Add pairingheap_initialize() for shared memory usage

The existing pairingheap_allocate() uses palloc(), which allocates
from process-local memory. For shared memory use cases, the pairingheap
structure must be allocated via ShmemAlloc() or embedded in a shared
memory struct. Add pairingheap_initialize() to initialize an already-
allocated pairingheap structure in-place, enabling shared memory usage.

Discussion: https://www.postgresql.org/message-id/flat/CAPpHfdsjtZLVzxjGT8rJHCYbM0D5dwkO+BBjcirozJ6nYbOW8Q@mail.gmail.com
Discussion: https://www.postgresql.org/message-id/flat/CABPTF7UNft368x-RgOXkfj475OwEbp%2BVVO-wEXz7StgjD_%3D6sw%40mail.gmail.com
Author: Kartyshov Ivan <i.kartyshov@postgrespro.ru>
Author: Alexander Korotkov <aekorotkov@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Reviewed-by: Euler Taveira <euler@eulerto.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>

2 files changed, 19 insertions, 2 deletions

diff --git a/src/backend/lib/pairingheap.c b/src/backend/lib/pairingheap.c
index 0aef8a88f1b..fa8431f7946 100644
--- a/src/backend/lib/pairingheap.c
+++ b/src/backend/lib/pairingheap.c
@@ -44,12 +44,26 @@ pairingheap_allocate(pairingheap_comparator compare, void *arg)
 	pairingheap *heap;
 
 	heap = (pairingheap *) palloc(sizeof(pairingheap));
+	pairingheap_initialize(heap, compare, arg);
+
+	return heap;
+}
+
+/*
+ * pairingheap_initialize
+ *
+ * Same as pairingheap_allocate(), but initializes the pairing heap in-place
+ * rather than allocating a new chunk of memory.  Useful to store the pairing
+ * heap in a shared memory.
+ */
+void
+pairingheap_initialize(pairingheap *heap, pairingheap_comparator compare,
+					   void *arg)
+{
 	heap->ph_compare = compare;
 	heap->ph_arg = arg;
 
 	heap->ph_root = NULL;
-
-	return heap;
 }
 
 /*
diff --git a/src/include/lib/pairingheap.h b/src/include/lib/pairingheap.h
index 3c57d3fda1b..567586f2ecf 100644
--- a/src/include/lib/pairingheap.h
+++ b/src/include/lib/pairingheap.h
@@ -77,6 +77,9 @@ typedef struct pairingheap
 
 extern pairingheap *pairingheap_allocate(pairingheap_comparator compare,
 										 void *arg);
+extern void pairingheap_initialize(pairingheap *heap,
+								   pairingheap_comparator compare,
+								   void *arg);
 extern void pairingheap_free(pairingheap *heap);
 extern void pairingheap_add(pairingheap *heap, pairingheap_node *node);
 extern pairingheap_node *pairingheap_first(pairingheap *heap);