Add a txid_status function.

If your connection to the database server is lost while a COMMIT is
in progress, it may be difficult to figure out whether the COMMIT was
successful or not.  This function will tell you, provided that you
don't wait too long to ask.  It may be useful in other situations,
too.

Craig Ringer, reviewed by Simon Riggs and by me

Discussion: http://postgr.es/m/CAMsr+YHQiWNEi0daCTboS40T+V5s_+dst3PYv_8v2wNVH+Xx4g@mail.gmail.com

doc/src/sgml/func.sgml

@@ -17523,6 +17523,10 @@ SELECT collation for ('foo' COLLATE "de_DE");
     <primary>txid_visible_in_snapshot</primary>
    </indexterm>
 
+   <indexterm>
+    <primary>txid_status</primary>
+   </indexterm>
+
    <para>
     The functions shown in <xref linkend="functions-txid-snapshot">
     provide server transaction information in an exportable form.  The main
@@ -17573,6 +17577,11 @@ SELECT collation for ('foo' COLLATE "de_DE");
        <entry><type>boolean</type></entry>
        <entry>is transaction ID visible in snapshot? (do not use with subtransaction ids)</entry>
       </row>
+      <row>
+       <entry><literal><function>txid_status(<parameter>bigint</parameter>)</function></literal></entry>
+       <entry><type>txid_status</type></entry>
+       <entry>report the status of the given xact - <literal>committed</literal>, <literal>aborted</literal>, <literal>in progress</literal>, or NULL if the txid is too old</entry>
+      </row>
      </tbody>
     </tgroup>
    </table>
@@ -17642,6 +17651,24 @@ SELECT collation for ('foo' COLLATE "de_DE");
     <literal>xmin=10, xmax=20, xip_list=10, 14, 15</literal>.
    </para>
 
+   <para>
+    <function>txid_status(bigint)</> reports the commit status of a recent
+    transaction.  Applications may use it to determine whether a transaction
+    committed or aborted when the application and database server become
+    disconnected while a <literal>COMMIT</literal> is in progress.
+    The status of a transaction will be reported as either
+    <literal>in progress</>,
+    <literal>committed</>, or <literal>aborted</>, provided that the
+    transaction is recent enough that the system retains the commit status
+    of that transaction.  If is old enough that no references to that
+    transaction survive in the system and the commit status information has
+    been discarded, this function will return NULL.  Note that prepared
+    transactions are reported as <literal>in progress</>; applications must
+    check <link
+    linkend="view-pg-prepared-xacts"><literal>pg_prepared_xacts</></> if they
+    need to determine whether the txid is a prepared transaction.
+   </para>
+
    <para>
     The functions shown in <xref linkend="functions-commit-timestamp">
     provide information about transactions that have been already committed.

src/backend/utils/adt/txid.c

@@ -21,6 +21,7 @@
 
 #include "postgres.h"
 
+#include "access/clog.h"
 #include "access/transam.h"
 #include "access/xact.h"
 #include "access/xlog.h"
@@ -28,6 +29,7 @@
 #include "miscadmin.h"
 #include "libpq/pqformat.h"
 #include "postmaster/postmaster.h"
+#include "storage/lwlock.h"
 #include "utils/builtins.h"
 #include "utils/memutils.h"
 #include "utils/snapmgr.h"
@@ -92,6 +94,70 @@ load_xid_epoch(TxidEpoch *state)
 	GetNextXidAndEpoch(&state->last_xid, &state->epoch);
 }
 
+/*
+ * Helper to get a TransactionId from a 64-bit xid with wraparound detection.
+ *
+ * It is an ERROR if the xid is in the future.  Otherwise, returns true if
+ * the transaction is still new enough that we can determine whether it
+ * committed and false otherwise.  If *extracted_xid is not NULL, it is set
+ * to the low 32 bits of the transaction ID (i.e. the actual XID, without the
+ * epoch).
+ *
+ * The caller must hold CLogTruncationLock since it's dealing with arbitrary
+ * XIDs, and must continue to hold it until it's done with any clog lookups
+ * relating to those XIDs.
+ */
+static bool
+TransactionIdInRecentPast(uint64 xid_with_epoch, TransactionId *extracted_xid)
+{
+	uint32		xid_epoch = (uint32) (xid_with_epoch >> 32);
+	TransactionId xid = (TransactionId) xid_with_epoch;
+	uint32		now_epoch;
+	TransactionId now_epoch_last_xid;
+
+	GetNextXidAndEpoch(&now_epoch_last_xid, &now_epoch);
+
+	if (extracted_xid != NULL)
+		*extracted_xid = xid;
+
+	if (!TransactionIdIsValid(xid))
+		return false;
+
+	/* For non-normal transaction IDs, we can ignore the epoch. */
+	if (!TransactionIdIsNormal(xid))
+		return true;
+
+	/* If the transaction ID is in the future, throw an error. */
+	if (xid_epoch > now_epoch
+		|| (xid_epoch == now_epoch && xid > now_epoch_last_xid))
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("transaction ID " UINT64_FORMAT " is in the future",
+						xid_with_epoch)));
+
+	/*
+	 * ShmemVariableCache->oldestClogXid is protected by CLogTruncationLock,
+	 * but we don't acquire that lock here.  Instead, we require the caller to
+	 * acquire it, because the caller is presumably going to look up the
+	 * returned XID.  If we took and released the lock within this function, a
+	 * CLOG truncation could occur before the caller finished with the XID.
+	 */
+	Assert(LWLockHeldByMe(CLogTruncationLock));
+
+	/*
+	 * If the transaction ID has wrapped around, it's definitely too old to
+	 * determine the commit status.  Otherwise, we can compare it to
+	 * ShmemVariableCache->oldestClogXid to determine whether the relevant CLOG
+	 * entry is guaranteed to still exist.
+	 */
+	if (xid_epoch + 1 < now_epoch
+		|| (xid_epoch + 1 == now_epoch && xid < now_epoch_last_xid)
+		|| TransactionIdPrecedes(xid, ShmemVariableCache->oldestClogXid))
+		return false;
+
+	return true;
+}
+
 /*
  * do a TransactionId -> txid conversion for an XID near the given epoch
  */
@@ -354,6 +420,9 @@ bad_format:
  *
  *	Return the current toplevel transaction ID as TXID
  *	If the current transaction does not have one, one is assigned.
+ *
+ *	This value has the epoch as the high 32 bits and the 32-bit xid
+ *	as the low 32 bits.
  */
 Datum
 txid_current(PG_FUNCTION_ARGS)
@@ -658,3 +727,66 @@ txid_snapshot_xip(PG_FUNCTION_ARGS)
 		SRF_RETURN_DONE(fctx);
 	}
 }
+
+/*
+ * Report the status of a recent transaction ID, or null for wrapped,
+ * truncated away or otherwise too old XIDs.
+ *
+ * The passed epoch-qualified xid is treated as a normal xid, not a
+ * multixact id.
+ *
+ * If it points to a committed subxact the result is the subxact status even
+ * though the parent xact may still be in progress or may have aborted.
+ */
+Datum
+txid_status(PG_FUNCTION_ARGS)
+{
+	const char	   *status;
+	uint64			xid_with_epoch = PG_GETARG_INT64(0);
+	TransactionId	xid;
+
+	/*
+	 * We must protect against concurrent truncation of clog entries to avoid
+	 * an I/O error on SLRU lookup.
+	 */
+	LWLockAcquire(CLogTruncationLock, LW_SHARED);
+	if (TransactionIdInRecentPast(xid_with_epoch, &xid))
+	{
+		Assert(TransactionIdIsValid(xid));
+
+		if (TransactionIdIsCurrentTransactionId(xid))
+			status = gettext_noop("in progress");
+		else if (TransactionIdDidCommit(xid))
+			status = gettext_noop("committed");
+		else if (TransactionIdDidAbort(xid))
+			status = gettext_noop("aborted");
+		else
+		{
+			/*
+			 * The xact is not marked as either committed or aborted in clog.
+			 *
+			 * It could be a transaction that ended without updating clog or
+			 * writing an abort record due to a crash. We can safely assume
+			 * it's aborted if it isn't committed and is older than our
+			 * snapshot xmin.
+			 *
+			 * Otherwise it must be in-progress (or have been at the time
+			 * we checked commit/abort status).
+			 */
+			if (TransactionIdPrecedes(xid, GetActiveSnapshot()->xmin))
+				status = gettext_noop("aborted");
+			else
+				status = gettext_noop("in progress");
+		}
+	}
+	else
+	{
+		status = NULL;
+	}
+	LWLockRelease(CLogTruncationLock);
+
+	if (status == NULL)
+		PG_RETURN_NULL();
+	else
+		PG_RETURN_TEXT_P(cstring_to_text(status));
+}

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201703231
+#define CATALOG_VERSION_NO	201703241
 
 #endif

src/include/catalog/pg_proc.h

@@ -4978,6 +4978,8 @@ DATA(insert OID = 2947 (  txid_snapshot_xip			PGNSP PGUID 12 1 50 0 0 f f f f t
 DESCR("get set of in-progress txids in snapshot");
 DATA(insert OID = 2948 (  txid_visible_in_snapshot	PGNSP PGUID 12 1  0 0 0 f f f f t f i s 2 0 16 "20 2970" _null_ _null_ _null_ _null_ _null_ txid_visible_in_snapshot _null_ _null_ _null_ ));
 DESCR("is txid visible in snapshot?");
+DATA(insert OID = 3360 (  txid_status				PGNSP PGUID 12 1  0 0 0 f f f f t f v s 1 0 25 "20" _null_ _null_ _null_ _null_ _null_ txid_status _null_ _null_ _null_ ));
+DESCR("commit status of transaction");
 
 /* record comparison using normal comparison rules */
 DATA(insert OID = 2981 (  record_eq		   PGNSP PGUID 12 1 0 0 0 f f f f t f i s 2 0 16 "2249 2249" _null_ _null_ _null_ _null_ _null_ record_eq _null_ _null_ _null_ ));

src/test/recovery/t/011_crash_recovery.pl

@@ -0,0 +1,46 @@
+#
+# Tests relating to PostgreSQL crash recovery and redo
+#
+use strict;
+use warnings;
+use PostgresNode;
+use TestLib;
+use Test::More tests => 3;
+
+my $node = get_new_node('master');
+$node->init(allows_streaming => 1);
+$node->start;
+
+my ($stdin, $stdout, $stderr) = ('', '', '');
+
+# Ensure that txid_status reports 'aborted' for xacts
+# that were in-progress during crash. To do that, we need
+# an xact to be in-progress when we crash and we need to know
+# its xid.
+my $tx = IPC::Run::start(
+	['psql', '-qAt', '-v', 'ON_ERROR_STOP=1', '-f', '-', '-d', $node->connstr('postgres')],
+	'<', \$stdin, '>', \$stdout, '2>', \$stderr);
+$stdin .= q[
+BEGIN;
+CREATE TABLE mine(x integer);
+SELECT txid_current();
+];
+$tx->pump until $stdout =~ /[[:digit:]]+[\r\n]$/;
+
+# Status should be in-progress
+my $xid = $stdout;
+chomp($xid);
+
+is($node->safe_psql('postgres', qq[SELECT txid_status('$xid');]), 'in progress', 'own xid is in-progres');
+
+# Crash and restart the postmaster
+$node->stop('immediate');
+$node->start;
+
+# Make sure we really got a new xid
+cmp_ok($node->safe_psql('postgres', 'SELECT txid_current()'), '>', $xid,
+	'new xid after restart is greater');
+# and make sure we show the in-progress xact as aborted
+is($node->safe_psql('postgres', qq[SELECT txid_status('$xid');]), 'aborted', 'xid is aborted after crash');
+
+$tx->kill_kill;

src/test/regress/expected/txid.out

@@ -254,3 +254,71 @@ SELECT txid_current_if_assigned() IS NOT DISTINCT FROM BIGINT :'txid_current';
 (1 row)
 
 COMMIT;
+-- test xid status functions
+BEGIN;
+SELECT txid_current() AS committed \gset
+COMMIT;
+BEGIN;
+SELECT txid_current() AS rolledback \gset
+ROLLBACK;
+BEGIN;
+SELECT txid_current() AS inprogress \gset
+SELECT txid_status(:committed) AS committed;
+ committed 
+-----------
+ committed
+(1 row)
+
+SELECT txid_status(:rolledback) AS rolledback;
+ rolledback 
+------------
+ aborted
+(1 row)
+
+SELECT txid_status(:inprogress) AS inprogress;
+ inprogress  
+-------------
+ in progress
+(1 row)
+
+SELECT txid_status(1); -- BootstrapTransactionId is always committed
+ txid_status 
+-------------
+ committed
+(1 row)
+
+SELECT txid_status(2); -- FrozenTransactionId is always committed
+ txid_status 
+-------------
+ committed
+(1 row)
+
+SELECT txid_status(3); -- in regress testing FirstNormalTransactionId will always be behind oldestXmin
+ txid_status 
+-------------
+ 
+(1 row)
+
+COMMIT;
+BEGIN;
+CREATE FUNCTION test_future_xid_status(bigint)
+RETURNS void
+LANGUAGE plpgsql
+AS
+$$
+BEGIN
+  PERFORM txid_status($1);
+  RAISE EXCEPTION 'didn''t ERROR at xid in the future as expected';
+EXCEPTION
+  WHEN invalid_parameter_value THEN
+    RAISE NOTICE 'Got expected error for xid in the future';
+END;
+$$;
+SELECT test_future_xid_status(:inprogress + 10000);
+NOTICE:  Got expected error for xid in the future
+ test_future_xid_status 
+------------------------
+ 
+(1 row)
+
+ROLLBACK;

src/test/regress/sql/txid.sql

@@ -59,3 +59,41 @@ SELECT txid_current_if_assigned() IS NULL;
 SELECT txid_current() \gset
 SELECT txid_current_if_assigned() IS NOT DISTINCT FROM BIGINT :'txid_current';
 COMMIT;
+
+-- test xid status functions
+BEGIN;
+SELECT txid_current() AS committed \gset
+COMMIT;
+
+BEGIN;
+SELECT txid_current() AS rolledback \gset
+ROLLBACK;
+
+BEGIN;
+SELECT txid_current() AS inprogress \gset
+
+SELECT txid_status(:committed) AS committed;
+SELECT txid_status(:rolledback) AS rolledback;
+SELECT txid_status(:inprogress) AS inprogress;
+SELECT txid_status(1); -- BootstrapTransactionId is always committed
+SELECT txid_status(2); -- FrozenTransactionId is always committed
+SELECT txid_status(3); -- in regress testing FirstNormalTransactionId will always be behind oldestXmin
+
+COMMIT;
+
+BEGIN;
+CREATE FUNCTION test_future_xid_status(bigint)
+RETURNS void
+LANGUAGE plpgsql
+AS
+$$
+BEGIN
+  PERFORM txid_status($1);
+  RAISE EXCEPTION 'didn''t ERROR at xid in the future as expected';
+EXCEPTION
+  WHEN invalid_parameter_value THEN
+    RAISE NOTICE 'Got expected error for xid in the future';
+END;
+$$;
+SELECT test_future_xid_status(:inprogress + 10000);
+ROLLBACK;