Prevent spurious "indexes on virtual generated columns are not supported".

Both of the checks in DefineIndex() that can produce this error
message have a guard against negative attribute numbers, but lack a
guard to ensure that attno is non-zero. As a result, we can index
off the beginning of the TupleDesc and read a garbage byte for
attgenerated. If that byte happens to be 'v', we'll incorrectly
produce the error mentioned above.

The first call site is easy to hit: any attempt to create an
expression index does so. The second one is not currently hit in
the regression tests, but can be hit by something like
CREATE INDEX ON some_table ((some_function(some_table))).

Found by study of a test_plan_advice failure on buildfarm member
skink, though this issue has nothing to do with test_plan_advice
and seems to have only been revealed by happenstance.

Backpatch-through: 18
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: http://postgr.es/m/CA+TgmoacixUZVvi00hOjk_d9B4iYKswWP1gNqQ8Vfray-AcOCA@mail.gmail.com

diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index b89c6855364947de86b787aaf0ff22bd3a5c26f6..dd593ccbc1cb276b815768d3f54102a7093f63ef 100644 (file)
--- a/src/backend/commands/indexcmds.c
+++ b/src/backend/commands/indexcmds.c
@@ -1123,7 +1123,8 @@ DefineIndex(ParseState *pstate,
                                         errmsg("index creation on system columns is not supported")));
 
 
-               if (TupleDescAttr(RelationGetDescr(rel), attno - 1)->attgenerated == ATTRIBUTE_GENERATED_VIRTUAL)
+               if (attno > 0 &&
+                       TupleDescAttr(RelationGetDescr(rel), attno - 1)->attgenerated == ATTRIBUTE_GENERATED_VIRTUAL)
                        ereport(ERROR,
                                        errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                                        stmt->primary ?
@@ -1164,7 +1165,8 @@ DefineIndex(ParseState *pstate,
                {
                        AttrNumber      attno = j + FirstLowInvalidHeapAttributeNumber;
 
-                       if (TupleDescAttr(RelationGetDescr(rel), attno - 1)->attgenerated == ATTRIBUTE_GENERATED_VIRTUAL)
+                       if (attno > 0 &&
+                               TupleDescAttr(RelationGetDescr(rel), attno - 1)->attgenerated == ATTRIBUTE_GENERATED_VIRTUAL)
                                ereport(ERROR,
                                                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                                                 stmt->isconstraint ?