v23-0017-Hard-code-TBMIterateResult-offsets-array-size.patch

From e691bf84f837d0875cc98e6d2af9852e8a4a346f Mon Sep 17 00:00:00 2001
From: Melanie Plageman <melanieplageman@gmail.com>
Date: Wed, 19 Jun 2024 11:42:12 -0400
Subject: [PATCH v23 17/19] Hard-code TBMIterateResult offsets array size

TIDBitmap's TBMIterateResult had a flexible sized array of tuple offsets
but the API always allocated MAX_TUPLES_PER_PAGE OffsetNumbers. Creating
a fixed-size aray of size MAX_TUPLES_PER_PAGE is more clear for the API
user.
---
 src/backend/nodes/tidbitmap.c | 19 ++++---------------
 src/include/nodes/tidbitmap.h | 12 ++++++++++--
 2 files changed, 14 insertions(+), 17 deletions(-)

diff --git a/src/backend/nodes/tidbitmap.c b/src/backend/nodes/tidbitmap.c
index d035de7f95..4dd49fd48a 100644
--- a/src/backend/nodes/tidbitmap.c
+++ b/src/backend/nodes/tidbitmap.c
@@ -40,7 +40,6 @@
 
 #include <limits.h>
 
-#include "access/htup_details.h"
 #include "common/hashfn.h"
 #include "common/int.h"
 #include "nodes/bitmapset.h"
@@ -48,14 +47,6 @@
 #include "storage/lwlock.h"
 #include "utils/dsa.h"
 
-/*
- * The maximum number of tuples per page is not large (typically 256 with
- * 8K pages, or 1024 with 32K pages).  So there's not much point in making
- * the per-page bitmaps variable size.  We just legislate that the size
- * is this:
- */
-#define MAX_TUPLES_PER_PAGE  MaxHeapTuplesPerPage
-
 /*
  * When we have to switch over to lossy storage, we use a data structure
  * with one bit per page, where all pages having the same number DIV
@@ -181,7 +172,7 @@ struct TBMPrivateIterator
 	int			spageptr;		/* next spages index */
 	int			schunkptr;		/* next schunks index */
 	int			schunkbit;		/* next bit to check in current schunk */
-	TBMIterateResult output;	/* MUST BE LAST (because variable-size) */
+	TBMIterateResult output;
 };
 
 /*
@@ -222,7 +213,7 @@ struct TBMSharedIterator
 	PTEntryArray *ptbase;		/* pagetable element array */
 	PTIterationArray *ptpages;	/* sorted exact page index list */
 	PTIterationArray *ptchunks; /* sorted lossy page index list */
-	TBMIterateResult output;	/* MUST BE LAST (because variable-size) */
+	TBMIterateResult output;
 };
 
 /* Local function prototypes */
@@ -696,8 +687,7 @@ tbm_begin_private_iterate(TIDBitmap *tbm)
 	 * Create the TBMPrivateIterator struct, with enough trailing space to
 	 * serve the needs of the TBMIterateResult sub-struct.
 	 */
-	iterator = (TBMPrivateIterator *) palloc(sizeof(TBMPrivateIterator) +
-											 MAX_TUPLES_PER_PAGE * sizeof(OffsetNumber));
+	iterator = palloc(sizeof(TBMPrivateIterator));
 	iterator->tbm = tbm;
 
 	/*
@@ -1467,8 +1457,7 @@ tbm_attach_shared_iterate(dsa_area *dsa, dsa_pointer dp)
 	 * Create the TBMSharedIterator struct, with enough trailing space to
 	 * serve the needs of the TBMIterateResult sub-struct.
 	 */
-	iterator = (TBMSharedIterator *) palloc0(sizeof(TBMSharedIterator) +
-											 MAX_TUPLES_PER_PAGE * sizeof(OffsetNumber));
+	iterator = (TBMSharedIterator *) palloc0(sizeof(TBMSharedIterator));
 
 	istate = (TBMSharedIteratorState *) dsa_get_address(dsa, dp);
 
diff --git a/src/include/nodes/tidbitmap.h b/src/include/nodes/tidbitmap.h
index 9dbc7bab7a..c108e4f3d5 100644
--- a/src/include/nodes/tidbitmap.h
+++ b/src/include/nodes/tidbitmap.h
@@ -22,9 +22,17 @@
 #ifndef TIDBITMAP_H
 #define TIDBITMAP_H
 
+#include "access/htup_details.h"
 #include "storage/itemptr.h"
 #include "utils/dsa.h"
 
+/*
+ * The maximum number of tuples per page is not large (typically 256 with
+ * 8K pages, or 1024 with 32K pages).  So there's not much point in making
+ * the per-page bitmaps variable size.  We just legislate that the size
+ * is this:
+ */
+#define MAX_TUPLES_PER_PAGE  MaxHeapTuplesPerPage
 
 /*
  * Actual bitmap representation is private to tidbitmap.c.  Callers can
@@ -52,9 +60,9 @@ typedef struct TBMIterateResult
 {
 	BlockNumber blockno;		/* page number containing tuples */
 	int			ntuples;		/* -1 indicates lossy result */
-	bool		recheck;		/* should the tuples be rechecked? */
 	/* Note: recheck is always true if ntuples < 0 */
-	OffsetNumber offsets[FLEXIBLE_ARRAY_MEMBER];
+	bool		recheck;		/* should the tuples be rechecked? */
+	OffsetNumber offsets[MAX_TUPLES_PER_PAGE];
 } TBMIterateResult;
 
 /* function prototypes in nodes/tidbitmap.c */
-- 
2.34.1