Move the shared memory size calculation to its own function

This change refactors the shared memory size calculation in
CreateSharedMemoryAndSemaphores() to its own function.  This is intended
for use in a future change related to the setup of huge pages and shared
memory with some GUCs, while useful on its own for extensions.

Author: Nathan Bossart
Discussion: https://postgr.es/m/F2772387-CE0F-46BF-B5F1-CC55516EB885@amazon.com

src/backend/storage/ipc/ipci.c

@@ -75,30 +75,19 @@ RequestAddinShmemSpace(Size size)
 	total_addin_request = add_size(total_addin_request, size);
 }
 
-
 /*
- * CreateSharedMemoryAndSemaphores
- *		Creates and initializes shared memory and semaphores.
+ * CalculateShmemSize
+ *		Calculates the amount of shared memory and number of semaphores needed.
  *
- * This is called by the postmaster or by a standalone backend.
- * It is also called by a backend forked from the postmaster in the
- * EXEC_BACKEND case.  In the latter case, the shared memory segment
- * already exists and has been physically attached to, but we have to
- * initialize pointers in local memory that reference the shared structures,
- * because we didn't inherit the correct pointer values from the postmaster
- * as we do in the fork() scenario.  The easiest way to do that is to run
- * through the same code as before.  (Note that the called routines mostly
- * check IsUnderPostmaster, rather than EXEC_BACKEND, to detect this case.
- * This is a bit code-wasteful and could be cleaned up.)
+ * If num_semaphores is not NULL, it will be set to the number of semaphores
+ * required.
+ *
+ * Note that this function freezes the additional shared memory request size
+ * from loadable modules.
  */
-void
-CreateSharedMemoryAndSemaphores(void)
+Size
+CalculateShmemSize(int *num_semaphores)
 {
-	PGShmemHeader *shim = NULL;
-
-	if (!IsUnderPostmaster)
-	{
-		PGShmemHeader *seghdr;
 	Size		size;
 	int			numSemas;
 
@@ -106,14 +95,18 @@ CreateSharedMemoryAndSemaphores(void)
 	numSemas = ProcGlobalSemas();
 	numSemas += SpinlockSemas();
 
+	/* Return the number of semaphores if requested by the caller */
+	if (num_semaphores)
+		*num_semaphores = numSemas;
+
 	/*
-		 * Size of the Postgres shared-memory block is estimated via
-		 * moderately-accurate estimates for the big hogs, plus 100K for the
-		 * stuff that's too small to bother with estimating.
+	 * Size of the Postgres shared-memory block is estimated via moderately-
+	 * accurate estimates for the big hogs, plus 100K for the stuff that's too
+	 * small to bother with estimating.
 	 *
-		 * We take some care during this phase to ensure that the total size
-		 * request doesn't overflow size_t.  If this gets through, we don't
-		 * need to be so careful during the actual allocation phase.
+	 * We take some care to ensure that the total size request doesn't
+	 * overflow size_t.  If this gets through, we don't need to be so careful
+	 * during the actual allocation phase.
 	 */
 	size = 100000;
 	size = add_size(size, PGSemaphoreShmemSize(numSemas));
@@ -161,6 +154,37 @@ CreateSharedMemoryAndSemaphores(void)
 	/* might as well round it off to a multiple of a typical page size */
 	size = add_size(size, 8192 - (size % 8192));
 
+	return size;
+}
+
+/*
+ * CreateSharedMemoryAndSemaphores
+ *		Creates and initializes shared memory and semaphores.
+ *
+ * This is called by the postmaster or by a standalone backend.
+ * It is also called by a backend forked from the postmaster in the
+ * EXEC_BACKEND case.  In the latter case, the shared memory segment
+ * already exists and has been physically attached to, but we have to
+ * initialize pointers in local memory that reference the shared structures,
+ * because we didn't inherit the correct pointer values from the postmaster
+ * as we do in the fork() scenario.  The easiest way to do that is to run
+ * through the same code as before.  (Note that the called routines mostly
+ * check IsUnderPostmaster, rather than EXEC_BACKEND, to detect this case.
+ * This is a bit code-wasteful and could be cleaned up.)
+ */
+void
+CreateSharedMemoryAndSemaphores(void)
+{
+	PGShmemHeader *shim = NULL;
+
+	if (!IsUnderPostmaster)
+	{
+		PGShmemHeader *seghdr;
+		Size		size;
+		int			numSemas;
+
+		/* Compute the size of the shared-memory block */
+		size = CalculateShmemSize(&numSemas);
 		elog(DEBUG3, "invoking IpcMemoryCreate(size=%zu)", size);
 
 		/*

src/include/storage/ipc.h

@@ -77,6 +77,7 @@ extern void check_on_shmem_exit_lists_are_empty(void);
 /* ipci.c */
 extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
+extern Size CalculateShmemSize(int *num_semaphores);
 extern void CreateSharedMemoryAndSemaphores(void);
 
 #endif							/* IPC_H */