Allow passing a pointer to GetNamedDSMSegment()'s init callback.

This commit adds a new "void *arg" parameter to
GetNamedDSMSegment() that is passed to the initialization callback
function.  This is useful for reusing an initialization callback
function for multiple DSM segments.

Author: Zsolt Parragi <zsolt.parragi@percona.com>
Reviewed-by: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CAN4CZFMjh8TrT9ZhWgjVTzBDkYZi2a84BnZ8bM%2BfLPuq7Cirzg%40mail.gmail.com

contrib/pg_prewarm/autoprewarm.c

@@ -858,7 +858,7 @@ autoprewarm_dump_now(PG_FUNCTION_ARGS)
 }
 
 static void
-apw_init_state(void *ptr)
+apw_init_state(void *ptr, void *arg)
 {
 	AutoPrewarmSharedState *state = (AutoPrewarmSharedState *) ptr;
 
@@ -880,7 +880,7 @@ apw_init_shmem(void)
 	apw_state = GetNamedDSMSegment("autoprewarm",
 								   sizeof(AutoPrewarmSharedState),
 								   apw_init_state,
-								   &found);
+								   &found, NULL);
 
 	return found;
 }

doc/src/sgml/xfunc.sgml

@@ -3696,15 +3696,18 @@ LWLockRelease(AddinShmemInitLock);
       use the shared memory should obtain a pointer to it by calling:
 <programlisting>
 void *GetNamedDSMSegment(const char *name, size_t size,
-                         void (*init_callback) (void *ptr),
+                         void (*init_callback) (void *ptr, void *arg),
-                         bool *found)
+                         bool *found, void *arg)
 </programlisting>
       If a dynamic shared memory segment with the given name does not yet
       exist, this function will allocate it and initialize it with the provided
       <function>init_callback</function> callback function.  If the segment has
       already been allocated and initialized by another backend, this function
       simply attaches the existing dynamic shared memory segment to the current
-      backend.
+      backend.  In the former case, <function>GetNamedDSMSegment</function>
+      passes the <literal>void *arg</literal> argument to the
+      <function>init_callback</function>.  This is particularly useful for
+      reusing an initialization callback function for multiple DSM segments.
      </para>
 
      <para>

src/backend/storage/ipc/dsm_registry.c

@@ -180,11 +180,13 @@ init_dsm_registry(void)
  * Initialize or attach a named DSM segment.
  *
  * This routine returns the address of the segment.  init_callback is called to
- * initialize the segment when it is first created.
+ * initialize the segment when it is first created.  'arg' is passed through to
+ * the initialization callback function.
  */
 void *
 GetNamedDSMSegment(const char *name, size_t size,
-				   void (*init_callback) (void *ptr), bool *found)
+				   void (*init_callback) (void *ptr, void *arg),
+				   bool *found, void *arg)
 {
 	DSMRegistryEntry *entry;
 	MemoryContext oldcontext;
@@ -235,7 +237,7 @@ GetNamedDSMSegment(const char *name, size_t size,
 		seg = dsm_create(size, 0);
 
 		if (init_callback)
-			(*init_callback) (dsm_segment_address(seg));
+			(*init_callback) (dsm_segment_address(seg), arg);
 
 		dsm_pin_segment(seg);
 		dsm_pin_mapping(seg);

src/include/storage/dsm_registry.h

@@ -16,8 +16,8 @@
 #include "lib/dshash.h"
 
 extern void *GetNamedDSMSegment(const char *name, size_t size,
-								void (*init_callback) (void *ptr),
+								void (*init_callback) (void *ptr, void *arg),
-								bool *found);
+								bool *found, void *arg);
 extern dsa_area *GetNamedDSA(const char *name, bool *found);
 extern dshash_table *GetNamedDSHash(const char *name,
 									const dshash_parameters *params,

src/test/modules/injection_points/injection_points.c

@@ -115,7 +115,7 @@ static shmem_startup_hook_type prev_shmem_startup_hook = NULL;
  * when initializing dynamically with a DSM or when loading the module.
  */
 static void
-injection_point_init_state(void *ptr)
+injection_point_init_state(void *ptr, void *arg)
 {
 	InjectionPointSharedState *state = (InjectionPointSharedState *) ptr;
 
@@ -159,7 +159,7 @@ injection_shmem_startup(void)
 		 * First time through, so initialize.  This is shared with the dynamic
 		 * initialization using a DSM.
 		 */
-		injection_point_init_state(inj_state);
+		injection_point_init_state(inj_state, NULL);
 	}
 
 	LWLockRelease(AddinShmemInitLock);
@@ -179,7 +179,7 @@ injection_init_shmem(void)
 	inj_state = GetNamedDSMSegment("injection_points",
 								   sizeof(InjectionPointSharedState),
 								   injection_point_init_state,
-								   &found);
+								   &found, NULL);
 }
 
 /*

src/test/modules/test_dsa/test_dsa.c

@@ -21,7 +21,7 @@
 PG_MODULE_MAGIC;
 
 static void
-init_tranche(void *ptr)
+init_tranche(void *ptr, void *arg)
 {
 	int		   *tranche_id = (int *) ptr;
 
@@ -39,7 +39,7 @@ test_dsa_basic(PG_FUNCTION_ARGS)
 	dsa_pointer p[100];
 
 	tranche_id = GetNamedDSMSegment("test_dsa", sizeof(int),
-									init_tranche, &found);
+									init_tranche, &found, NULL);
 
 	a = dsa_create(*tranche_id);
 	for (int i = 0; i < 100; i++)
@@ -80,7 +80,7 @@ test_dsa_resowners(PG_FUNCTION_ARGS)
 	ResourceOwner childowner;
 
 	tranche_id = GetNamedDSMSegment("test_dsa", sizeof(int),
-									init_tranche, &found);
+									init_tranche, &found, NULL);
 
 	/* Create DSA in parent resource owner */
 	a = dsa_create(*tranche_id);

src/test/modules/test_dsm_registry/test_dsm_registry.c

@@ -44,10 +44,13 @@ static const dshash_parameters dsh_params = {
 };
 
 static void
-init_tdr_dsm(void *ptr)
+init_tdr_dsm(void *ptr, void *arg)
 {
 	TestDSMRegistryStruct *dsm = (TestDSMRegistryStruct *) ptr;
 
+	if ((int) (intptr_t) arg != 5432)
+		elog(ERROR, "unexpected arg value %d", (int) (intptr_t) arg);
+
 	LWLockInitialize(&dsm->lck, LWLockNewTrancheId("test_dsm_registry"));
 	dsm->val = 0;
 }
@@ -60,7 +63,7 @@ tdr_attach_shmem(void)
 	tdr_dsm = GetNamedDSMSegment("test_dsm_registry_dsm",
 								 sizeof(TestDSMRegistryStruct),
 								 init_tdr_dsm,
-								 &found);
+								 &found, (void *) (intptr_t) 5432);
 
 	if (tdr_dsa == NULL)
 		tdr_dsa = GetNamedDSA("test_dsm_registry_dsa", &found);