Add "Reason code" prefix to internal SSI error messages

This makes it clearer that the error message is perhaps not supposed
to be understood by users, and it also makes it somewhat clearer that
it was not accidentally omitted from translation.

Idea from Heikki Linnakangas, except that we don't mark "Reason code"
for translation at this point, because that would make the
implementation too cumbersome.

src/backend/storage/lmgr/predicate.c

@@ -3776,7 +3776,7 @@ CheckForSerializableConflictOut(bool visible, Relation relation,
 		ereport(ERROR,
 				(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
 				 errmsg("could not serialize access due to read/write dependencies among transactions"),
-				 errdetail_internal("Canceled on identification as a pivot, during conflict out checking."),
+				 errdetail_internal("Reason code: Canceled on identification as a pivot, during conflict out checking."),
 				 errhint("The transaction might succeed if retried.")));
 	}
 
@@ -3865,7 +3865,7 @@ CheckForSerializableConflictOut(bool visible, Relation relation,
 				ereport(ERROR,
 						(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
 						 errmsg("could not serialize access due to read/write dependencies among transactions"),
-						 errdetail_internal("Canceled on conflict out to old pivot %u.", xid),
+						 errdetail_internal("Reason code: Canceled on conflict out to old pivot %u.", xid),
 					  errhint("The transaction might succeed if retried.")));
 
 			if (SxactHasSummaryConflictIn(MySerializableXact)
@@ -3873,7 +3873,7 @@ CheckForSerializableConflictOut(bool visible, Relation relation,
 				ereport(ERROR,
 						(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
 						 errmsg("could not serialize access due to read/write dependencies among transactions"),
-						 errdetail_internal("Canceled on identification as a pivot, with conflict out to old committed transaction %u.", xid),
+						 errdetail_internal("Reason code: Canceled on identification as a pivot, with conflict out to old committed transaction %u.", xid),
 					  errhint("The transaction might succeed if retried.")));
 
 			MySerializableXact->flags |= SXACT_FLAG_SUMMARY_CONFLICT_OUT;
@@ -3912,7 +3912,7 @@ CheckForSerializableConflictOut(bool visible, Relation relation,
 			ereport(ERROR,
 					(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
 					 errmsg("could not serialize access due to read/write dependencies among transactions"),
-				errdetail_internal("Canceled on conflict out to old pivot."),
+				errdetail_internal("Reason code: Canceled on conflict out to old pivot."),
 					 errhint("The transaction might succeed if retried.")));
 		}
 	}
@@ -4151,7 +4151,7 @@ CheckForSerializableConflictIn(Relation relation, HeapTuple tuple,
 		ereport(ERROR,
 				(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
 				 errmsg("could not serialize access due to read/write dependencies among transactions"),
-				 errdetail_internal("Canceled on identification as a pivot, during conflict in checking."),
+				 errdetail_internal("Reason code: Canceled on identification as a pivot, during conflict in checking."),
 				 errhint("The transaction might succeed if retried.")));
 
 	/*
@@ -4489,7 +4489,7 @@ OnConflict_CheckForSerializationFailure(const SERIALIZABLEXACT *reader,
 			ereport(ERROR,
 					(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
 					 errmsg("could not serialize access due to read/write dependencies among transactions"),
-					 errdetail_internal("Canceled on identification as a pivot, during write."),
+					 errdetail_internal("Reason code: Canceled on identification as a pivot, during write."),
 					 errhint("The transaction might succeed if retried.")));
 		}
 		else if (SxactIsPrepared(writer))
@@ -4501,7 +4501,7 @@ OnConflict_CheckForSerializationFailure(const SERIALIZABLEXACT *reader,
 			ereport(ERROR,
 					(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
 					 errmsg("could not serialize access due to read/write dependencies among transactions"),
-					 errdetail_internal("Canceled on conflict out to pivot %u, during read.", writer->topXid),
+					 errdetail_internal("Reason code: Canceled on conflict out to pivot %u, during read.", writer->topXid),
 					 errhint("The transaction might succeed if retried.")));
 		}
 		writer->flags |= SXACT_FLAG_DOOMED;
@@ -4543,7 +4543,7 @@ PreCommit_CheckForSerializationFailure(void)
 		ereport(ERROR,
 				(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
 				 errmsg("could not serialize access due to read/write dependencies among transactions"),
-				 errdetail_internal("Canceled on identification as a pivot, during commit attempt."),
+				 errdetail_internal("Reason code: Canceled on identification as a pivot, during commit attempt."),
 				 errhint("The transaction might succeed if retried.")));
 	}
 
@@ -4581,7 +4581,7 @@ PreCommit_CheckForSerializationFailure(void)
 						ereport(ERROR,
 								(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
 								 errmsg("could not serialize access due to read/write dependencies among transactions"),
-								 errdetail_internal("Canceled on commit attempt with conflict in from prepared pivot."),
+								 errdetail_internal("Reason code: Canceled on commit attempt with conflict in from prepared pivot."),
 								 errhint("The transaction might succeed if retried.")));
 					}
 					nearConflict->sxactOut->flags |= SXACT_FLAG_DOOMED;