Handle duplicate XIDs in txid_snapshot.

The proc array can contain duplicate XIDs, when a transaction is just being
prepared for two-phase commit. To cope, remove any duplicates in
txid_current_snapshot(). Also ignore duplicates in the input functions, so
that if e.g. you have an old pg_dump file that already contains duplicates,
it will be accepted.

Report and fix by Jan Wieck. Backpatch to all supported versions.

2 files changed, 7 insertions, 5 deletions

diff --git a/src/test/regress/expected/txid.out b/src/test/regress/expected/txid.out
index 930b86a6562..7750b7b98f9 100644
--- a/src/test/regress/expected/txid.out
+++ b/src/test/regress/expected/txid.out
@@ -12,6 +12,12 @@ select '12:18:14,16'::txid_snapshot;
  12:18:14,16
 (1 row)
 
+select '12:16:14,14'::txid_snapshot;
+ txid_snapshot 
+---------------
+ 12:16:14
+(1 row)
+
 -- errors
 select '31:12:'::txid_snapshot;
 ERROR:  invalid input for txid_snapshot: "31:12:"
@@ -29,10 +35,6 @@ select '12:16:14,13'::txid_snapshot;
 ERROR:  invalid input for txid_snapshot: "12:16:14,13"
 LINE 1: select '12:16:14,13'::txid_snapshot;
                ^
-select '12:16:14,14'::txid_snapshot;
-ERROR:  invalid input for txid_snapshot: "12:16:14,14"
-LINE 1: select '12:16:14,14'::txid_snapshot;
-               ^
 create temp table snapshot_test (
 	nr	integer,
 	snap	txid_snapshot
diff --git a/src/test/regress/sql/txid.sql b/src/test/regress/sql/txid.sql
index ecae10e024d..b6650b922e6 100644
--- a/src/test/regress/sql/txid.sql
+++ b/src/test/regress/sql/txid.sql
@@ -3,13 +3,13 @@
 -- i/o
 select '12:13:'::txid_snapshot;
 select '12:18:14,16'::txid_snapshot;
+select '12:16:14,14'::txid_snapshot;
 
 -- errors
 select '31:12:'::txid_snapshot;
 select '0:1:'::txid_snapshot;
 select '12:13:0'::txid_snapshot;
 select '12:16:14,13'::txid_snapshot;
-select '12:16:14,14'::txid_snapshot;
 
 create temp table snapshot_test (
 	nr	integer,