Try to get some info about Windows can't-reattach-to-shared-memory errors.

Add some debug printouts focused on the idea that MapViewOfFileEx might
be rounding its virtual memory allocation up more than we expect (and,
in particular, more than VirtualAllocEx does).

Once we've seen what this reports in one of the failures on buildfarm
members dory or jacana, we might revert this ... or perhaps just
decrease the log level.

Discussion: https://postgr.es/m/25495.1524517820@sss.pgh.pa.us

src/backend/port/win32_shmem.c

@@ -191,6 +191,7 @@ PGSharedMemoryCreate(Size size, bool makePrivate, int port,
 	SIZE_T		largePageSize = 0;
 	Size		orig_size = size;
 	DWORD		flProtect = PAGE_READWRITE;
+	MEMORY_BASIC_INFORMATION info;
 
 	/* Room for a header? */
 	Assert(size > MAXALIGN(sizeof(PGShmemHeader)));
@@ -359,6 +360,14 @@ retry:
 	/* Register on-exit routine to delete the new segment */
 	on_shmem_exit(pgwin32_SharedMemoryDelete, PointerGetDatum(hmap2));
 
+	/* Log information about the segment's virtual memory use */
+	if (VirtualQuery(memAddress, &info, sizeof(info)) != 0)
+		elog(LOG, "mapped shared memory segment at %p, requested size %zu, mapped size %zu",
+			 memAddress, size, info.RegionSize);
+	else
+		elog(LOG, "VirtualQuery(%p) failed: error code %lu",
+			 memAddress, GetLastError());
+
 	*shim = hdr;
 	return hdr;
 }
@@ -392,8 +401,21 @@ PGSharedMemoryReAttach(void)
 
 	hdr = (PGShmemHeader *) MapViewOfFileEx(UsedShmemSegID, FILE_MAP_READ | FILE_MAP_WRITE, 0, 0, 0, UsedShmemSegAddr);
 	if (!hdr)
+	{
+		DWORD		maperr = GetLastError();
+		MEMORY_BASIC_INFORMATION info;
+
+		if (VirtualQuery(UsedShmemSegAddr, &info, sizeof(info)) != 0)
+			elog(LOG, "VirtualQuery(%p) reports region of size %zu, base %p, has state 0x%lx",
+				 UsedShmemSegAddr, info.RegionSize,
+				 info.AllocationBase, info.State);
+		else
+			elog(LOG, "VirtualQuery(%p) failed: error code %lu",
+				 UsedShmemSegAddr, GetLastError());
+
 		elog(FATAL, "could not reattach to shared memory (key=%p, addr=%p): error code %lu",
-			 UsedShmemSegID, UsedShmemSegAddr, GetLastError());
+			 UsedShmemSegID, UsedShmemSegAddr, maperr);
+	}
 	if (hdr != origUsedShmemSegAddr)
 		elog(FATAL, "reattaching to shared memory returned unexpected address (got %p, expected %p)",
 			 hdr, origUsedShmemSegAddr);