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Commit5940ffb

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Avoid update conflict out serialization anomalies.
SSI's HeapCheckForSerializableConflictOut() test failed to correctlyhandle conditions involving a concurrently inserted tuple which is laterconcurrently updated by a separate transaction . A SELECT statementthat called HeapCheckForSerializableConflictOut() could end up using thesame XID (updater's XID) for both the original tuple, and the successortuple, missing the XID of the xact that created the original tupleentirely. This only happened when neither tuple from the chain wasvisible to the transaction's MVCC snapshot.The observable symptoms of this bug were subtle. A pair of transactionscould commit, with the later transaction failing to observe the effectsof the earlier transaction (because of the confusion created by theupdate to the non-visible row). This bug dates all the way back tocommitdafaa3e, which added SSI.To fix, make sure that we check the xmin of concurrently inserted tuplesthat happen to also have been updated concurrently.Author: Peter GeogheganReported-By: Kyle KingsburyReviewed-By: Thomas MunroDiscussion:https://postgr.es/m/db7b729d-0226-d162-a126-8a8ab2dc4443@jepsen.ioBackpatch: All supported versions
1 parentd9fa17a commit5940ffb

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4 files changed

+98
-5
lines changed

4 files changed

+98
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lines changed

‎src/backend/access/heap/heapam.c

Lines changed: 16 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -9007,6 +9007,10 @@ HeapCheckForSerializableConflictOut(bool visible, Relation relation,
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* while it is visible to us. The "visible" bool indicates whether the
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* tuple is visible to us, while HeapTupleSatisfiesVacuum checks what else
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* is going on with it.
9010+
*
9011+
* In the event of a concurrently inserted tuple that also happens to have
9012+
* been concurrently updated (by a separate transaction), the xmin of the
9013+
* tuple will be used -- not the updater's xid.
90109014
*/
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htsvResult=HeapTupleSatisfiesVacuum(tuple,TransactionXmin,buffer);
90129016
switch (htsvResult)
@@ -9017,17 +9021,24 @@ HeapCheckForSerializableConflictOut(bool visible, Relation relation,
90179021
xid=HeapTupleHeaderGetXmin(tuple->t_data);
90189022
break;
90199023
caseHEAPTUPLE_RECENTLY_DEAD:
9020-
if (!visible)
9021-
return;
9022-
xid=HeapTupleHeaderGetUpdateXid(tuple->t_data);
9023-
break;
90249024
caseHEAPTUPLE_DELETE_IN_PROGRESS:
9025-
xid=HeapTupleHeaderGetUpdateXid(tuple->t_data);
9025+
if (visible)
9026+
xid=HeapTupleHeaderGetUpdateXid(tuple->t_data);
9027+
else
9028+
xid=HeapTupleHeaderGetXmin(tuple->t_data);
9029+
9030+
if (TransactionIdPrecedes(xid,TransactionXmin))
9031+
{
9032+
/* This is like the HEAPTUPLE_DEAD case */
9033+
Assert(!visible);
9034+
return;
9035+
}
90269036
break;
90279037
caseHEAPTUPLE_INSERT_IN_PROGRESS:
90289038
xid=HeapTupleHeaderGetXmin(tuple->t_data);
90299039
break;
90309040
caseHEAPTUPLE_DEAD:
9041+
Assert(!visible);
90319042
return;
90329043
default:
90339044

Lines changed: 27 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,27 @@
1+
Parsed test spec with 3 sessions
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starting permutation: foo_select bar_insert foo_insert foo_commit trouble_update bar_select bar_commit trouble_abort
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step foo_select: SELECT * FROM txn0 WHERE id = 42;
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id val
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step bar_insert: INSERT INTO txn0 SELECT 42, 'bar_insert';
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step foo_insert: INSERT INTO txn1 SELECT 7, 'foo_insert';
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step foo_commit: COMMIT;
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step trouble_update: UPDATE txn1 SET val = 'add physical version for "bar_select"' WHERE id = 7;
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step bar_select: SELECT * FROM txn1 WHERE id = 7;
12+
ERROR: could not serialize access due to read/write dependencies among transactions
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step bar_commit: COMMIT;
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step trouble_abort: ABORT;
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starting permutation: foo_select bar_insert foo_insert foo_commit trouble_delete bar_select bar_commit trouble_abort
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step foo_select: SELECT * FROM txn0 WHERE id = 42;
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id val
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step bar_insert: INSERT INTO txn0 SELECT 42, 'bar_insert';
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step foo_insert: INSERT INTO txn1 SELECT 7, 'foo_insert';
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step foo_commit: COMMIT;
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step trouble_delete: DELETE FROM txn1 WHERE id = 7;
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step bar_select: SELECT * FROM txn1 WHERE id = 7;
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ERROR: could not serialize access due to read/write dependencies among transactions
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step bar_commit: COMMIT;
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step trouble_abort: ABORT;

‎src/test/isolation/isolation_schedule

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -18,6 +18,7 @@ test: two-ids
1818
test: multiple-row-versions
1919
test: index-only-scan
2020
test: predicate-lock-hot-tuple
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test: update-conflict-out
2122
test: deadlock-simple
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test: deadlock-hard
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test: deadlock-soft
Lines changed: 54 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,54 @@
1+
# Test for interactions between SSI's "conflict out" handling for heapam and
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# concurrently updated tuple
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#
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# See bug report:
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# https://postgr.es/m/db7b729d-0226-d162-a126-8a8ab2dc4443%40jepsen.io
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setup
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{
9+
CREATETABLEtxn0(idint4PRIMARYKEY,valTEXT);
10+
CREATETABLEtxn1(idint4PRIMARYKEY,valTEXT);
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}
12+
13+
teardown
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{
15+
DROPTABLEtxn0;
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DROPTABLEtxn1;
17+
}
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session"foo"
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setup {BEGINTRANSACTIONISOLATIONLEVELSERIALIZABLE; }
21+
step"foo_select" {SELECT*FROMtxn0WHEREid=42; }
22+
step"foo_insert" {INSERTINTOtxn1SELECT7,'foo_insert'; }
23+
step"foo_commit" {COMMIT; }
24+
25+
session"bar"
26+
setup {BEGINTRANSACTIONISOLATIONLEVELSERIALIZABLE; }
27+
step"bar_select" {SELECT*FROMtxn1WHEREid=7; }
28+
step"bar_insert" {INSERTINTOtxn0SELECT42,'bar_insert'; }
29+
step"bar_commit" {COMMIT; }
30+
31+
# This session creates the conditions that confused bar's "conflict out"
32+
# handling in old releases affected by bug:
33+
session"trouble"
34+
setup {BEGINTRANSACTIONISOLATIONLEVELSERIALIZABLE; }
35+
step"trouble_update" {UPDATEtxn1SETval='add physical version for "bar_select"'WHEREid=7; }
36+
step"trouble_delete" {DELETEFROMtxn1WHEREid=7; }
37+
step"trouble_abort" {ABORT; }
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39+
permutation"foo_select"
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"bar_insert"
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"foo_insert""foo_commit"
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"trouble_update"# Updates tuple...
43+
"bar_select"# Should observe one distinct XID per version
44+
"bar_commit"# "bar" should fail here at the latest
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"trouble_abort"
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47+
# Same as above, but "trouble" session DELETEs this time around
48+
permutation"foo_select"
49+
"bar_insert"
50+
"foo_insert""foo_commit"
51+
"trouble_delete"# Deletes tuple...
52+
"bar_select"# Should observe foo's XID
53+
"bar_commit"# "bar" should fail here at the latest
54+
"trouble_abort"

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