Refuse to return the "dead" pseudo column during recovery

GetOldestXmin() asserts !RecoveryInProgress().
Spotted by Andreas Seltenreich, thanks!

Close #2.

Author: df7cb

README.md

@@ -67,6 +67,8 @@ System Columns
 System columns such as `xmax` and `ctid` can be retrieved by including them in
 the table alias attached to the `pg_dirtyread()` call. A special column `dead` of
 type boolean is available to report dead rows (as by `HeapTupleIsSurelyDead`).
+The `dead` column is not usable during recovery, i.e. most notably not on
+standby servers.
 
   ```sql
     SELECT * FROM pg_dirtyread('foo'::regclass)

debian/changelog

@@ -1,3 +1,11 @@
+pg-dirtyread (1.2) unstable; urgency=medium
+
+  * Refuse to return the "dead" pseudo column during recovery,
+    because GetOldestXmin() asserts !RecoveryInProgress().
+    Spotted by Andreas Seltenreich, thanks!
+
+ -- Christoph Berg <myon@debian.org>  Sun, 06 Aug 2017 16:57:41 +0200
+
 pg-dirtyread (1.1) unstable; urgency=medium
 
   * Initial release.

debian/control

@@ -4,7 +4,7 @@ Priority: optional
 Maintainer: Debian PostgreSQL Maintainers <pkg-postgresql-public@lists.alioth.debian.org>
 Uploaders: Christoph Berg <myon@debian.org>
 Build-Depends: debhelper (>= 9), postgresql-server-dev-all (>= 153~)
-Standards-Version: 3.9.8
+Standards-Version: 4.0.1
 Vcs-Browser: https://github.com/ChristophBerg/pg_dirtyread
 Vcs-Git: https://github.com/ChristophBerg/pg_dirtyread.git
 

debian/control.in

@@ -4,7 +4,7 @@ Priority: optional
 Maintainer: Debian PostgreSQL Maintainers <pkg-postgresql-public@lists.alioth.debian.org>
 Uploaders: Christoph Berg <myon@debian.org>
 Build-Depends: debhelper (>= 9), postgresql-server-dev-all (>= 153~)
-Standards-Version: 3.9.8
+Standards-Version: 4.0.1
 Vcs-Browser: https://github.com/ChristophBerg/pg_dirtyread
 Vcs-Git: https://github.com/ChristophBerg/pg_dirtyread.git
 

dirtyread_tupconvert.c

@@ -25,6 +25,7 @@
 #endif
 #include "access/tupconvert.h"
 #include "access/sysattr.h"
+#include "access/xlog.h" /* RecoveryInProgress */
 #include "catalog/pg_type.h" /* *OID */
 #include "utils/builtins.h"
 #include "utils/tqual.h" /* HeapTupleIsSurelyDead */
@@ -221,6 +222,12 @@ dirtyread_convert_tuples_by_name_map(TupleDesc indesc,
 										   attname,
 										   format_type_be(system_columns[j].atttypid),
 										   format_type_be(indesc->tdtypeid))));
+					/* GetOldestXmin() is not available during recovery */
+					if (system_columns[j].attnum == DeadFakeAttributeNumber &&
+							RecoveryInProgress())
+						ereport(ERROR,
+								(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+								 errmsg("Cannot use \"dead\" column during recovery")));
 					attrMap[i] = system_columns[j].attnum;
 					break;
 				}

pg_dirtyread.c

@@ -42,6 +42,7 @@
 #if PG_VERSION_NUM >= 90300
 #include "access/htup_details.h"
 #endif
+#include "access/xlog.h" /* RecoveryInProgress */
 #include "miscadmin.h" /* superuser */
 #include "storage/procarray.h" /* GetOldestXmin */
 
@@ -97,7 +98,9 @@ pg_dirtyread(PG_FUNCTION_ARGS)
         usr_ctx->map = dirtyread_convert_tuples_by_name(usr_ctx->reltupdesc,
                 funcctx->tuple_desc, "Error converting tuple descriptors!");
         usr_ctx->scan = heap_beginscan(usr_ctx->rel, SnapshotAny, 0, NULL);
-        usr_ctx->oldest_xmin = GetOldestXmin(
+        /* only call GetOldestXmin while not in recovery */
+        if (!RecoveryInProgress())
+            usr_ctx->oldest_xmin = GetOldestXmin(
 #if PG_VERSION_NUM >= 90400
                 usr_ctx->rel
 #else