From 0789b82a97924cec14f651f678eebc56c09da17f Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Thu, 29 Jun 2023 10:19:10 -0400 Subject: [PATCH] Fix order of operations in ExecEvalFieldStoreDeForm(). If the given composite datum is toasted out-of-line, DatumGetHeapTupleHeader will perform database accesses to detoast it. That can invalidate the result of get_cached_rowtype, as documented (perhaps not plainly enough) in that function's API spec; which leads to strange errors or crashes when we try to use the TupleDesc to read the tuple. In short then, trying to update a field of a composite column could fail intermittently if the overall column value is wide enough to require toasting. We can fix the bug at no cost by just changing the order of operations, since we don't need the TupleDesc until after detoasting. (Other callers of get_cached_rowtype appear to get this right already, so there's only one bug.) Note that the added regression test case reveals this bug reliably only with debug_discard_caches/CLOBBER_CACHE_ALWAYS. Per bug #17994 from Alexander Lakhin. Sadly, this patch does not fix the missing-values issue revealed in the bug discussion; we'll need some more work to cover that. Discussion: https://postgr.es/m/17994-5c7100b51b4790e9@postgresql.org --- src/backend/executor/execExprInterp.c | 29 +++++++++++++++----------- src/test/regress/expected/rowtypes.out | 9 ++++++++ src/test/regress/sql/rowtypes.sql | 5 +++++ 3 files changed, 31 insertions(+), 12 deletions(-) diff --git a/src/backend/executor/execExprInterp.c b/src/backend/executor/execExprInterp.c index bdf59a10fcf..4b2c48ba602 100644 --- a/src/backend/executor/execExprInterp.c +++ b/src/backend/executor/execExprInterp.c @@ -1969,7 +1969,8 @@ CheckOpSlotCompatibility(ExprEvalStep *op, TupleTableSlot *slot) * changed: if not NULL, *changed is set to true on any update * * The returned TupleDesc is not guaranteed pinned; caller must pin it - * to use it across any operation that might incur cache invalidation. + * to use it across any operation that might incur cache invalidation, + * including for example detoasting of input tuples. * (The TupleDesc is always refcounted, so just use IncrTupleDescRefCount.) * * NOTE: because composite types can change contents, we must be prepared @@ -3128,17 +3129,6 @@ ExecEvalFieldSelect(ExprState *state, ExprEvalStep *op, ExprContext *econtext) void ExecEvalFieldStoreDeForm(ExprState *state, ExprEvalStep *op, ExprContext *econtext) { - TupleDesc tupDesc; - - /* Lookup tupdesc if first time through or if type changes */ - tupDesc = get_cached_rowtype(op->d.fieldstore.fstore->resulttype, -1, - op->d.fieldstore.rowcache, NULL); - - /* Check that current tupdesc doesn't have more fields than we allocated */ - if (unlikely(tupDesc->natts > op->d.fieldstore.ncolumns)) - elog(ERROR, "too many columns in composite type %u", - op->d.fieldstore.fstore->resulttype); - if (*op->resnull) { /* Convert null input tuple into an all-nulls row */ @@ -3154,6 +3144,7 @@ ExecEvalFieldStoreDeForm(ExprState *state, ExprEvalStep *op, ExprContext *econte Datum tupDatum = *op->resvalue; HeapTupleHeader tuphdr; HeapTupleData tmptup; + TupleDesc tupDesc; tuphdr = DatumGetHeapTupleHeader(tupDatum); tmptup.t_len = HeapTupleHeaderGetDatumLength(tuphdr); @@ -3161,6 +3152,20 @@ ExecEvalFieldStoreDeForm(ExprState *state, ExprEvalStep *op, ExprContext *econte tmptup.t_tableOid = InvalidOid; tmptup.t_data = tuphdr; + /* + * Lookup tupdesc if first time through or if type changes. Because + * we don't pin the tupdesc, we must not do this lookup until after + * doing DatumGetHeapTupleHeader: that could do database access while + * detoasting the datum. + */ + tupDesc = get_cached_rowtype(op->d.fieldstore.fstore->resulttype, -1, + op->d.fieldstore.rowcache, NULL); + + /* Check that current tupdesc doesn't have more fields than allocated */ + if (unlikely(tupDesc->natts > op->d.fieldstore.ncolumns)) + elog(ERROR, "too many columns in composite type %u", + op->d.fieldstore.fstore->resulttype); + heap_deform_tuple(&tmptup, tupDesc, op->d.fieldstore.values, op->d.fieldstore.nulls); diff --git a/src/test/regress/expected/rowtypes.out b/src/test/regress/expected/rowtypes.out index a4cc2d8c126..4a05a730d96 100644 --- a/src/test/regress/expected/rowtypes.out +++ b/src/test/regress/expected/rowtypes.out @@ -145,6 +145,15 @@ select (fn).first, substr((fn).last, 1, 20), length((fn).last) from people; Jim | abcdefghijklabcdefgh | 1200000 (2 rows) +-- try an update on a toasted composite value, too +update people set fn.first = 'Jack'; +select (fn).first, substr((fn).last, 1, 20), length((fn).last) from people; + first | substr | length +-------+----------------------+--------- + Jack | Blow | 4 + Jack | abcdefghijklabcdefgh | 1200000 +(2 rows) + -- Test row comparison semantics. Prior to PG 8.2 we did this in a totally -- non-spec-compliant way. select ROW(1,2) < ROW(1,3) as true; diff --git a/src/test/regress/sql/rowtypes.sql b/src/test/regress/sql/rowtypes.sql index ad5b7e128f9..2ea5e25525a 100644 --- a/src/test/regress/sql/rowtypes.sql +++ b/src/test/regress/sql/rowtypes.sql @@ -80,6 +80,11 @@ insert into people select ('Jim', f1, null)::fullname, current_date from pp; select (fn).first, substr((fn).last, 1, 20), length((fn).last) from people; +-- try an update on a toasted composite value, too +update people set fn.first = 'Jack'; + +select (fn).first, substr((fn).last, 1, 20), length((fn).last) from people; + -- Test row comparison semantics. Prior to PG 8.2 we did this in a totally -- non-spec-compliant way.