PGPRO-5646: remove compiler warning (gcc 11.2.0)

Marina Polyakova · Marina Polyakova · commit 033471d2bbfc · 2021-10-15T20:28:36.000+03:00
In function ‘ptrack_write_chunk’,
    inlined from ‘ptrackCheckpoint’ at engine.c:397:2:
engine.c:78:13: warning: ‘write’ reading 8 bytes from a region of size 4
[-Wstringop-overread]
   78 |         if (write(fd, chunk, size) != size)
      |             ^~~~~~~~~~~~~~~~~~~~~~
In file included from engine.c:47:
engine.c: In function ‘ptrackCheckpoint’:
engine.h:55:25: note: source object ‘magic’ of size 4
   55 |         char            magic[PTRACK_MAGIC_SIZE];
      |                         ^~~~~
In file included from engine.c:22:
/usr/include/unistd.h:378:16: note: in a call to function ‘write’ declared with
attribute ‘access (read_only, 2, 3)’
  378 | extern ssize_t write (int __fd, const void *__buf, size_t __n) __wur
      |                ^~~~~

To avoid changing other code for the fields magic and version_num in the
structure PtrackMapHdr, use two calls to ptrack_write_chunk to write them to the
file. (This function is called a few times later, so why not add another call?)
Add a compile-time assertion check that nothing has actually changed (e.g.
alignment is fine).
diff --git a/engine.c b/engine.c
@@ -391,11 +391,22 @@ ptrackCheckpoint(void)
 	 * We are writing ptrack map values to file, but we want to simply map it
 	 * into the memory with mmap after a crash/restart. That way, we have to
 	 * write values taking into account all paddings/alignments.
-	 *
-	 * Write both magic and varsion_num at once.
 	 */
+
+	/*
+	 * Earlier we wrote both magic and varsion_num at once. Make sure nothing
+	 * has changed since then (e.g. alignment is fine).
+	 */
+	StaticAssertStmt(
+		sizeof(ptrack_map->magic) + sizeof(ptrack_map->version_num) ==
+		offsetof(PtrackMapHdr, init_lsn),
+		"old write format for PtrackMapHdr.magic and PtrackMapHdr.version_num "
+		"is not upward-compatible");
+
 	ptrack_write_chunk(ptrack_tmp_fd, &crc, (char *) &ptrack_map->magic,
-					   offsetof(PtrackMapHdr, init_lsn));
+					   sizeof(ptrack_map->magic));
+	ptrack_write_chunk(ptrack_tmp_fd, &crc, (char *) &ptrack_map->version_num,
+					   sizeof(ptrack_map->version_num));
 
 	init_lsn = pg_atomic_read_u64(&ptrack_map->init_lsn);
 
