Improve checks and error messages of pgstat_register_kind()

pgstat_register_kind() did not validate that required callbacks are
set, which could lead to NULL pointer dereferences when trying to
register a stats kind.  This adds a couple of checks:
- Fox fixed-sized kinds, init_shmem_cb, reset_all_cb, and snapshot_cb
are required.
- For variable-sized kinds, flush_pending_cb is called when there is
pending data, pending_size being required.

These issues should be easy to notice for someone developing an
extension that relies on the custom pgstats APIs.  No backpatch is done
as it is mainly a life improvement.

Author: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CAA5RZ0uNoe=xT7QsU1K0mMRg-QAwPtupPWZ2J3weM2PjVL2tiA@mail.gmail.com

diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index c4fa14f138fa7f31b51b664dbcce08a28099d8b5..8b2a5ec367503f8290acb6ef3cc3c1dba76f0a55 100644 (file)
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -1511,18 +1511,19 @@ pgstat_register_kind(PgStat_Kind kind, const PgStat_KindInfo *kind_info)
 
        if (kind_info->name == NULL || strlen(kind_info->name) == 0)
                ereport(ERROR,
-                               (errmsg("custom cumulative statistics name is invalid"),
+                               (errmsg("failed to register custom cumulative statistics with ID %u", kind),
                                 errhint("Provide a non-empty name for the custom cumulative statistics.")));
 
        if (!pgstat_is_kind_custom(kind))
-               ereport(ERROR, (errmsg("custom cumulative statistics ID %u is out of range", kind),
+               ereport(ERROR, (errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
                                                errhint("Provide a custom cumulative statistics ID between %u and %u.",
                                                                PGSTAT_KIND_CUSTOM_MIN, PGSTAT_KIND_CUSTOM_MAX)));
 
        if (!process_shared_preload_libraries_in_progress)
                ereport(ERROR,
                                (errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
-                                errdetail("Custom cumulative statistics must be registered while initializing modules in \"shared_preload_libraries\".")));
+                                errdetail("Custom cumulative statistics must be registered while initializing modules in \"%s\".",
+                                                  "shared_preload_libraries")));
 
        /*
         * Check some data for fixed-numbered stats.
@@ -1531,13 +1532,36 @@ pgstat_register_kind(PgStat_Kind kind, const PgStat_KindInfo *kind_info)
        {
                if (kind_info->shared_size == 0)
                        ereport(ERROR,
-                                       (errmsg("custom cumulative statistics property is invalid"),
+                                       (errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
                                         errhint("Custom cumulative statistics require a shared memory size for fixed-numbered objects.")));
+               if (kind_info->init_shmem_cb == NULL)
+                       ereport(ERROR,
+                                       (errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+                                        errhint("Custom cumulative statistics require a \"%s\" callback for fixed-numbered objects.",
+                                                        "init_shmem_cb")));
+               if (kind_info->reset_all_cb == NULL)
+                       ereport(ERROR,
+                                       (errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+                                        errhint("Custom cumulative statistics require a \"%s\" callback for fixed-numbered objects.",
+                                                        "reset_all_cb")));
+               if (kind_info->snapshot_cb == NULL)
+                       ereport(ERROR,
+                                       (errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+                                        errhint("Custom cumulative statistics require a \"%s\" callback for fixed-numbered objects.",
+                                                        "snapshot_cb")));
                if (kind_info->track_entry_count)
                        ereport(ERROR,
-                                       (errmsg("custom cumulative statistics property is invalid"),
+                                       (errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
                                         errhint("Custom cumulative statistics cannot use entry count tracking for fixed-numbered objects.")));
        }
+       else
+       {
+               if (kind_info->pending_size > 0 && kind_info->flush_pending_cb == NULL)
+                       ereport(ERROR,
+                                       (errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+                                        errhint("Custom cumulative statistics require a \"%s\" callback when pending size is set.",
+                                                        "flush_pending_cb")));
+       }
 
        /*
         * If pgstat_kind_custom_infos is not available yet, allocate it.