ST_CollectionHomogenize sometimes returns reference to argument

[dbaston]

I noticed this situation when digging into a question posted on Stack Exchange: ​http://gis.stackexchange.com/questions/195915/weird-behavior-with-st-clusterwithin

That question itself isn't relevant here, but the some of the test data can be used to make a reproducible (for me) test case of the problem:

CREATE TABLE cluster_inputs (
    name text,
    geom geometry
);

CREATE TABLE cluster (
    geom geometry
);

INSERT INTO cluster (geom) VALUES ('010700002031BF0D000100000001030000000100000005000000713D0AD76D4D11C1A4703D9AE5955941713D0AD76D4D11C1A4703D9A94965941713D0AD77D4211C1A4703D9A94965941713D0AD77D4211C1A4703D9AE5955941713D0AD76D4D11C1A4703D9AE5955941');
INSERT INTO cluster (geom) VALUES ('010700002031BF0D00010000000103000000010000000D0000009A9999998B3211C114AE47F13D965941295C8FC2863211C1A4703D5A569659418FC2F528D33011C1EC51B84E5A9659417B14AE47B62F11C1B81E851B5A965941295C8FC2BC2F11C18FC2F5F85D96594114AE47E10D2C11C1295C8FE26F965941EC51B81EF32B11C1666666D6699659417B14AE47D12C11C1F6285CFF139659418FC2F528BA2F11C1F6285C7F14965941B81E85EBBE2F11C11F85EB01279659415C8FC2F5DB2F11C1D7A3702D2C965941F6285C8F2F3011C15C8FC235309659419A9999998B3211C114AE47F13D965941');


INSERT INTO cluster_inputs (name, geom) VALUES ('a', '010300002031BF0D000100000005000000713D0AD76D4D11C1A4703D9AE5955941713D0AD76D4D11C1A4703D9A94965941713D0AD77D4211C1A4703D9A94965941713D0AD77D4211C1A4703D9AE5955941713D0AD76D4D11C1A4703D9AE5955941');
INSERT INTO cluster_inputs (name, geom) VALUES ('b', '010300002031BF0D00010000000D0000009A9999998B3211C114AE47F13D965941295C8FC2863211C1A4703D5A569659418FC2F528D33011C1EC51B84E5A9659417B14AE47B62F11C1B81E851B5A965941295C8FC2BC2F11C18FC2F5F85D96594114AE47E10D2C11C1295C8FE26F965941EC51B81EF32B11C1666666D6699659417B14AE47D12C11C1F6285CFF139659418FC2F528BA2F11C1F6285C7F14965941B81E85EBBE2F11C11F85EB01279659415C8FC2F5DB2F11C1D7A3702D2C965941F6285C8F2F3011C15C8FC235309659419A9999998B3211C114AE47F13D965941');

For me, running the following query then produces an error:

SELECT
  cluster_inputs.name, 
  ST_Intersects(ST_CollectionHomogenize(cluster.geom), cluster_inputs.geom)
FROM cluster_inputs, cluster
ORDER BY name;

ERROR:  First argument geometry could not be converted to GEOS: IllegalArgumentException: Points of LinearRing do not form a closed linestring

which suggests a memory issue. The problem can be resolved by wrapping the return of lwgeom_homogenize with an lwgeom_clone_deep, which seems to confirm this. Since the return of lwgeom_homogenize references portions of its argument, and ST_CollectionHomogenize releases the serialized input before serializing the output, we seem to have a problem.

[Mike Taves]

Here's a simplified version of the issue:

WITH cluster AS (
  SELECT ST_Polygonize(ST_MakeEnvelope(1, 2, 3, 4)) AS geom
)
SELECT ST_GeometryType(geom),
  ST_CollectionHomogenize(geom)::box2d,
  ST_CollectionHomogenize(geom::text)::box2d
FROM cluster;

-[ RECORD 1 ]-----------+----------------------
st_geometrytype         | ST_GeometryCollection
st_collectionhomogenize | BOX(0 0,3 4)
st_collectionhomogenize | BOX(1 2,3 4)

The error is shown from the second field, where the X and Y coords are 0. A workaround is shown in the last example, where the geometry is recreated.

[robe]

Okay this is a problem in both 2.2 and 2.3.

Does anyone have issue with applying mtoews suggestion. If not I'm going to go ahead and do that.

[pramsey]

Fixing homogenize to return a full copy seems preferable to deep cloning the return from homogenize, ISTM.

[dbaston]

Replying to pramsey:

Fixing homogenize to return a full copy seems preferable to deep cloning the return from homogenize, ISTM.

Agreed. I don't think we actually need a clone though, I think it's sufficient to just avoid freeing our input GSERIALIZED before we've serialized our output. If there are no concerns with the attached patch that does this, I'll commit it.

[pramsey]

Sure. Maybe add a TODO comment in the code indicating we're guarding against this problem and should maybe change the liblwgeom contract.

[dbaston]

Fixed in trunk at r15105 and 2.2 at r15107.