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Fix datumSerialize infrastructure to not crash on non-varlena data.

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Commit 1efc7e538 did a poor job of emulating existing logic for touching
Datums that might be expanded-object pointers.  It didn't check for typlen
being -1 first, which meant it could crash on fixed-length pass-by-ref
values, and probably on cstring values as well.  It also didn't use
DatumGetPointer before VARATT_IS_EXTERNAL_EXPANDED, which while currently
harmless is not according to documentation nor prevailing style.

I also think the lack of any explanation as to why datumSerialize makes
these particular nonobvious choices is pretty awful, so fix that.

Per report from Jarred Ward.  Back-patch to 9.6 where this code came in.

Discussion: https://postgr.es/m/6F61E6D2-2F5E-4794-9479-A429BE1CEA4B@simple.com
This commit is contained in:
Tom Lane 2017-08-08 19:18:12 -04:00
parent 1924ef4166
commit fe578cbd4b
1 changed files with 13 additions and 5 deletions
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18
src/backend/utils/adt/datum.c
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@ -264,11 +264,11 @@ datumEstimateSpace(Datum value, bool isnull, bool typByVal, int typLen)
/* no need to use add_size, can't overflow */
if (typByVal)
sz += sizeof(Datum);
else if (VARATT_IS_EXTERNAL_EXPANDED(value))
else if (typLen == -1 &&
VARATT_IS_EXTERNAL_EXPANDED(DatumGetPointer(value)))
{
ExpandedObjectHeader *eoh = DatumGetEOHP(value);
sz += EOH_get_flat_size(eoh);
/* Expanded objects need to be flattened, see comment below */
sz += EOH_get_flat_size(DatumGetEOHP(value));
}
else
sz += datumGetSize(value, typByVal, typLen);
@ -282,6 +282,13 @@ datumEstimateSpace(Datum value, bool isnull, bool typByVal, int typLen)
*
* Serialize a possibly-NULL datum into caller-provided storage.
*
* Note: "expanded" objects are flattened so as to produce a self-contained
* representation, but other sorts of toast pointers are transferred as-is.
* This is because the intended use of this function is to pass the value
* to another process within the same database server. The other process
* could not access an "expanded" object within this process's memory, but
* we assume it can dereference the same TOAST pointers this one can.
*
* The format is as follows: first, we write a 4-byte header word, which
* is either the length of a pass-by-reference datum, -1 for a
* pass-by-value datum, or -2 for a NULL. If the value is NULL, nothing
@ -306,7 +313,8 @@ datumSerialize(Datum value, bool isnull, bool typByVal, int typLen,
header = -2;
else if (typByVal)
header = -1;
else if (VARATT_IS_EXTERNAL_EXPANDED(value))
else if (typLen == -1 &&
VARATT_IS_EXTERNAL_EXPANDED(DatumGetPointer(value)))
{
eoh = DatumGetEOHP(value);
header = EOH_get_flat_size(eoh);