Modify pg_basebackup to use a new COPY subprotocol for base backups.

In the new approach, all files across all tablespaces are sent in a
single COPY OUT operation. The CopyData messages are no longer raw
archive content; rather, each message is prefixed with a type byte
that describes its purpose, e.g. 'n' signifies the start of a new
archive and 'd' signifies archive or manifest data. This protocol
is significantly more extensible than the old approach, since we can
later create more message types, though not without concern for
backward compatibility.

The new protocol sends a few things to the client that the old one
did not. First, it sends the name of each archive explicitly, instead
of letting the client compute it. This is intended to make it easier
to write future patches that might send archives in a format other
that tar (e.g. cpio, pax, tar.gz). Second, it sends explicit progress
messages rather than allowing the client to assume that progress is
defined by the number of bytes received. This will help with future
features where the server compresses the data, or sends it someplace
directly rather than transmitting it to the client.

The old protocol is still supported for compatibility with previous
releases. The new protocol is selected by means of a new
TARGET option to the BASE_BACKUP command. Currently, the
only supported target is 'client'. Support for additional
targets will be added in a later commit.

Patch by me. The patch set of which this is a part has had review
and/or testing from Jeevan Ladhe, Tushar Ahuja, Suraj Kharage,
Dipesh Pandit, and Mark Dilger.

Discussion: http://postgr.es/m/CA+TgmoaYZbz0=Yk797aOJwkGJC-LK3iXn+wzzMx7KdwNpZhS5g@mail.gmail.com

doc/src/sgml/protocol.sgml

@@ -2630,6 +2630,22 @@ The commands accepted in replication mode are:
         </listitem>
        </varlistentry>
 
+       <varlistentry>
+        <term><literal>TARGET</literal> <replaceable>'target'</replaceable></term>
+        <listitem>
+         <para>
+          Tells the server where to send the backup. If not specified,
+          the legacy base backup protocol will be used. Otherwise, the new
+          protocol will be used, as described below.
+         </para>
+
+         <para>
+          At present, the only supported value for this parameter is
+          <literal>client</literal>.
+         </para>
+        </listitem>
+       </varlistentry>
+
        <varlistentry>
         <term><literal>PROGRESS [ <replaceable class="parameter">boolean</replaceable> ]</literal></term>
         <listitem>
@@ -2805,19 +2821,113 @@ The commands accepted in replication mode are:
 
      <para>
       After the second regular result set, one or more CopyOutResponse results
-      will be sent, one for the main data directory and one for each additional tablespace other
-      than <literal>pg_default</literal> and <literal>pg_global</literal>. The data in
-      the CopyOutResponse results will be a tar format (following the
-      <quote>ustar interchange format</quote> specified in the POSIX 1003.1-2008
-      standard) dump of the tablespace contents. Prior to
+      will be sent. If the <literal>TARGET</literal> option is not specified,
+      the legacy base backup protocol will be used. In this mode,
+      there will be one CopyOutResponse for the main directory, one for each
+      additional tablespace other than <literal>pg_default</literal> and
+      <literal>pg_global</literal>, and one for the backup manifested if
+      requested. The main data directory and any additional tablespaces will
+      be sent in tar format (following the <quote>ustar interchange
+      format</quote> specified in the POSIX 1003.1-2008 standard), and
+      the manifest will sent as a plain file.  Prior to
       <literal>PostgreSQL</literal> 15, the server omitted the two trailing
       blocks of zeroes specified in the standard, but this is no longer the
       case.
-      After the tar data is complete, and if a backup manifest was requested,
-      another CopyOutResponse result is sent, containing the manifest data for the
-      current base backup. In any case, a final ordinary result set will be
-      sent, containing the WAL end position of the backup, in the same format as
-      the start position.
+     </para>
+ 
+     <para>
+      New applications should specify the <literal>TARGET</literal> option.
+      When that option is used, a single CopyOutResponse will be sent, and
+      the payload of each CopyData message will contain a message in one of
+      the following formats:
+     </para>
+
+     <para>
+      <variablelist>
+
+      <varlistentry>
+        <term>new archive (B)</term>
+        <listitem><para><variablelist>
+          <varlistentry>
+            <term>Byte1('n')</term>
+            <listitem><para>
+              Identifes the messaage as indicating the start of a new archive.
+            </para></listitem>
+          </varlistentry>
+          <varlistentry>
+            <term>String</term>
+            <listitem><para>
+              The file name for this archive.
+            </para></listitem>
+          </varlistentry>
+          <varlistentry>
+            <term>String</term>
+            <listitem><para>
+              For the main data directory, an empty string. For other
+              tablespaces, the full path to the directory from which this
+              archive was created.
+            </para></listitem>
+          </varlistentry>
+        </variablelist></para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term>manifest (B)</term>
+        <listitem><para><variablelist>
+          <varlistentry>
+            <term>Byte1('m')</term>
+            <listitem><para>
+              Identifes the message as indicating the start of the backup
+              manifest.
+            </para></listitem>
+          </varlistentry>
+        </variablelist></para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term>archive or manifest data (B)</term>
+        <listitem><para><variablelist>
+          <varlistentry>
+            <term>Byte1('d')</term>
+            <listitem><para>
+              Identifes the message as containing archive or manifest data.
+            </para></listitem>
+          </varlistentry>
+          <varlistentry>
+            <term>Byte<replaceable>n</replaceable></term>
+            <listitem><para>
+              Data bytes.
+            </para></listitem>
+          </varlistentry>
+        </variablelist></para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term>progress report (B)</term>
+        <listitem><para><variablelist>
+          <varlistentry>
+            <term>Byte1('p')</term>
+            <listitem><para>
+              Identifes the message as a progress report.
+            </para></listitem>
+          </varlistentry>
+          <varlistentry>
+            <term>Int64</term>
+            <listitem><para>
+              The number of bytes from the current tablespace for which
+              processing has been completed.
+            </para></listitem>
+          </varlistentry>
+        </variablelist></para></listitem>
+      </varlistentry>
+
+      </variablelist>
+     </para>
+
+     <para>
+      After the CopyOutResponse, or all such responses, have been sent, a
+      final ordinary result set will be sent, containing the WAL end position
+      of the backup, in the same format as the start position.
      </para>
 
      <para>