pg_walinspect: pg_get_wal_fpi_info() -> pg_get_wal_block_info()

This commit reworks pg_get_wal_fpi_info() to become aware of all the
block information that can be attached to a record rather than just its
full-page writes:
- Addition of the block id as assigned by XLogRegisterBuffer(),
XLogRegisterBlock() or XLogRegisterBufData().
- Addition of the block data, as bytea, or NULL if none.  The length of
the block data can be guessed with length(), so there is no need to
store its length in a separate field.
- Addition of the full-page image length, as counted without a hole or
even compressed.
- Modification of the handling of the full-page image data.  This is
still a bytea, but it could become NULL if none is assigned to a block.
- Addition of the full-page image flags, tracking if a page is stored
with a hole, if it needs to be applied and the type of compression
applied to it, as of all the BKPIMAGE_* values in xlogrecord.h.

The information of each block is returned as one single record, with the
record's ReadRecPtr included to be able to join the block information
with the existing pg_get_wal_records_info().  Note that it is perfectly
possible for a block to hold both data and full-page image.

Thanks also to Kyotaro Horiguchi and Matthias van de Meent for the
discussion.

This commit uses some of the work proposed by Melanie, though it has
been largely redesigned and rewritten by me.  Bharath has helped in
refining a bit the whole.

Reported-by: Melanie Plageman
Author: Michael Paquier, Melanie Plageman, Bharath Rupireddy
Discussion: https://postgr.es/m/CAAKRu_bORebdZmcV8V4cZBzU8M_C6tDDdbiPhCZ6i-iuSXW9TA@mail.gmail.com

contrib/pg_walinspect/sql/pg_walinspect.sql

@@ -53,20 +53,25 @@ SELECT COUNT(*) >= 1 AS ok FROM pg_get_wal_records_info(:'wal_lsn1', :'wal_lsn2'
 			WHERE resource_manager = 'Heap' AND record_type = 'INSERT';
 
 -- ===================================================================
--- Tests to get full page image (FPI) from WAL record
+-- Tests to get block information from WAL record
 -- ===================================================================
+
+-- Update table to generate some block data
 SELECT pg_current_wal_lsn() AS wal_lsn3 \gset
-
--- Force FPI on the next update.
-CHECKPOINT;
-
--- Update table to generate an FPI.
-UPDATE sample_tbl SET col1 = col1 * 100 WHERE col1 = 1;
+UPDATE sample_tbl SET col1 = col1 + 1 WHERE col1 = 1;
 SELECT pg_current_wal_lsn() AS wal_lsn4 \gset
+-- Check if we get block data from WAL record.
+SELECT COUNT(*) >= 1 AS ok FROM pg_get_wal_block_info(:'wal_lsn3', :'wal_lsn4')
+  WHERE relfilenode = :'sample_tbl_oid' AND blockdata IS NOT NULL;
 
+-- Force full-page image on the next update.
+SELECT pg_current_wal_lsn() AS wal_lsn5 \gset
+CHECKPOINT;
+UPDATE sample_tbl SET col1 = col1 + 1 WHERE col1 = 2;
+SELECT pg_current_wal_lsn() AS wal_lsn6 \gset
 -- Check if we get FPI from WAL record.
-SELECT COUNT(*) >= 1 AS ok FROM pg_get_wal_fpi_info(:'wal_lsn3', :'wal_lsn4')
-  WHERE relfilenode = :'sample_tbl_oid';
+SELECT COUNT(*) >= 1 AS ok FROM pg_get_wal_block_info(:'wal_lsn5', :'wal_lsn6')
+  WHERE relfilenode = :'sample_tbl_oid' AND fpi IS NOT NULL;
 
 -- ===================================================================
 -- Tests for permissions
@@ -83,7 +88,7 @@ SELECT has_function_privilege('regress_pg_walinspect',
   'pg_get_wal_stats(pg_lsn, pg_lsn, boolean) ', 'EXECUTE'); -- no
 
 SELECT has_function_privilege('regress_pg_walinspect',
-  'pg_get_wal_fpi_info(pg_lsn, pg_lsn) ', 'EXECUTE'); -- no
+  'pg_get_wal_block_info(pg_lsn, pg_lsn) ', 'EXECUTE'); -- no
 
 -- Functions accessible by users with role pg_read_server_files
 
@@ -99,7 +104,7 @@ SELECT has_function_privilege('regress_pg_walinspect',
   'pg_get_wal_stats(pg_lsn, pg_lsn, boolean) ', 'EXECUTE'); -- yes
 
 SELECT has_function_privilege('regress_pg_walinspect',
-  'pg_get_wal_fpi_info(pg_lsn, pg_lsn) ', 'EXECUTE'); -- yes
+  'pg_get_wal_block_info(pg_lsn, pg_lsn) ', 'EXECUTE'); -- yes
 
 REVOKE pg_read_server_files FROM regress_pg_walinspect;
 
@@ -113,7 +118,7 @@ GRANT EXECUTE ON FUNCTION pg_get_wal_records_info(pg_lsn, pg_lsn)
 GRANT EXECUTE ON FUNCTION pg_get_wal_stats(pg_lsn, pg_lsn, boolean)
   TO regress_pg_walinspect;
 
-GRANT EXECUTE ON FUNCTION pg_get_wal_fpi_info(pg_lsn, pg_lsn)
+GRANT EXECUTE ON FUNCTION pg_get_wal_block_info(pg_lsn, pg_lsn)
   TO regress_pg_walinspect;
 
 SELECT has_function_privilege('regress_pg_walinspect',
@@ -126,7 +131,7 @@ SELECT has_function_privilege('regress_pg_walinspect',
   'pg_get_wal_stats(pg_lsn, pg_lsn, boolean) ', 'EXECUTE'); -- yes
 
 SELECT has_function_privilege('regress_pg_walinspect',
-  'pg_get_wal_fpi_info(pg_lsn, pg_lsn) ', 'EXECUTE'); -- yes
+  'pg_get_wal_block_info(pg_lsn, pg_lsn) ', 'EXECUTE'); -- yes
 
 REVOKE EXECUTE ON FUNCTION pg_get_wal_record_info(pg_lsn)
   FROM regress_pg_walinspect;
@@ -137,7 +142,7 @@ REVOKE EXECUTE ON FUNCTION pg_get_wal_records_info(pg_lsn, pg_lsn)
 REVOKE EXECUTE ON FUNCTION pg_get_wal_stats(pg_lsn, pg_lsn, boolean)
   FROM regress_pg_walinspect;
 
-REVOKE EXECUTE ON FUNCTION pg_get_wal_fpi_info(pg_lsn, pg_lsn)
+REVOKE EXECUTE ON FUNCTION pg_get_wal_block_info(pg_lsn, pg_lsn)
   FROM regress_pg_walinspect;
 
 -- ===================================================================