Make pg_receivexlog and pg_basebackup -X stream work across timeline switches.

This mirrors the changes done earlier to the server in standby mode. When
receivelog reaches the end of a timeline, as reported by the server, it
fetches the timeline history file of the next timeline, and restarts
streaming from the new timeline by issuing a new START_STREAMING command.

When pg_receivexlog crosses a timeline, it leaves the .partial suffix on the
last segment on the old timeline. This helps you to tell apart a partial
segment left in the directory because of a timeline switch, and a completed
segment. If you just follow a single server, it won't make a difference, but
it can be significant in more complicated scenarios where new WAL is still
generated on the old timeline.

This includes two small changes to the streaming replication protocol:
First, when you reach the end of timeline while streaming, the server now
sends the TLI of the next timeline in the server's history to the client.
pg_receivexlog uses that as the next timeline, so that it doesn't need to
parse the timeline history file like a standby server does. Second, when
BASE_BACKUP command sends the begin and end WAL positions, it now also sends
the timeline IDs corresponding the positions.

1 files changed, 10 insertions, 4 deletions

diff --git a/doc/src/sgml/protocol.sgml b/doc/src/sgml/protocol.sgml
index e14627c201e..baae59de6e9 100644
--- a/doc/src/sgml/protocol.sgml
+++ b/doc/src/sgml/protocol.sgml
@@ -1418,8 +1418,10 @@ The commands accepted in walsender mode are:
      <para>
       After streaming all the WAL on a timeline that is not the latest one,
       the server will end streaming by exiting the COPY mode. When the client
-      acknowledges this by also exiting COPY mode, the server responds with a
-      CommandComplete message, and is ready to accept a new command.
+      acknowledges this by also exiting COPY mode, the server sends a
+      single-row, single-column result set indicating the next timeline in
+      this server's history. That is followed by a CommandComplete message,
+      and the server is ready to accept a new command.
      </para>
 
      <para>
@@ -1784,7 +1786,9 @@ The commands accepted in walsender mode are:
      </para>
      <para>
       The first ordinary result set contains the starting position of the
-      backup, given in XLogRecPtr format as a single column in a single row.
+      backup, in a single row with two columns. The first column contains
+      the start position given in XLogRecPtr format, and the second column
+      contains the corresponding timeline ID.
      </para>
      <para>
       The second ordinary result set has one row for each tablespace.
@@ -1827,7 +1831,9 @@ The commands accepted in walsender mode are:
       <quote>ustar interchange format</> specified in the POSIX 1003.1-2008
       standard) dump of the tablespace contents, except that the two trailing
       blocks of zeroes specified in the standard are omitted.
-      After the tar data is complete, a final ordinary result set will be sent.
+      After the tar data is complete, 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>