Improve InitShmemAccess() prototype

The code comment said, 'the argument should be declared "PGShmemHeader
*seghdr", but we use void to avoid having to include ipc.h in
shmem.h.'  We can achieve the original goal with a struct forward
declaration.  (ipc.h was also not the correct header file.)

Discussion: https://www.postgresql.org/message-id/flat/cnthxg2eekacrejyeonuhiaezc7vd7o2uowlsbenxqfkjwgvwj@qgzu6eoqrglb

Author: petere

src/backend/storage/ipc/shmem.c

@@ -92,18 +92,13 @@ static HTAB *ShmemIndex = NULL; /* primary index hashtable for shmem */
 
 /*
  *	InitShmemAccess() --- set up basic pointers to shared memory.
- *
- * Note: the argument should be declared "PGShmemHeader *seghdr",
- * but we use void to avoid having to include ipc.h in shmem.h.
  */
 void
-InitShmemAccess(void *seghdr)
+InitShmemAccess(PGShmemHeader *seghdr)
 {
-	PGShmemHeader *shmhdr = (PGShmemHeader *) seghdr;
-
-	ShmemSegHdr = shmhdr;
-	ShmemBase = (void *) shmhdr;
-	ShmemEnd = (char *) ShmemBase + shmhdr->totalsize;
+	ShmemSegHdr = seghdr;
+	ShmemBase = seghdr;
+	ShmemEnd = (char *) ShmemBase + seghdr->totalsize;
 }
 
 /*

src/include/storage/shmem.h

@@ -27,7 +27,8 @@
 
 /* shmem.c */
 extern PGDLLIMPORT slock_t *ShmemLock;
-extern void InitShmemAccess(void *seghdr);
+struct PGShmemHeader;			/* avoid including storage/pg_shmem.h here */
+extern void InitShmemAccess(struct PGShmemHeader *seghdr);
 extern void InitShmemAllocation(void);
 extern void *ShmemAlloc(Size size);
 extern void *ShmemAllocNoError(Size size);