Silence complaints involving dlist_node lists.

Put the dlist_node fields of catctup and catclist structs first.
This ensures that the dlist pointers point to the starts of these
palloc blocks, and thus that Valgrind won't consider them
"possibly lost".

The postmaster's PMChild structs and the autovac launcher's avl_dbase
structs also have the dlist_node-is-not-first problem, but putting it
first still wouldn't silence the warning because we bulk-allocate
those structs in an array, so that Valgrind sees a single allocation.
Commonly the first array element will be pointed to only from some
later element, so that the reference would be an interior pointer even
if it pointed to the array start.  (This is the same issue fixed in
the previous patch for dynahash elements.)  Since these are pretty
simple data structures, I don't feel too bad about faking out Valgrind
by just keeping a static pointer to the array start.

This is all quite hacky, and it's not hard to imagine usages where
we'd need some other idea in order to have reasonable leak tracking of
structures that are only accessible via dlist_node lists.  But this
patch seems to be enough to silence this class of leakage complaints
for the moment.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/285483.1746756246@sss.pgh.pa.us

Author: tglsfdc

src/backend/postmaster/autovacuum.c

@@ -310,6 +310,16 @@ static AutoVacuumShmemStruct *AutoVacuumShmem;
 static dlist_head DatabaseList = DLIST_STATIC_INIT(DatabaseList);
 static MemoryContext DatabaseListCxt = NULL;
 
+/*
+ * Dummy pointer to persuade Valgrind that we've not leaked the array of
+ * avl_dbase structs.  Make it global to ensure the compiler doesn't
+ * optimize it away.
+ */
+#ifdef USE_VALGRIND
+extern avl_dbase *avl_dbase_array;
+avl_dbase  *avl_dbase_array;
+#endif
+
 /* Pointer to my own WorkerInfo, valid on each worker */
 static WorkerInfo MyWorkerInfo = NULL;
 
@@ -1020,6 +1030,10 @@ rebuild_database_list(Oid newdb)
 
 		/* put all the hash elements into an array */
 		dbary = palloc(nelems * sizeof(avl_dbase));
+		/* keep Valgrind quiet */
+#ifdef USE_VALGRIND
+		avl_dbase_array = dbary;
+#endif
 
 		i = 0;
 		hash_seq_init(&seq, dbhash);

src/backend/postmaster/pmchild.c

@@ -59,6 +59,17 @@ NON_EXEC_STATIC int num_pmchild_slots = 0;
  */
 dlist_head	ActiveChildList;
 
+/*
+ * Dummy pointer to persuade Valgrind that we've not leaked the array of
+ * PMChild structs.  Make it global to ensure the compiler doesn't
+ * optimize it away.
+ */
+#ifdef USE_VALGRIND
+extern PMChild *pmchild_array;
+PMChild    *pmchild_array;
+#endif
+
+
 /*
  * MaxLivePostmasterChildren
  *
@@ -125,8 +136,13 @@ InitPostmasterChildSlots(void)
 	for (int i = 0; i < BACKEND_NUM_TYPES; i++)
 		num_pmchild_slots += pmchild_pools[i].size;
 
-	/* Initialize them */
+	/* Allocate enough slots, and make sure Valgrind doesn't complain */
 	slots = palloc(num_pmchild_slots * sizeof(PMChild));
+#ifdef USE_VALGRIND
+	pmchild_array = slots;
+#endif
+
+	/* Initialize them */
 	slotno = 0;
 	for (int btype = 0; btype < BACKEND_NUM_TYPES; btype++)
 	{

src/include/utils/catcache.h

@@ -87,6 +87,14 @@ typedef struct catcache
 
 typedef struct catctup
 {
+	/*
+	 * Each tuple in a cache is a member of a dlist that stores the elements
+	 * of its hash bucket.  We keep each dlist in LRU order to speed repeated
+	 * lookups.  Keep the dlist_node field first so that Valgrind understands
+	 * the struct is reachable.
+	 */
+	dlist_node	cache_elem;		/* list member of per-bucket list */
+
 	int			ct_magic;		/* for identifying CatCTup entries */
 #define CT_MAGIC   0x57261502
 
@@ -98,13 +106,6 @@ typedef struct catctup
 	 */
 	Datum		keys[CATCACHE_MAXKEYS];
 
-	/*
-	 * Each tuple in a cache is a member of a dlist that stores the elements
-	 * of its hash bucket.  We keep each dlist in LRU order to speed repeated
-	 * lookups.
-	 */
-	dlist_node	cache_elem;		/* list member of per-bucket list */
-
 	/*
 	 * A tuple marked "dead" must not be returned by subsequent searches.
 	 * However, it won't be physically deleted from the cache until its
@@ -158,13 +159,17 @@ typedef struct catctup
  */
 typedef struct catclist
 {
+	/*
+	 * Keep the dlist_node field first so that Valgrind understands the struct
+	 * is reachable.
+	 */
+	dlist_node	cache_elem;		/* list member of per-catcache list */
+
 	int			cl_magic;		/* for identifying CatCList entries */
 #define CL_MAGIC   0x52765103
 
 	uint32		hash_value;		/* hash value for lookup keys */
 
-	dlist_node	cache_elem;		/* list member of per-catcache list */
-
 	/*
 	 * Lookup keys for the entry, with the first nkeys elements being valid.
 	 * All by-reference are separately allocated.