Fix confusion between "size" and "AnonymousShmemSize".

Noted by Andres Freund.  Also improve a couple of comments.

diff --git a/src/backend/port/sysv_shmem.c b/src/backend/port/sysv_shmem.c
index 8f1da7a7ef4310225972ad53c5e29debd0020965..e040400bb15b124c93be8fcce12e9bed264c61a4 100644 (file)
--- a/src/backend/port/sysv_shmem.c
+++ b/src/backend/port/sysv_shmem.c
@@ -408,13 +408,14 @@ PGSharedMemoryCreate(Size size, bool makePrivate, int port)
        long    pagesize = sysconf(_SC_PAGE_SIZE);
 
        /*
+        * Ensure request size is a multiple of pagesize.
+        *
         * pagesize will, for practical purposes, always be a power of two.
         * But just in case it isn't, we do it this way instead of using
         * TYPEALIGN().
         */
-       AnonymousShmemSize = size;
-       if (size % pagesize != 0)
-           AnonymousShmemSize += pagesize - (size % pagesize);
+       if (pagesize > 0 && size % pagesize != 0)
+           size += pagesize - (size % pagesize);
 
        /*
         * We assume that no one will attempt to run PostgreSQL 9.3 or later
@@ -435,9 +436,10 @@ PGSharedMemoryCreate(Size size, bool makePrivate, int port)
                       "%lu bytes), reduce PostgreSQL's shared memory usage, "
                       "perhaps by reducing shared_buffers or "
                       "max_connections.",
-                   (unsigned long) AnonymousShmemSize) : 0));
+                      (unsigned long) size) : 0));
+       AnonymousShmemSize = size;
 
-       /* Now we can allocate a minimal SHM block. */
+       /* Now we need only allocate a minimal-sized SysV shmem block. */
        allocsize = sizeof(PGShmemHeader);
    }
 #endif