recovery between the
first-stage and second-stage WAL records of a deletion.  Back-patch to
v11 (all supported versions).

Reviewed by Peter Geoghegan.

Discussion: https://postgr.es/m/20231005025232.c7.nmisch@google.com
This commit is contained in:
Noah Misch 2023-10-30 14:46:05 -07:00
parent c804ffb562
commit e04509f324
1 changed files with 72 additions and 4 deletions
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76
contrib/amcheck/verify_nbtree.c
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@ -127,6 +127,9 @@ static void bt_check_every_level(Relation rel, Relation heaprel,
bool readonly, bool heapallindexed); bool readonly, bool heapallindexed);
static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state, static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
BtreeLevel level); BtreeLevel level);
static bool bt_leftmost_ignoring_half_dead(BtreeCheckState *state,
BlockNumber start,
BTPageOpaque start_opaque);
static void bt_target_page_check(BtreeCheckState *state); static void bt_target_page_check(BtreeCheckState *state);
static ScanKey bt_right_page_check_scankey(BtreeCheckState *state); static ScanKey bt_right_page_check_scankey(BtreeCheckState *state);
static void bt_downlink_check(BtreeCheckState *state, BlockNumber childblock, static void bt_downlink_check(BtreeCheckState *state, BlockNumber childblock,
@ -716,7 +719,7 @@ bt_check_level_from_leftmost(BtreeCheckState *state, BtreeLevel level)
*/ */
if (state->readonly) if (state->readonly)
{ {
if (!P_LEFTMOST(opaque)) if (!bt_leftmost_ignoring_half_dead(state, current, opaque))
ereport(ERROR, ereport(ERROR,
(errcode(ERRCODE_INDEX_CORRUPTED), (errcode(ERRCODE_INDEX_CORRUPTED),
errmsg("block %u is not leftmost in index \"%s\"", errmsg("block %u is not leftmost in index \"%s\"",
@ -769,10 +772,14 @@ bt_check_level_from_leftmost(BtreeCheckState *state, BtreeLevel level)
} }
/* /*
* readonly mode can only ever land on live pages and half-dead pages, * Sibling links should be in mutual agreement. There arises
* so sibling pointers should always be in mutual agreement * leftcurrent == P_NONE && btpo_prev != P_NONE when the left sibling
* of the parent's low-key downlink is half-dead. (A half-dead page
* has no downlink from its parent.) Under heavyweight locking, the
* last bt_leftmost_ignoring_half_dead() validated this btpo_prev.
*/ */
if (state->readonly && opaque->btpo_prev != leftcurrent) if (state->readonly &&
opaque->btpo_prev != leftcurrent && leftcurrent != P_NONE)
ereport(ERROR, ereport(ERROR,
(errcode(ERRCODE_INDEX_CORRUPTED), (errcode(ERRCODE_INDEX_CORRUPTED),
errmsg("left link/right link pair in index \"%s\" not in agreement", errmsg("left link/right link pair in index \"%s\" not in agreement",
@ -822,6 +829,67 @@ nextpage:
return nextleveldown; return nextleveldown;
} }
/*
* Like P_LEFTMOST(start_opaque), but accept an arbitrarily-long chain of
* half-dead, sibling-linked pages to the left. If a half-dead page appears
* under state->readonly, the database exited recovery between the first-stage
* and second-stage WAL records of a deletion.
*/
static bool
bt_leftmost_ignoring_half_dead(BtreeCheckState *state,
BlockNumber start,
BTPageOpaque start_opaque)
{
BlockNumber reached = start_opaque->btpo_prev,
reached_from = start;
bool all_half_dead = true;
/*
* To handle the !readonly case, we'd need to accept BTP_DELETED pages and
* potentially observe nbtree/README "Page deletion and backwards scans".
*/
Assert(state->readonly);
while (reached != P_NONE && all_half_dead)
{
Page page = palloc_btree_page(state, reached);
BTPageOpaque reached_opaque = (BTPageOpaque) PageGetSpecialPointer(page);
CHECK_FOR_INTERRUPTS();
/*
* Try to detect btpo_prev circular links. _bt_unlink_halfdead_page()
* writes that side-links will continue to point to the siblings.
* Check btpo_next for that property.
*/
all_half_dead = P_ISHALFDEAD(reached_opaque) &&
reached != start &&
reached != reached_from &&
reached_opaque->btpo_next == reached_from;
if (all_half_dead)
{
XLogRecPtr pagelsn = PageGetLSN(page);
/* pagelsn should point to an XLOG_BTREE_MARK_PAGE_HALFDEAD */
ereport(DEBUG1,
(errcode(ERRCODE_NO_DATA),
errmsg_internal("harmless interrupted page deletion detected in index \"%s\"",
RelationGetRelationName(state->rel)),
errdetail_internal("Block=%u right block=%u page lsn=%X/%X.",
reached, reached_from,
(uint32) (pagelsn >> 32),
(uint32) pagelsn)));
reached_from = reached;
reached = reached_opaque->btpo_prev;
}
pfree(page);
}
return all_half_dead;
}
/* /*
* Function performs the following checks on target page, or pages ancillary to * Function performs the following checks on target page, or pages ancillary to
* target page: * target page: