[MERGE 15] YSQL: Fix name conflict in aset realloc

Summary:
    In the master branch, for memory tracking, we added `oldsize` for the block.
    It overwrites the variable with the same name in the outer scope, defined for the chunk.
    However, in PG upstream, the `oldsize` in the outer scope is referenced in the inner scope.
    Because of the naming overwritting, after merging, the reference used becomes incorrect.

    **Solution**
    I basically "backport" how the upstream creates the variable for "block old size" and use
    it is for YB memory tracking.

    Along with this, it also fixes two potential overflow issues in the existing code.
    The fixes are needed in the master as well. So a backport to the PG13 branch will be committed later
    in a separate diff.

Original commit: 83c6b6a/D21128

Test Plan: Jenkins: rebase: pg15

Reviewers: neil, mbautin, telgersma

Reviewed By: telgersma

Subscribers: smishra, yql

Differential Revision: https://phabricator.dev.yugabyte.com/D24000

src/postgres/src/backend/utils/mmgr/aset.c

@@ -1136,7 +1136,6 @@ AllocSetRealloc(MemoryContext context, void *pointer, Size size)
 		AllocBlock	block = (AllocBlock) (((char *) chunk) - ALLOC_BLOCKHDRSZ);
 		Size		chksize;
 		Size		blksize;
-		Size		oldsize = block->endptr - ((char *) block);
 		Size		oldblksize;
 
 		/*
@@ -1176,7 +1175,8 @@ AllocSetRealloc(MemoryContext context, void *pointer, Size size)
 		context->mem_allocated += blksize;
 
 		block->freeptr = block->endptr = ((char *) block) + blksize;
-		YbPgMemAddConsumption(blksize - oldsize);
+		YbPgMemSubConsumption(oldblksize);
+		YbPgMemAddConsumption(blksize);
 
 		/* Update pointers since block has likely been moved */
 		chunk = (AllocChunk) (((char *) block) + ALLOC_BLOCKHDRSZ);

src/postgres/src/backend/utils/mmgr/mcxt.c

@@ -98,7 +98,9 @@ YbPgMemSubConsumption(Size sz)
 		return;
 
 	// Avoid overflow when subtracting sz.
-	PgMemTracker.pg_cur_mem_bytes = Max(PgMemTracker.pg_cur_mem_bytes - sz, 0);
+	PgMemTracker.pg_cur_mem_bytes = PgMemTracker.pg_cur_mem_bytes >= sz ?
+										PgMemTracker.pg_cur_mem_bytes - sz :
+										0;
 	// Only call release if pggate is alive, and update its liveness from the
 	// return value.
 	if (PgMemTracker.pggate_alive)