Use a nicer pattern for wrapping transactions

src/github.com/matrix-org/dendrite/common/sql.go

@@ -18,6 +18,24 @@ import (
 	"database/sql"
 )
 
+// A Transaction is something that can be committed or rolledback.
+type Transaction interface {
+	// Commit the transaction
+	Commit() error
+	// Rollback the transaction.
+	Rollback() error
+}
+
+// EndTransaction ends a transaction.
+// If the transaction succeeded then it is committed, otherwise it is rolledback.
+func EndTransaction(txn Transaction, succeeded *bool) {
+	if *succeeded {
+		txn.Commit()
+	} else {
+		txn.Rollback()
+	}
+}
+
 // WithTransaction runs a block of code passing in an SQL transaction
 // If the code returns an error or panics then the transactions is rolledback
 // Otherwise the transaction is committed.
@@ -26,16 +44,14 @@ func WithTransaction(db *sql.DB, fn func(txn *sql.Tx) error) (err error) {
 	if err != nil {
 		return
 	}
-	defer func() {
-		if r := recover(); r != nil {
-			txn.Rollback()
-			panic(r)
-		} else if err != nil {
-			txn.Rollback()
-		} else {
-			err = txn.Commit()
-		}
-	}()
+	succeeded := false
+	defer EndTransaction(txn, &succeeded)
+
 	err = fn(txn)
+	if err != nil {
+		return
+	}
+
+	succeeded = true
 	return
 }

src/github.com/matrix-org/dendrite/roomserver/input/events.go

@@ -17,6 +17,7 @@ package input
 import (
 	"fmt"
 
+	"github.com/matrix-org/dendrite/common"
 	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/roomserver/state"
 	"github.com/matrix-org/dendrite/roomserver/types"
@@ -122,8 +123,8 @@ func processInviteEvent(db RoomEventDatabase, ow OutputRoomEventWriter, input ap
 	if err != nil {
 		return err
 	}
-
-	return withTransaction(updater, func() error {
+	succeeded := false
+	defer common.EndTransaction(updater, &succeeded)
 
 	if updater.IsJoin() {
 		// If the user is joined to the room then that takes precedence over this
@@ -161,30 +162,10 @@ func processInviteEvent(db RoomEventDatabase, ow OutputRoomEventWriter, input ap
 		return err
 	}
 
-		return ow.WriteOutputEvents(roomID, outputUpdates)
-	})
+	if err = ow.WriteOutputEvents(roomID, outputUpdates); err != nil {
+		return err
 	}
 
-// withTransaction runs a function inside a transaction.
-// Commits the transaction if the function succeeds.
-// Rollsback the transaction if the function fails or panics.
-func withTransaction(t types.Transaction, f func() error) (err error) {
-	defer func() {
-		if r := recover(); r != nil {
-			t.Rollback()
-			panic(r)
-		}
-		if err != nil {
-			// Ignore any error we get rolling back since we don't want to
-			// clobber the current error
-			// TODO: log the error here.
-			t.Rollback()
-		} else {
-			// Commit if there wasn't an error.
-			// Set the returned err value if we encounter an error committing.
-			// This only works because err is a named return.
-			err = t.Commit()
-		}
-	}()
-	return f()
+	succeeded = true
+	return nil
 }

src/github.com/matrix-org/dendrite/roomserver/input/latest_events.go

@@ -17,6 +17,7 @@ package input
 import (
 	"bytes"
 
+	"github.com/matrix-org/dendrite/common"
 	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/roomserver/state"
 	"github.com/matrix-org/dendrite/roomserver/types"
@@ -52,12 +53,19 @@ func updateLatestEvents(
 	if err != nil {
 		return
 	}
+	succeeded := false
+	defer common.EndTransaction(updater, &succeeded)
 
 	u := latestEventsUpdater{
 		db: db, updater: updater, ow: ow, roomNID: roomNID,
 		stateAtEvent: stateAtEvent, event: event, sendAsServer: sendAsServer,
 	}
-	return withTransaction(updater, u.doUpdateLatestEvents)
+	if err = u.doUpdateLatestEvents(); err != nil {
+		return err
+	}
+
+	succeeded = true
+	return
 }
 
 // latestEventsUpdater tracks the state used to update the latest events in the