Be more wary about 32-bit integer overflow in pg_stat_statements.

We've heard a couple of reports of people having trouble with
multi-gigabyte-sized query-texts files.  It occurred to me that on
32-bit platforms, there could be an issue with integer overflow
of calculations associated with the total query text size.
Address that with several changes:

1. Limit pg_stat_statements.max to INT_MAX / 2 not INT_MAX.
The hashtable code will bound it to that anyway unless "long"
is 64 bits.  We still need overflow guards on its use, but
this helps.

2. Add a check to prevent extending the query-texts file to
more than MaxAllocHugeSize.  If it got that big, qtext_load_file
would certainly fail, so there's not much point in allowing it.
Without this, we'd need to consider whether extent, query_offset,
and related variables shouldn't be off_t not size_t.

3. Adjust the comparisons in need_gc_qtexts() to be done in 64-bit
arithmetic on all platforms.  It appears possible that under duress
those multiplications could overflow 32 bits, yielding a false
conclusion that we need to garbage-collect the texts file, which
could lead to repeatedly garbage-collecting after every hash table
insertion.

Per report from Bruno da Silva.  I'm not convinced that these
issues fully explain his problem; there may be some other bug that's
contributing to the query-texts file becoming so large in the first
place.  But it did get that big, so #2 is a reasonable defense,
and #3 could explain the reported performance difficulties.

(See also commit 8bbe4cbd9, which addressed some related bugs.
The second Discussion: link is the thread that led up to that.)

This issue is old, and is primarily a problem for old platforms,
so back-patch.

Discussion: https://postgr.es/m/CAB+Nuk93fL1Q9eLOCotvLP07g7RAv4vbdrkm0cVQohDVMpAb9A@mail.gmail.com
Discussion: https://postgr.es/m/5601D354.5000703@BlueTreble.com

contrib/pg_stat_statements/pg_stat_statements.c

@@ -397,7 +397,7 @@ _PG_init(void)
 							&pgss_max,
 							5000,
 							100,
-							INT_MAX,
+							INT_MAX / 2,
 							PGC_POSTMASTER,
 							0,
 							NULL,
@@ -2086,6 +2086,18 @@ qtext_store(const char *query, int query_len,
 
 	*query_offset = off;
 
+	/*
+	 * Don't allow the file to grow larger than what qtext_load_file can
+	 * (theoretically) handle.  This has been seen to be reachable on 32-bit
+	 * platforms.
+	 */
+	if (unlikely(query_len >= MaxAllocHugeSize - off))
+	{
+		errno = EFBIG;			/* not quite right, but it'll do */
+		fd = -1;
+		goto error;
+	}
+
 	/* Now write the data into the successfully-reserved part of the file */
 	fd = OpenTransientFile(PGSS_TEXT_FILE, O_RDWR | O_CREAT | PG_BINARY);
 	if (fd < 0)
@@ -2271,8 +2283,14 @@ need_gc_qtexts(void)
 		SpinLockRelease(&s->mutex);
 	}
 
-	/* Don't proceed if file does not exceed 512 bytes per possible entry */
-	if (extent < 512 * pgss_max)
+	/*
+	 * Don't proceed if file does not exceed 512 bytes per possible entry.
+	 *
+	 * Here and in the next test, 32-bit machines have overflow hazards if
+	 * pgss_max and/or mean_query_len are large.  Force the multiplications
+	 * and comparisons to be done in uint64 arithmetic to forestall trouble.
+	 */
+	if ((uint64) extent < (uint64) 512 * pgss_max)
 		return false;
 
 	/*
@@ -2282,7 +2300,7 @@ need_gc_qtexts(void)
 	 * query length in order to prevent garbage collection from thrashing
 	 * uselessly.
 	 */
-	if (extent < pgss->mean_query_len * pgss_max * 2)
+	if ((uint64) extent < (uint64) pgss->mean_query_len * pgss_max * 2)
 		return false;
 
 	return true;