Dept. of second thoughts: add comments cautioning against using

tglsfdc · tglsfdc · commit 63735ca81577 · 2007-05-02T23:34:48.000Z
ReadOrZeroBuffer to fetch pages from beyond physical EOF.  This would
usually work, but would cause problems for md.c if writes occurred
beyond a segment boundary when the previous segment file hadn't been
fully extended.
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.217 2007/05/02 23:18:03 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.218 2007/05/02 23:34:48 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -21,7 +21,9 @@
  *		cache we don't read it, but just return a zeroed-out buffer.  Useful
  *		when the caller intends to fill the page from scratch, since this
  *		saves I/O and avoids unnecessary failure if the page-on-disk has
- *		corrupt page headers.
+ *		corrupt page headers.  Caution: do not use this to read a page that
+ *		is beyond the relation's current physical EOF; that is likely to
+ *		cause problems in md.c when the page is modified and written out.
  *
  * ReleaseBuffer() -- unpin a buffer
  *
@@ -137,6 +139,9 @@ ReadBuffer(Relation reln, BlockNumber blockNum)
  *		cache already, it's filled with zeros instead of reading it from
  *		disk. The caller is expected to overwrite the whole buffer,
  *		so that the current page contents are not interesting.
+ *		Caution: do not use this to read a page that is beyond the relation's
+ *		current physical EOF; that is likely to cause problems in md.c when
+ *		the page is modified and written out.  P_NEW is OK, though.
  */
 Buffer
 ReadOrZeroBuffer(Relation reln, BlockNumber blockNum)

Original file line number	Diff line number	Diff line change
`@@ -8,7 +8,7 @@`
`8`	`8`	`*`
`9`	`9`	`*`
`10`	`10`	`* IDENTIFICATION`
`11`		`- * $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.217 2007/05/02 23:18:03 tgl Exp $`
	`11`	`+ * $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.218 2007/05/02 23:34:48 tgl Exp $`
`12`	`12`	`*`
`13`	`13`	`*-------------------------------------------------------------------------`
`14`	`14`	`*/`
`@@ -21,7 +21,9 @@`
`21`	`21`	`* cache we don't read it, but just return a zeroed-out buffer. Useful`
`22`	`22`	`* when the caller intends to fill the page from scratch, since this`
`23`	`23`	`* saves I/O and avoids unnecessary failure if the page-on-disk has`
`24`		`- * corrupt page headers.`
	`24`	`+ * corrupt page headers. Caution: do not use this to read a page that`
	`25`	`+ * is beyond the relation's current physical EOF; that is likely to`
	`26`	`+ * cause problems in md.c when the page is modified and written out.`
`25`	`27`	`*`
`26`	`28`	`* ReleaseBuffer() -- unpin a buffer`
`27`	`29`	`*`
`@@ -137,6 +139,9 @@ ReadBuffer(Relation reln, BlockNumber blockNum)`
`137`	`139`	`* cache already, it's filled with zeros instead of reading it from`
`138`	`140`	`* disk. The caller is expected to overwrite the whole buffer,`
`139`	`141`	`* so that the current page contents are not interesting.`
	`142`	`+ * Caution: do not use this to read a page that is beyond the relation's`
	`143`	`+ * current physical EOF; that is likely to cause problems in md.c when`
	`144`	`+ * the page is modified and written out. P_NEW is OK, though.`
`140`	`145`	`*/`
`141`	`146`	`Buffer`
`142`	`147`	`ReadOrZeroBuffer(Relation reln, BlockNumber blockNum)`