Add pg_rewind, for re-synchronizing a master server after failback.

Earlier versions of this tool were available (and still are) on github.

Thanks to Michael Paquier, Alvaro Herrera, Peter Eisentraut, Amit Kapila,
and Satoshi Nagayasu for review.

src/bin/pg_rewind/t/001_basic.pl

@@ -0,0 +1,80 @@
+use strict;
+use warnings;
+use TestLib;
+use Test::More tests => 4;
+
+use RewindTest;
+
+my $testmode = shift;
+
+RewindTest::init_rewind_test('basic', $testmode);
+RewindTest::setup_cluster();
+
+# Create a test table and insert a row in master.
+master_psql("CREATE TABLE tbl1 (d text)");
+master_psql("INSERT INTO tbl1 VALUES ('in master')");
+
+# This test table will be used to test truncation, i.e. the table
+# is extended in the old master after promotion
+master_psql("CREATE TABLE trunc_tbl (d text)");
+master_psql("INSERT INTO trunc_tbl VALUES ('in master')");
+
+# This test table will be used to test the "copy-tail" case, i.e. the
+# table is truncated in the old master after promotion
+master_psql("CREATE TABLE tail_tbl (id integer, d text)");
+master_psql("INSERT INTO tail_tbl VALUES (0, 'in master')");
+
+
+master_psql("CHECKPOINT");
+
+RewindTest::create_standby();
+
+# Insert additional data on master that will be replicated to standby
+master_psql("INSERT INTO tbl1 values ('in master, before promotion')");
+master_psql("INSERT INTO trunc_tbl values ('in master, before promotion')");
+master_psql("INSERT INTO tail_tbl SELECT g, 'in master, before promotion: ' || g FROM generate_series(1, 10000) g");
+
+master_psql('CHECKPOINT');
+
+RewindTest::promote_standby();
+
+# Insert a row in the old master. This causes the master and standby
+# to have "diverged", it's no longer possible to just apply the
+# standy's logs over master directory - you need to rewind.
+master_psql("INSERT INTO tbl1 VALUES ('in master, after promotion')");
+
+# Also insert a new row in the standby, which won't be present in the
+# old master.
+standby_psql("INSERT INTO tbl1 VALUES ('in standby, after promotion')");
+
+# Insert enough rows to trunc_tbl to extend the file. pg_rewind should
+# truncate it back to the old size.
+master_psql("INSERT INTO trunc_tbl SELECT 'in master, after promotion: ' || g FROM generate_series(1, 10000) g");
+
+# Truncate tail_tbl. pg_rewind should copy back the truncated part
+# (We cannot use an actual TRUNCATE command here, as that creates a
+# whole new relfilenode)
+master_psql("DELETE FROM tail_tbl WHERE id > 10");
+master_psql("VACUUM tail_tbl");
+
+RewindTest::run_pg_rewind();
+
+check_query('SELECT * FROM tbl1',
+	qq(in master
+in master, before promotion
+in standby, after promotion
+),
+	'table content');
+
+check_query('SELECT * FROM trunc_tbl',
+	qq(in master
+in master, before promotion
+),
+	'truncation');
+
+check_query('SELECT count(*) FROM tail_tbl',
+	qq(10001
+),
+	'tail-copy');
+
+exit(0);