Add pg_basebackup tool for streaming base backups

This tool makes it possible to do the pg_start_backup/
copy files/pg_stop_backup step in a single command.

There are still some steps to be done before this is a
complete backup solution, such as the ability to stream
the required WAL logs, but it's still usable, and
could do with some buildfarm coverage.

In passing, make the checkpoint request optionally
fast instead of hardcoding it.

Magnus Hagander, reviewed by Fujii Masao and Dimitri Fontaine

Author: mhagander

doc/src/sgml/backup.sgml

@@ -812,6 +812,16 @@ SELECT pg_stop_backup();
   </orderedlist>
    </para>
 
+   <para>
+    You can also use the <xref linkend="app-pgbasebackup"> tool to take
+    the backup, instead of manually copying the files. This tool will take
+    care of the <function>pg_start_backup()</>, copy and
+    <function>pg_stop_backup()</> steps automatically, and transfers the
+    backup over a regular <productname>PostgreSQL</productname> connection
+    using the replication protocol, instead of requiring filesystem level
+    access.
+   </para>
+
    <para>
     Some file system backup tools emit warnings or errors
     if the files they are trying to copy change while the copy proceeds.

doc/src/sgml/config.sgml

@@ -1921,7 +1921,8 @@ SET ENABLE_SEQSCAN TO OFF;
        <listitem>
        <para>
         Specifies the maximum number of concurrent connections from standby
-        servers (i.e., the maximum number of simultaneously running WAL sender
+        servers or streaming base backup clients (i.e., the maximum number of
+        simultaneously running WAL sender
         processes). The default is zero. This parameter can only be set at
         server start. <varname>wal_level</> must be set to <literal>archive</>
         or <literal>hot_standby</> to allow connections from standby servers.

doc/src/sgml/protocol.sgml

@@ -1460,7 +1460,7 @@ The commands accepted in walsender mode are:
   </varlistentry>
 
   <varlistentry>
-    <term>BASE_BACKUP [<literal>LABEL</literal> <replaceable>'label'</replaceable>] [<literal>PROGRESS</literal>]</term>
+    <term>BASE_BACKUP [<literal>LABEL</literal> <replaceable>'label'</replaceable>] [<literal>PROGRESS</literal>] [<literal>FAST</literal>]</term>
     <listitem>
      <para>
       Instructs the server to start streaming a base backup.
@@ -1496,6 +1496,15 @@ The commands accepted in walsender mode are:
          </para>
         </listitem>
        </varlistentry>
+
+       <varlistentry>
+        <term><literal>FAST</></term>
+        <listitem>
+         <para>
+          Request a fast checkpoint.
+         </para>
+        </listitem>
+       </varlistentry>
       </variablelist>
      </para>
      <para>

doc/src/sgml/ref/allfiles.sgml

@@ -160,6 +160,7 @@ Complete list of usable sgml source files in this directory.
 <!entity dropuser           system "dropuser.sgml">
 <!entity ecpgRef            system "ecpg-ref.sgml">
 <!entity initdb             system "initdb.sgml">
+<!entity pgBasebackup       system "pg_basebackup.sgml">
 <!entity pgConfig           system "pg_config-ref.sgml">
 <!entity pgControldata      system "pg_controldata.sgml">
 <!entity pgCtl              system "pg_ctl-ref.sgml">