Get still more info about Windows can't-reattach-to-shared-memory errors.

After some thought about the info captured so far, it seems possible
that MapViewOfFileEx is itself causing some DLL to get loaded into
the space just freed by VirtualFree.  The previous commit here didn't
capture enough info to really prove the case for that, so let's add
one more VirtualQuery in between those steps.  Also, be sure to
capture the post-Map state before we emit any log entries, just in
case elog() is invoking some code not previously loaded.

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

src/backend/port/win32_shmem.c

@@ -389,16 +389,18 @@ PGSharedMemoryReAttach(void)
 	PGShmemHeader *hdr;
 	void	   *origUsedShmemSegAddr = UsedShmemSegAddr;
 	MEMORY_BASIC_INFORMATION previnfo;
-	DWORD		queryerr;
+	MEMORY_BASIC_INFORMATION afterinfo;
+	DWORD		preverr;
+	DWORD		aftererr;
 
 	Assert(UsedShmemSegAddr != NULL);
 	Assert(IsUnderPostmaster);
 
 	/* Preliminary probe of region we intend to release */
 	if (VirtualQuery(UsedShmemSegAddr, &previnfo, sizeof(previnfo)) != 0)
-		queryerr = 0;
+		preverr = 0;
 	else
-		queryerr = GetLastError();
+		preverr = GetLastError();
 
 	/*
 	 * Release memory region reservation that was made by the postmaster
@@ -407,27 +409,48 @@ PGSharedMemoryReAttach(void)
 		elog(FATAL, "failed to release reserved memory region (addr=%p): error code %lu",
 			 UsedShmemSegAddr, GetLastError());
 
+	/* Verify post-release state */
+	if (VirtualQuery(UsedShmemSegAddr, &afterinfo, sizeof(afterinfo)) != 0)
+		aftererr = 0;
+	else
+		aftererr = GetLastError();
+
 	hdr = (PGShmemHeader *) MapViewOfFileEx(UsedShmemSegID, FILE_MAP_READ | FILE_MAP_WRITE, 0, 0, 0, UsedShmemSegAddr);
 	if (!hdr)
 	{
 		DWORD		maperr = GetLastError();
-		MEMORY_BASIC_INFORMATION info;
+		MEMORY_BASIC_INFORMATION postinfo;
+		DWORD		posterr;
+
+		/* Capture post-failure state */
+		if (VirtualQuery(UsedShmemSegAddr, &postinfo, sizeof(postinfo)) != 0)
+			posterr = 0;
+		else
+			posterr = GetLastError();
 
-		if (queryerr == 0)
+		if (preverr == 0)
 			elog(LOG, "VirtualQuery(%p) before free reports region of size %zu, base %p, has state 0x%lx",
 				 UsedShmemSegAddr, previnfo.RegionSize,
 				 previnfo.AllocationBase, previnfo.State);
 		else
 			elog(LOG, "VirtualQuery(%p) before free failed: error code %lu",
-				 UsedShmemSegAddr, queryerr);
+				 UsedShmemSegAddr, preverr);
 
-		if (VirtualQuery(UsedShmemSegAddr, &info, sizeof(info)) != 0)
+		if (aftererr == 0)
 			elog(LOG, "VirtualQuery(%p) after free reports region of size %zu, base %p, has state 0x%lx",
-				 UsedShmemSegAddr, info.RegionSize,
-				 info.AllocationBase, info.State);
+				 UsedShmemSegAddr, afterinfo.RegionSize,
+				 afterinfo.AllocationBase, afterinfo.State);
 		else
 			elog(LOG, "VirtualQuery(%p) after free failed: error code %lu",
-				 UsedShmemSegAddr, GetLastError());
+				 UsedShmemSegAddr, aftererr);
+
+		if (posterr == 0)
+			elog(LOG, "VirtualQuery(%p) after map reports region of size %zu, base %p, has state 0x%lx",
+				 UsedShmemSegAddr, postinfo.RegionSize,
+				 postinfo.AllocationBase, postinfo.State);
+		else
+			elog(LOG, "VirtualQuery(%p) after map failed: error code %lu",
+				 UsedShmemSegAddr, posterr);
 
 		elog(FATAL, "could not reattach to shared memory (key=%p, addr=%p): error code %lu",
 			 UsedShmemSegID, UsedShmemSegAddr, maperr);