amcheck: editorialize variable name & comment.
No exclusive lock is taken anymore...
This commit is contained in:
Andres Freund
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56018bf26e
commit
fcd8d25d38
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@ -55,8 +55,8 @@ typedef struct BtreeCheckState
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/* B-Tree Index Relation */
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Relation rel;
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/* ExclusiveLock held? */
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bool exclusivelylocked;
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/* ShareLock held on heap/index, rather than AccessShareLock? */
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bool readonly;
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/* Per-page context */
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MemoryContext targetcontext;
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/* Buffer access strategy */
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@ -94,7 +94,7 @@ PG_FUNCTION_INFO_V1(bt_index_parent_check);
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static void bt_index_check_internal(Oid indrelid, bool parentcheck);
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static inline void btree_index_checkable(Relation rel);
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static void bt_check_every_level(Relation rel, bool exclusivelylocked);
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static void bt_check_every_level(Relation rel, bool readonly);
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static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
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BtreeLevel level);
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static void bt_target_page_check(BtreeCheckState *state);
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@ -256,23 +256,23 @@ btree_index_checkable(Relation rel)
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* logical order, verifying invariants as it goes.
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*
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* It is the caller's responsibility to acquire appropriate heavyweight lock on
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* the index relation, and advise us if extra checks are safe when an
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* ExclusiveLock is held.
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* the index relation, and advise us if extra checks are safe when a ShareLock
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* is held.
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*
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* An ExclusiveLock is generally assumed to prevent any kind of physical
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* A ShareLock is generally assumed to prevent any kind of physical
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* modification to the index structure, including modifications that VACUUM may
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* make. This does not include setting of the LP_DEAD bit by concurrent index
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* scans, although that is just metadata that is not able to directly affect
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* any check performed here. Any concurrent process that might act on the
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* LP_DEAD bit being set (recycle space) requires a heavyweight lock that
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* cannot be held while we hold an ExclusiveLock. (Besides, even if that could
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* cannot be held while we hold a ShareLock. (Besides, even if that could
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* happen, the ad-hoc recycling when a page might otherwise split is performed
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* per-page, and requires an exclusive buffer lock, which wouldn't cause us
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* trouble. _bt_delitems_vacuum() may only delete leaf items, and so the extra
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* parent/child check cannot be affected.)
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*/
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static void
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bt_check_every_level(Relation rel, bool exclusivelylocked)
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bt_check_every_level(Relation rel, bool readonly)
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{
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BtreeCheckState *state;
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Page metapage;
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@ -291,7 +291,7 @@ bt_check_every_level(Relation rel, bool exclusivelylocked)
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*/
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state = palloc(sizeof(BtreeCheckState));
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state->rel = rel;
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state->exclusivelylocked = exclusivelylocked;
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state->readonly = readonly;
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/* Create context for page */
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state->targetcontext = AllocSetContextCreate(CurrentMemoryContext,
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"amcheck context",
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@ -353,7 +353,7 @@ bt_check_every_level(Relation rel, bool exclusivelylocked)
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/*
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* Given a left-most block at some level, move right, verifying each page
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* individually (with more verification across pages for "exclusivelylocked"
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* individually (with more verification across pages for "readonly"
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* callers). Caller should pass the true root page as the leftmost initially,
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* working their way down by passing what is returned for the last call here
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* until level 0 (leaf page level) was reached.
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@ -428,7 +428,7 @@ bt_check_level_from_leftmost(BtreeCheckState *state, BtreeLevel level)
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* locking, check that the first valid page meets caller's
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* expectations.
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*/
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if (state->exclusivelylocked)
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if (state->readonly)
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{
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if (!P_LEFTMOST(opaque))
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ereport(ERROR,
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@ -482,7 +482,7 @@ bt_check_level_from_leftmost(BtreeCheckState *state, BtreeLevel level)
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*/
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}
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if (state->exclusivelylocked && opaque->btpo_prev != leftcurrent)
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if (state->readonly && opaque->btpo_prev != leftcurrent)
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ereport(ERROR,
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(errcode(ERRCODE_INDEX_CORRUPTED),
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errmsg("left link/right link pair in index \"%s\" not in agreement",
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@ -541,7 +541,7 @@ nextpage:
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* "real" data item on the page to the right (if such a first item is
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* available).
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*
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* Furthermore, when state passed shows ExclusiveLock held, and target page is
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* Furthermore, when state passed shows ShareLock held, and target page is
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* internal page, function also checks:
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*
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* - That all child pages respect downlinks lower bound.
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@ -597,8 +597,8 @@ bt_target_page_check(BtreeCheckState *state)
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* page items.
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*
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* We prefer to check all items against high key rather than checking
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* just the first and trusting that the operator class obeys the
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* transitive law (which implies that all subsequent items also
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* just the last and trusting that the operator class obeys the
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* transitive law (which implies that all previous items also
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* respected the high key invariant if they pass the item order
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* check).
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*
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@ -705,12 +705,12 @@ bt_target_page_check(BtreeCheckState *state)
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{
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/*
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* As explained at length in bt_right_page_check_scankey(),
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* there is a known !exclusivelylocked race that could account
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* for apparent violation of invariant, which we must check
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* for before actually proceeding with raising error. Our
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* canary condition is that target page was deleted.
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* there is a known !readonly race that could account for
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* apparent violation of invariant, which we must check for
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* before actually proceeding with raising error. Our canary
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* condition is that target page was deleted.
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*/
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if (!state->exclusivelylocked)
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if (!state->readonly)
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{
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/* Get fresh copy of target page */
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state->target = palloc_btree_page(state, state->targetblock);
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@ -718,8 +718,7 @@ bt_target_page_check(BtreeCheckState *state)
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topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
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/*
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* All !exclusivelylocked checks now performed; just
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* return
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* All !readonly checks now performed; just return
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*/
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if (P_IGNORE(topaque))
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return;
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@ -740,11 +739,11 @@ bt_target_page_check(BtreeCheckState *state)
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* * Downlink check *
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*
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* Additional check of child items iff this is an internal page and
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* caller holds an ExclusiveLock. This happens for every downlink
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* (item) in target excluding the negative-infinity downlink (again,
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* this is because it has no useful value to compare).
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* caller holds a ShareLock. This happens for every downlink (item)
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* in target excluding the negative-infinity downlink (again, this is
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* because it has no useful value to compare).
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*/
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if (!P_ISLEAF(topaque) && state->exclusivelylocked)
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if (!P_ISLEAF(topaque) && state->readonly)
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{
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BlockNumber childblock = ItemPointerGetBlockNumber(&(itup->t_tid));
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@ -766,7 +765,7 @@ bt_target_page_check(BtreeCheckState *state)
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* with different parent page). If no such valid item is available, return
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* NULL instead.
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*
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* Note that !exclusivelylocked callers must reverify that target page has not
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* Note that !readonly callers must reverify that target page has not
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* been concurrently deleted.
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*/
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static ScanKey
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@ -833,14 +832,14 @@ bt_right_page_check_scankey(BtreeCheckState *state)
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}
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/*
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* No ExclusiveLock held case -- why it's safe to proceed.
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* No ShareLock held case -- why it's safe to proceed.
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*
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* Problem:
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*
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* We must avoid false positive reports of corruption when caller treats
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* item returned here as an upper bound on target's last item. In
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* general, false positives are disallowed. Avoiding them here when
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* caller is !exclusivelylocked is subtle.
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* caller is !readonly is subtle.
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*
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* A concurrent page deletion by VACUUM of the target page can result in
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* the insertion of items on to this right sibling page that would
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@ -892,7 +891,7 @@ bt_right_page_check_scankey(BtreeCheckState *state)
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*
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* Top level tree walk caller moves on to next page (makes it the new
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* target) following recovery from this race. (cf. The rationale for
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* child/downlink verification needing an ExclusiveLock within
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* child/downlink verification needing a ShareLock within
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* bt_downlink_check(), where page deletion is also the main source of
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* trouble.)
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*
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@ -984,7 +983,7 @@ bt_downlink_check(BtreeCheckState *state, BlockNumber childblock,
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BTPageOpaque copaque;
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/*
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* Caller must have ExclusiveLock on target relation, because of
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* Caller must have ShareLock on target relation, because of
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* considerations around page deletion by VACUUM.
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*
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* NB: In general, page deletion deletes the right sibling's downlink, not
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@ -994,8 +993,8 @@ bt_downlink_check(BtreeCheckState *state, BlockNumber childblock,
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* page's rightmost child unless it is the last child page, and we intend
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* to also delete the parent itself.)
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*
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* If this verification happened without an ExclusiveLock, the following
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* race condition could cause false positives:
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* If this verification happened without a ShareLock, the following race
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* condition could cause false positives:
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*
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* In general, concurrent page deletion might occur, including deletion of
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* the left sibling of the child page that is examined here. If such a
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@ -1009,11 +1008,11 @@ bt_downlink_check(BtreeCheckState *state, BlockNumber childblock,
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* part of deletion's first phase.)
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*
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* Note that while the cross-page-same-level last item check uses a trick
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* that allows it to perform verification for !exclusivelylocked callers,
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* a similar trick seems difficult here. The trick that that other check
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* uses is, in essence, to lock down race conditions to those that occur
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* due to concurrent page deletion of the target; that's a race that can
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* be reliably detected before actually reporting corruption.
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* that allows it to perform verification for !readonly callers, a similar
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* trick seems difficult here. The trick that that other check uses is,
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* in essence, to lock down race conditions to those that occur due to
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* concurrent page deletion of the target; that's a race that can be
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* reliably detected before actually reporting corruption.
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*
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* On the other hand, we'd need to lock down race conditions involving
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* deletion of child's left page, for long enough to read the child page
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@ -1021,7 +1020,7 @@ bt_downlink_check(BtreeCheckState *state, BlockNumber childblock,
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* locks on both child and left-of-child pages). That's unacceptable for
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* amcheck functions on general principle, though.
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*/
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Assert(state->exclusivelylocked);
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Assert(state->readonly);
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/*
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* Verify child page has the downlink key from target page (its parent) as
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