sql/backwards_extremities: Shift to table format and share code (#985)

* sql/backwards_extremities: Shift to table format and share code

This is an initial cut to reduce boilerplate at the storage layer.
It removes the need for 2x `_table.go` files, one for each DB engine,
replacing it with a single struct which has an interface which
implements the raw SQL statements.

The actual impl sits alongside the interface declaration which is
generally regarded as best practice (though no canonical sources).
Especially in this case where the impl is tiny (functions returning
strings) and relies heavily on the function signatures of the
table struct (for parameters), having the context in the same file
is useful.

* Remove _table redundancy

1 files changed, 10 insertions, 8 deletions

diff --git a/syncapi/storage/postgres/syncserver.go b/syncapi/storage/postgres/syncserver.go
index 1e078ef4..9d61ccfc 100644
--- a/syncapi/storage/postgres/syncserver.go
+++ b/syncapi/storage/postgres/syncserver.go
@@ -32,6 +32,7 @@ import (
 	_ "github.com/lib/pq"
 	"github.com/matrix-org/dendrite/common"
 	"github.com/matrix-org/dendrite/eduserver/cache"
+	"github.com/matrix-org/dendrite/syncapi/storage/tables"
 	"github.com/matrix-org/dendrite/syncapi/types"
 	"github.com/matrix-org/gomatrixserverlib"
 )
@@ -56,7 +57,7 @@ type SyncServerDatasource struct {
 	invites             inviteEventsStatements
 	eduCache            *cache.EDUCache
 	topology            outputRoomEventsTopologyStatements
-	backwardExtremities backwardExtremitiesStatements
+	backwardExtremities tables.BackwardsExtremities
 }
 
 // NewSyncServerDatasource creates a new sync server database
@@ -75,16 +76,17 @@ func NewSyncServerDatasource(dbDataSourceName string) (*SyncServerDatasource, er
 	if err = d.events.prepare(d.db); err != nil {
 		return nil, err
 	}
-	if err := d.roomstate.prepare(d.db); err != nil {
+	if err = d.roomstate.prepare(d.db); err != nil {
 		return nil, err
 	}
-	if err := d.invites.prepare(d.db); err != nil {
+	if err = d.invites.prepare(d.db); err != nil {
 		return nil, err
 	}
-	if err := d.topology.prepare(d.db); err != nil {
+	if err = d.topology.prepare(d.db); err != nil {
 		return nil, err
 	}
-	if err := d.backwardExtremities.prepare(d.db); err != nil {
+	d.backwardExtremities, err = tables.NewBackwardsExtremities(d.db, &tables.PostgresBackwardsExtremitiesStatements{})
+	if err != nil {
 		return nil, err
 	}
 	d.eduCache = cache.New()
@@ -116,7 +118,7 @@ func (d *SyncServerDatasource) Events(ctx context.Context, eventIDs []string) ([
 // the events listed in the event's 'prev_events'. This function also updates the backwards extremities table
 // to account for the fact that the given event is no longer a backwards extremity, but may be marked as such.
 func (d *SyncServerDatasource) handleBackwardExtremities(ctx context.Context, txn *sql.Tx, ev *gomatrixserverlib.HeaderedEvent) error {
-	if err := d.backwardExtremities.deleteBackwardExtremity(ctx, txn, ev.RoomID(), ev.EventID()); err != nil {
+	if err := d.backwardExtremities.DeleteBackwardExtremity(ctx, txn, ev.RoomID(), ev.EventID()); err != nil {
 		return err
 	}
 
@@ -137,7 +139,7 @@ func (d *SyncServerDatasource) handleBackwardExtremities(ctx context.Context, tx
 
 		// If the event is missing, consider it a backward extremity.
 		if !found {
-			if err = d.backwardExtremities.insertsBackwardExtremity(ctx, txn, ev.RoomID(), ev.EventID(), eID); err != nil {
+			if err = d.backwardExtremities.InsertsBackwardExtremity(ctx, txn, ev.RoomID(), ev.EventID(), eID); err != nil {
 				return err
 			}
 		}
@@ -314,7 +316,7 @@ func (d *SyncServerDatasource) SyncPosition(ctx context.Context) (types.Paginati
 func (d *SyncServerDatasource) BackwardExtremitiesForRoom(
 	ctx context.Context, roomID string,
 ) (backwardExtremities []string, err error) {
-	return d.backwardExtremities.selectBackwardExtremitiesForRoom(ctx, roomID)
+	return d.backwardExtremities.SelectBackwardExtremitiesForRoom(ctx, roomID)
 }
 
 // MaxTopologicalPosition returns the highest topological position for a given