Hi all
Following on from
bigint txids vs 'xid' type, new txid_recent(bigint) => xid
https://www.postgresql.org/message-id/camsr+yfdzmn_iz7krroe+j0kvlqvfvgvzxbcvxr-mljgtoz...@mail.gmail.com
here's a patch that implements a txid_status(bigint) function to report the
commit status of a function.
If an application is disconnected while a COMMIT request is in flight,
the backend crashes mid-commit, etc, then an application may not be
sure whether or not a commit completed successfully or was rolled
back. While two-phase commit solves this it does so at a considerable
overhead, so introduce a lighter alternative.
txid_status(bigint) lets an application determine the status of a
a commit based on an xid-with-epoch as returned by txid_current()
or similar. Status may be committed, aborted, in-progress (including
prepared xacts) or null if the xact is too old for its commit status
to still be retained because it has passed the wrap-around epoch
boundary.
Applications must call txid_current() in their transactions to make
much use of this since PostgreSQL does not automatically report an xid
to the client when one is assigned.
A future protocol enhancement to report txid assignment would be very
useful, but quite separate to this.
--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
From a298bb242716fbf1526bf9f949a94740a9d975fc Mon Sep 17 00:00:00 2001
From: Craig Ringer <cr...@2ndquadrant.com>
Date: Fri, 19 Aug 2016 14:44:15 +0800
Subject: [PATCH 1/5] Introduce txid_status(bigint) to get status of an xact
If an appliation is disconnected while a COMMIT request is in flight,
the backend crashes mid-commit, etc, then an application may not be
sure whether or not a commit completed successfully or was rolled
back. While two-phase commit solves this it does so at a considerable
overhead, so introduce a lighter alternative.
txid_status(bigint) lets an application determine the status of a
a commit based on an xid-with-epoch as returned by txid_current()
or similar. Status may be committed, aborted, in-progress (including
prepared xacts) or null if the xact is too old for its commit status
to still be retained because it has passed the wrap-around epoch
boundary.
Applications must call txid_current() in their transactions to make
much use of this since PostgreSQL does not automatically report an xid
to the client when one is assigned.
---
doc/src/sgml/func.sgml | 11 ++++-
src/backend/access/transam/clog.c | 23 ----------
src/backend/catalog/system_views.sql | 20 +++++++++
src/backend/utils/adt/txid.c | 82 ++++++++++++++++++++++++++++++++++++
src/include/access/clog.h | 23 ++++++++++
src/include/catalog/pg_proc.h | 2 +
src/test/regress/expected/txid.out | 50 ++++++++++++++++++++++
src/test/regress/sql/txid.sql | 36 ++++++++++++++++
8 files changed, 223 insertions(+), 24 deletions(-)
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 7830334..c2a0aeb 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -16911,6 +16911,10 @@ 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
@@ -16929,7 +16933,7 @@ SELECT collation for ('foo' COLLATE "de_DE");
<row>
<entry><literal><function>txid_current()</function></literal></entry>
<entry><type>bigint</type></entry>
- <entry>get current transaction ID, assigning a new one if the current transaction does not have one</entry>
+ <entry>get current 64-bit transaction ID with epoch, assigning a new one if the current transaction does not have one</entry>
</row>
<row>
<entry><literal><function>txid_current_snapshot()</function></literal></entry>
@@ -16956,6 +16960,11 @@ 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 - committed, aborted, in-progress, or null if the xid is too old</entry>
+ </row>
</tbody>
</tgroup>
</table>
diff --git a/src/backend/access/transam/clog.c b/src/backend/access/transam/clog.c
index 2634476..1a6e26d 100644
--- a/src/backend/access/transam/clog.c
+++ b/src/backend/access/transam/clog.c
@@ -41,29 +41,6 @@
#include "miscadmin.h"
#include "pg_trace.h"
-/*
- * Defines for CLOG page sizes. A page is the same BLCKSZ as is used
- * everywhere else in Postgres.
- *
- * Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF,
- * CLOG page numbering also wraps around at 0xFFFFFFFF/CLOG_XACTS_PER_PAGE,
- * and CLOG segment numbering at
- * 0xFFFFFFFF/CLOG_XACTS_PER_PAGE/SLRU_PAGES_PER_SEGMENT. We need take no
- * explicit notice of that fact in this module, except when comparing segment
- * and page numbers in TruncateCLOG (see CLOGPagePrecedes).
- */
-
-/* We need two bits per xact, so four xacts fit in a byte */
-#define CLOG_BITS_PER_XACT 2
-#define CLOG_XACTS_PER_BYTE 4
-#define CLOG_XACTS_PER_PAGE (BLCKSZ * CLOG_XACTS_PER_BYTE)
-#define CLOG_XACT_BITMASK ((1 << CLOG_BITS_PER_XACT) - 1)
-
-#define TransactionIdToPage(xid) ((xid) / (TransactionId) CLOG_XACTS_PER_PAGE)
-#define TransactionIdToPgIndex(xid) ((xid) % (TransactionId) CLOG_XACTS_PER_PAGE)
-#define TransactionIdToByte(xid) (TransactionIdToPgIndex(xid) / CLOG_XACTS_PER_BYTE)
-#define TransactionIdToBIndex(xid) ((xid) % (TransactionId) CLOG_XACTS_PER_BYTE)
-
/* We store the latest async LSN for each group of transactions */
#define CLOG_XACTS_PER_LSN_GROUP 32 /* keep this a power of 2 */
#define CLOG_LSNS_PER_PAGE (CLOG_XACTS_PER_PAGE / CLOG_XACTS_PER_LSN_GROUP)
diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index 4fc5d5a..4194195 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -1008,6 +1008,26 @@ LANGUAGE INTERNAL
STRICT IMMUTABLE PARALLEL SAFE
AS 'jsonb_insert';
+CREATE TYPE txid_status AS ENUM ('committed', 'in-progress', 'aborted');
+
+CREATE FUNCTION
+ txid_status(txid bigint)
+RETURNS txid_status
+LANGUAGE sql
+VOLATILE PARALLEL SAFE
+AS $$
+SELECT CASE
+ WHEN s IS NULL THEN NULL::txid_status
+ WHEN s = -1 THEN 'aborted'::txid_status
+ WHEN s = 0 THEN 'in-progress'::txid_status
+ WHEN s = 1 THEN 'committed'::txid_status
+END
+FROM pg_catalog.txid_status_internal($1) s;
+$$;
+
+COMMENT ON FUNCTION txid_status(bigint)
+IS 'get commit status of given recent xid or null if too old';
+
-- The default permissions for functions mean that anyone can execute them.
-- A number of functions shouldn't be executable by just anyone, but rather
-- than use explicit 'superuser()' checks in those functions, we use the GRANT
diff --git a/src/backend/utils/adt/txid.c b/src/backend/utils/adt/txid.c
index c2069a9..0480c99 100644
--- a/src/backend/utils/adt/txid.c
+++ b/src/backend/utils/adt/txid.c
@@ -21,9 +21,11 @@
#include "postgres.h"
+#include "access/clog.h"
#include "access/transam.h"
#include "access/xact.h"
#include "access/xlog.h"
+#include "catalog/pg_proc.h"
#include "funcapi.h"
#include "miscadmin.h"
#include "libpq/pqformat.h"
@@ -117,6 +119,50 @@ convert_xid(TransactionId xid, const TxidEpoch *state)
}
/*
+ * Helper to get a TransactionId from a 64-bit txid with wraparound detection.
+ * ERRORs if the txid is in the future. Returns permanent XIDs unchanged.
+ * Otherwise returns the 32-bit xid and sets the wraparound param to true
+ * if wraparound is detected, false otherwise.
+ */
+static TransactionId
+get_xid_in_recent_past(txid xid_with_epoch, bool *wraparound)
+{
+ uint32 xid_epoch = (uint32)(xid_with_epoch >>32);
+ TransactionId xid = (TransactionId)(xid_with_epoch);
+ TxidEpoch now_epoch;
+
+ load_xid_epoch(&now_epoch);
+
+ *wraparound = false;
+
+ if (!TransactionIdIsNormal(xid))
+ {
+ /* must be a permanent XID, ignore the epoch and return unchanged */
+ return xid;
+ }
+ else if (xid_epoch > now_epoch.epoch
+ || (xid_epoch == now_epoch.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)));
+ }
+ else if (xid_epoch + 1 < now_epoch.epoch
+ || (xid_epoch + 1 == now_epoch.epoch && xid < now_epoch.last_xid))
+ {
+ /* xid too far in the past */
+ *wraparound = true;
+ }
+ else
+ {
+ Assert(TransactionIdPrecedesOrEquals(xid, now_epoch.last_xid));
+ }
+
+ return xid;
+}
+
+/*
* txid comparator for qsort/bsearch
*/
static int
@@ -354,6 +400,9 @@ 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)
@@ -637,3 +686,36 @@ txid_snapshot_xip(PG_FUNCTION_ARGS)
SRF_RETURN_DONE(fctx);
}
}
+
+/*
+ * Underlying implementation of txid_status, which is mapped to an enum in
+ * system_views.sql.
+ */
+Datum
+txid_status_internal(PG_FUNCTION_ARGS)
+{
+ bool wraparound;
+ uint64 xid_with_epoch = PG_GETARG_INT64(0);
+ TransactionId xid = get_xid_in_recent_past(xid_with_epoch, &wraparound);
+
+ if (!TransactionIdIsValid(xid))
+ {
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("transaction ID "UINT64_FORMAT" is an invalid xid",
+ xid_with_epoch)));
+ }
+
+ if (wraparound)
+ PG_RETURN_NULL();
+ else if (TransactionIdIsCurrentTransactionId(xid) || TransactionIdIsInProgress(xid))
+ PG_RETURN_INT32(0);
+ else if (TransactionIdDidCommit(xid))
+ PG_RETURN_INT32(1);
+ else if (TransactionIdDidAbort(xid))
+ PG_RETURN_INT32(-1);
+ else
+ /* shouldn't happen */
+ ereport(ERROR,
+ (errmsg_internal("unable to determine commit status of xid "UINT64_FORMAT, xid)));
+}
diff --git a/src/include/access/clog.h b/src/include/access/clog.h
index 06c069a..a763dfb 100644
--- a/src/include/access/clog.h
+++ b/src/include/access/clog.h
@@ -28,6 +28,29 @@ typedef int XidStatus;
#define TRANSACTION_STATUS_ABORTED 0x02
#define TRANSACTION_STATUS_SUB_COMMITTED 0x03
+/*
+ * Defines for CLOG page sizes. A page is the same BLCKSZ as is used
+ * everywhere else in Postgres.
+ *
+ * Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF,
+ * CLOG page numbering also wraps around at 0xFFFFFFFF/CLOG_XACTS_PER_PAGE,
+ * and CLOG segment numbering at
+ * 0xFFFFFFFF/CLOG_XACTS_PER_PAGE/SLRU_PAGES_PER_SEGMENT. We need take no
+ * explicit notice of that fact in this module, except when comparing segment
+ * and page numbers in TruncateCLOG (see CLOGPagePrecedes).
+ */
+
+/* We need two bits per xact, so four xacts fit in a byte */
+#define CLOG_BITS_PER_XACT 2
+#define CLOG_XACTS_PER_BYTE 4
+#define CLOG_XACTS_PER_PAGE (BLCKSZ * CLOG_XACTS_PER_BYTE)
+#define CLOG_XACT_BITMASK ((1 << CLOG_BITS_PER_XACT) - 1)
+
+#define TransactionIdToPage(xid) ((xid) / (TransactionId) CLOG_XACTS_PER_PAGE)
+#define TransactionIdToPgIndex(xid) ((xid) % (TransactionId) CLOG_XACTS_PER_PAGE)
+#define TransactionIdToByte(xid) (TransactionIdToPgIndex(xid) / CLOG_XACTS_PER_BYTE)
+#define TransactionIdToBIndex(xid) ((xid) % (TransactionId) CLOG_XACTS_PER_BYTE)
+
extern void TransactionIdSetTreeStatus(TransactionId xid, int nsubxids,
TransactionId *subxids, XidStatus status, XLogRecPtr lsn);
diff --git a/src/include/catalog/pg_proc.h b/src/include/catalog/pg_proc.h
index 270dd21..b0b25e6 100644
--- a/src/include/catalog/pg_proc.h
+++ b/src/include/catalog/pg_proc.h
@@ -4903,6 +4903,8 @@ 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 = 3346 ( txid_status_internal PGNSP PGUID 12 1 0 0 0 f f f f t f v s 1 0 23 "20" _null_ _null_ _null_ _null_ _null_ txid_status_internal _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_ ));
diff --git a/src/test/regress/expected/txid.out b/src/test/regress/expected/txid.out
index ddd217e..61299c5 100644
--- a/src/test/regress/expected/txid.out
+++ b/src/test/regress/expected/txid.out
@@ -238,3 +238,53 @@ SELECT txid_snapshot '1:9223372036854775808:3';
ERROR: invalid input syntax for type txid_snapshot: "1:9223372036854775808:3"
LINE 1: SELECT txid_snapshot '1:9223372036854775808:3';
^
+-- 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)
+
+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;
diff --git a/src/test/regress/sql/txid.sql b/src/test/regress/sql/txid.sql
index b6650b9..50d4461 100644
--- a/src/test/regress/sql/txid.sql
+++ b/src/test/regress/sql/txid.sql
@@ -52,3 +52,39 @@ select txid_visible_in_snapshot('1000100010001015', '1000100010001000:1000100010
-- test 64bit overflow
SELECT txid_snapshot '1:9223372036854775807:3';
SELECT txid_snapshot '1:9223372036854775808:3';
+
+-- 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;
+
+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;
+
--
2.5.5
--
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