[SCM] PostGIS branch master updated. 3.6.0rc2-604-g5f207dbe5

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- Log -----------------------------------------------------------------
commit 5f207dbe59a552eba46617f8436d4478aa7045f7
Author: Darafei Praliaskouski <me at komzpa.net>
Date:   Thu Jun 18 19:36:32 2026 +0400

    Initialize skipped union-find cluster ids
    
    Initialize unassigned entries returned by UF_get_collapsed_cluster_ids to UINT32_MAX so callers and tests see deterministic values for elements outside the filtered cluster set.
    
    Closes https://github.com/postgis/postgis/pull/974
    
    References #5975

diff --git a/NEWS b/NEWS
index 578109e8d..249f6c879 100644
--- a/NEWS
+++ b/NEWS
@@ -53,6 +53,8 @@ To take advantage of all postgis_sfcgal extension features SFCGAL 2.3+ is needed
 
 * Bug Fixes *
 
+ - #5975, Initialize skipped union-find cluster ids deterministically
+          (Darafei Praliaskouski)
  - GH-885, [topology] Avoid GMP overflow in ring orientation for very large
           finite coordinates (Darafei Praliaskouski)
  - GH-892, Add OSS-Fuzz coverage for TWKB and serialized raster inputs,
diff --git a/liblwgeom/cunit/cu_unionfind.c b/liblwgeom/cunit/cu_unionfind.c
index fb00dc5c1..9deff2adb 100644
--- a/liblwgeom/cunit/cu_unionfind.c
+++ b/liblwgeom/cunit/cu_unionfind.c
@@ -4,6 +4,7 @@
  * http://postgis.net
  *
  * Copyright 2015 Daniel Baston
+ * Copyright 2026 Darafei Praliaskouski <me at komzpa.net>
  *
  * This is free software; you can redistribute and/or modify it under
  * the terms of the GNU General Public Licence. See the COPYING file.
@@ -148,15 +149,13 @@ static void test_unionfind_collapse_cluster_ids(void)
 	lwfree(collapsed_ids);
 
 	uint8_t is_in_cluster[] = {0, 1, 1, 1, 0, 1, 0, 0, 0, 0};
-	uint32_t expected_collapsed_ids2[] = { 8, 0, 0, 0, 7, 0, 8, 7, 8, 7 };
+	uint32_t expected_collapsed_ids2[] = {
+	    UINT32_MAX, 0, 0, 0, UINT32_MAX, 0, UINT32_MAX, UINT32_MAX, UINT32_MAX, UINT32_MAX};
 
 	collapsed_ids = UF_get_collapsed_cluster_ids(uf, is_in_cluster);
 	uint32_t i;
 	for (i = 0; i < uf->N; i++)
-	{
-		if (is_in_cluster[i])
-			ASSERT_INT_EQUAL(expected_collapsed_ids2[i], collapsed_ids[i]);
-	}
+		ASSERT_INT_EQUAL(expected_collapsed_ids2[i], collapsed_ids[i]);
 
 	lwfree(collapsed_ids);
 	UF_destroy(uf);
diff --git a/liblwgeom/lwunionfind.c b/liblwgeom/lwunionfind.c
index 9ca47a1c3..7e78d8064 100644
--- a/liblwgeom/lwunionfind.c
+++ b/liblwgeom/lwunionfind.c
@@ -19,10 +19,10 @@
  **********************************************************************
  *
  * Copyright 2015 Daniel Baston <dbaston at gmail.com>
+ * Copyright 2026 Darafei Praliaskouski <me at komzpa.net>
  *
  **********************************************************************/
 
-
 #include "liblwgeom.h"
 #include "lwunionfind.h"
 #include <string.h>
@@ -150,6 +150,8 @@ UF_get_collapsed_cluster_ids(UNIONFIND *uf, const uint8_t *is_in_cluster)
 	uint32_t last_old_id, current_new_id, i;
 	uint8_t encountered_cluster = LW_FALSE;
 
+	memset(new_ids, 0xFF, uf->N * sizeof(uint32_t));
+
 	current_new_id = 0; last_old_id = 0;
 	for (i = 0; i < uf->N; i++)
 	{

-----------------------------------------------------------------------

Summary of changes:
 NEWS                           | 2 ++
 liblwgeom/cunit/cu_unionfind.c | 9 ++++-----
 liblwgeom/lwunionfind.c        | 4 +++-
 3 files changed, 9 insertions(+), 6 deletions(-)


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