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

doc/src/sgml/pgwalinspect.sgml

@@ -190,31 +190,39 @@ combined_size_percentage     | 2.8634072910530795
 
    <varlistentry>
     <term>
-     <function>pg_get_wal_fpi_info(start_lsn pg_lsn, end_lsn pg_lsn) returns setof record</function>
+     <function>pg_get_wal_block_info(start_lsn pg_lsn, end_lsn pg_lsn) returns setof record</function>
     </term>
 
     <listitem>
      <para>
-      Gets a copy of full page images as <type>bytea</type> values (after
-      applying decompression when necessary) and their information associated
-      with all the valid WAL records between
+      Gets a copy of the block information stored in WAL records. This includes
+      copies of the block data (<literal>NULL</literal> if none) and full page
+      images as <type>bytea</type> values (after
+      applying decompression when necessary, or <literal>NULL</literal> if none)
+      and their information associated with all the valid WAL records between
       <replaceable>start_lsn</replaceable> and
-      <replaceable>end_lsn</replaceable>. Returns one row per full page image.
-      If <replaceable>start_lsn</replaceable> or
+      <replaceable>end_lsn</replaceable>. Returns one row per block registered
+      in a WAL record. If <replaceable>start_lsn</replaceable> or
       <replaceable>end_lsn</replaceable> are not yet available, the function
       will raise an error. For example:
 <screen>
-postgres=# SELECT lsn, reltablespace, reldatabase, relfilenode, relblocknumber,
-                  forkname, substring(fpi for 24) as fpi_trimmed
-             FROM pg_get_wal_fpi_info('0/1801690', '0/1825C60');
+postgres=# SELECT lsn, blockid, reltablespace, reldatabase, relfilenode,
+                  relblocknumber, forkname,
+                  substring(blockdata for 24) as block_trimmed,
+                  substring(fpi for 24) as fpi_trimmed, fpilen, fpiinfo
+             FROM pg_get_wal_block_info('0/1871080', '0/1871440');
 -[ RECORD 1 ]--+---------------------------------------------------
-lsn            | 0/1807E20
+lsn            | 0/18712F8
+blockid        | 0
 reltablespace  | 1663
-reldatabase    | 5
-relfilenode    | 16396
-relblocknumber | 43
+reldatabase    | 16384
+relfilenode    | 16392
+relblocknumber | 0
 forkname       | main
-fpi_trimmed    | \x00000000b89e660100000000a003c0030020042000000000
+block_trimmed  | \x02800128180164000000
+fpi_trimmed    | \x0000000050108701000000002c00601f00200420e0020000
+fpilen         | 204
+fpiinfo        | {HAS_HOLE,APPLY}
 </screen>
      </para>
     </listitem>