Fix GUC check_hook validation for synchronized_standby_slots.

Previously, the check_hook for synchronized_standby_slots attempted to
validate that each specified slot existed and was physical. However, these
checks were not performed during server startup. As a result, if users
configured non-existent slots before startup, the misconfiguration would
go undetected initially. This could later cause parallel query failures,
as newly launched workers would detect the issue and raise an ERROR.

This patch improves the check_hook by validating the syntax and format of
slot names. Validation of slot existence and type is deferred to the WAL
sender process, aligning with the behavior of the check_hook for
primary_slot_name.

Reported-by: Fabrice Chapuis <fabrice636861@gmail.com>
Author: Shlok Kyal <shlok.kyal.oss@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Ashutosh Sharma <ashu.coek88@gmail.com>
Reviewed-by: Rahila Syed <rahilasyed90@gmail.com>
Backpatch-through: 17, where it was introduced
Discussion: https://postgr.es/m/CAA5-nLCeO4MQzWipCXH58qf0arruiw0OeUc1+Q=Z=4GM+=v1NQ@mail.gmail.com

2 files changed, 25 insertions, 0 deletions

diff --git a/src/test/modules/unsafe_tests/expected/guc_privs.out b/src/test/modules/unsafe_tests/expected/guc_privs.out
index 6c0ad898341..3cf2f2fdb85 100644
--- a/src/test/modules/unsafe_tests/expected/guc_privs.out
+++ b/src/test/modules/unsafe_tests/expected/guc_privs.out
@@ -581,6 +581,21 @@ DROP ROLE regress_host_resource_newadmin;  -- ok, nothing was transferred
 -- Use "drop owned by" so we can drop the role
 DROP OWNED BY regress_host_resource_admin;  -- ok
 DROP ROLE regress_host_resource_admin;  -- ok
+-- Test for GUC synchronized standby slots
+-- Cannot set synchronized_standby_slots to a reserved slot name
+ALTER SYSTEM SET synchronized_standby_slots='pg_conflict_detection';
+ERROR:  invalid value for parameter "synchronized_standby_slots": "pg_conflict_detection"
+DETAIL:  replication slot name "pg_conflict_detection" is reserved
+HINT:  The name "pg_conflict_detection" is reserved for the conflict detection slot.
+-- Cannot set synchronized_standby_slots to an invalid slot name
+ALTER SYSTEM SET synchronized_standby_slots='invalid*';
+ERROR:  invalid value for parameter "synchronized_standby_slots": "invalid*"
+DETAIL:  replication slot name "invalid*" contains invalid character
+HINT:  Replication slot names may only contain lower case letters, numbers, and the underscore character.
+-- Can set synchronized_standby_slots to a non-existent slot name
+ALTER SYSTEM SET synchronized_standby_slots='missing';
+-- Reset the GUC
+ALTER SYSTEM RESET synchronized_standby_slots;
 -- Clean up
 RESET SESSION AUTHORIZATION;
 DROP ROLE regress_admin; -- ok
diff --git a/src/test/modules/unsafe_tests/sql/guc_privs.sql b/src/test/modules/unsafe_tests/sql/guc_privs.sql
index 9bcbbfa9040..d0d16f3c29f 100644
--- a/src/test/modules/unsafe_tests/sql/guc_privs.sql
+++ b/src/test/modules/unsafe_tests/sql/guc_privs.sql
@@ -262,6 +262,16 @@ DROP ROLE regress_host_resource_newadmin;  -- ok, nothing was transferred
 DROP OWNED BY regress_host_resource_admin;  -- ok
 DROP ROLE regress_host_resource_admin;  -- ok
 
+-- Test for GUC synchronized standby slots
+-- Cannot set synchronized_standby_slots to a reserved slot name
+ALTER SYSTEM SET synchronized_standby_slots='pg_conflict_detection';
+-- Cannot set synchronized_standby_slots to an invalid slot name
+ALTER SYSTEM SET synchronized_standby_slots='invalid*';
+-- Can set synchronized_standby_slots to a non-existent slot name
+ALTER SYSTEM SET synchronized_standby_slots='missing';
+-- Reset the GUC
+ALTER SYSTEM RESET synchronized_standby_slots;
+
 -- Clean up
 RESET SESSION AUTHORIZATION;
 DROP ROLE regress_admin; -- ok