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Commitc3656c9

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Fix locking a tuple updated by an aborted (sub)transaction
When heap_lock_tuple decides to follow the update chain, it tried toalso lock any version of the tuple that was created by an update thatwas subsequently rolled back. This is pointless, since for all intentsand purposes that tuple exists no more; and moreover it causesmisbehavior, as reported independently by Marko Tiikkaja and MartiRaudsepp: some SELECT FOR UPDATE/SHARE queries may fail to returnthe tuples, and assertion-enabled builds crash.Fix by having heap_lock_updated_tuple test the xmin and return successimmediately if the tuple was created by an aborted transaction.The condition where tuples become invisible occurs when an updated tuplechain is followed by heap_lock_updated_tuple, which reports the problemas HeapTupleSelfUpdated to its caller heap_lock_tuple, which in turnpropagates that code outwards possibly leading the calling code(ExecLockRows) to believe that the tuple exists no longer.Backpatch to 9.3. Only on 9.5 and newer this leads to a visiblefailure, because of commit27846f0; before that, heap_lock_tupleskips the whole dance when the tuple is already locked by the sametransaction, because of the ancient HeapTupleSatisfiesUpdate behavior.Still, the buggy condition may also exist in more convoluted scenariosinvolving concurrent transactions, so it seems safer to fix the bug inthe old branches too.Discussion:https://www.postgresql.org/message-id/CABRT9RC81YUf1=jsmWopcKJEro=VoeG2ou6sPwyOUTx_qteRsg@mail.gmail.comhttps://www.postgresql.org/message-id/48d3eade-98d3-8b9a-477e-1a8dc32a724d@joh.to
1 parent1fef3d5 commitc3656c9

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‎src/backend/access/heap/heapam.c

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@@ -5722,6 +5722,17 @@ heap_lock_updated_tuple_rec(Relation rel, ItemPointer tid, TransactionId xid,
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gotoout_locked;
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}
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/*
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* Also check Xmin: if this tuple was created by an aborted
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* (sub)transaction, then we already locked the last live one in the
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* chain, thus we're done, so return success.
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*/
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if (TransactionIdDidAbort(HeapTupleHeaderGetXmin(mytup.t_data)))
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{
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UnlockReleaseBuffer(buf);
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returnHeapTupleMayBeUpdated;
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}
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old_infomask=mytup.t_data->t_infomask;
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old_infomask2=mytup.t_data->t_infomask2;
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xmax=HeapTupleHeaderGetRawXmax(mytup.t_data);

‎src/test/regress/expected/combocid.out

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@@ -140,3 +140,30 @@ SELECT ctid,cmin,* FROM combocidtest;
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(0,6) | 0 | 444
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(3 rows)
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-- test for bug reported in
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-- CABRT9RC81YUf1=jsmWopcKJEro=VoeG2ou6sPwyOUTx_qteRsg@mail.gmail.com
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CREATE TABLE IF NOT EXISTS testcase(
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id int PRIMARY KEY,
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balance numeric
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);
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INSERT INTO testcase VALUES (1, 0);
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BEGIN;
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SELECT * FROM testcase WHERE testcase.id = 1 FOR UPDATE;
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id | balance
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----+---------
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1 | 0
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(1 row)
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UPDATE testcase SET balance = balance + 400 WHERE id=1;
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SAVEPOINT subxact;
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UPDATE testcase SET balance = balance - 100 WHERE id=1;
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ROLLBACK TO SAVEPOINT subxact;
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-- should return one tuple
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SELECT * FROM testcase WHERE id = 1 FOR UPDATE;
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id | balance
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----+---------
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1 | 400
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(1 row)
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ROLLBACK;
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DROP TABLE testcase;

‎src/test/regress/sql/combocid.sql

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@@ -91,3 +91,21 @@ SELECT ctid,cmin,* FROM combocidtest;
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COMMIT;
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SELECT ctid,cmin,*FROM combocidtest;
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-- test for bug reported in
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-- CABRT9RC81YUf1=jsmWopcKJEro=VoeG2ou6sPwyOUTx_qteRsg@mail.gmail.com
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CREATETABLEIF NOT EXISTS testcase(
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idintPRIMARY KEY,
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balancenumeric
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);
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INSERT INTO testcaseVALUES (1,0);
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BEGIN;
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SELECT*FROM testcaseWHEREtestcase.id=1 FORUPDATE;
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UPDATE testcaseSET balance= balance+400WHERE id=1;
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SAVEPOINT subxact;
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UPDATE testcaseSET balance= balance-100WHERE id=1;
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ROLLBACK TO SAVEPOINT subxact;
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-- should return one tuple
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SELECT*FROM testcaseWHERE id=1 FORUPDATE;
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ROLLBACK;
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DROPTABLE testcase;

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