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

@@ -1913,7 +1913,8 @@ CheckVarSlotCompatibility(TupleTableSlot *slot, int attnum, Oid vartype)
  * 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
@@ -3005,17 +3006,6 @@ 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 */
@@ -3031,6 +3021,7 @@ 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);
@@ -3038,6 +3029,20 @@ 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);

src/test/regress/expected/rowtypes.out

@@ -139,6 +139,15 @@ 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;

src/test/regress/sql/rowtypes.sql

@@ -76,6 +76,11 @@ 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.