Component-wide TransactionWriters (#1290)

* Offset updates take place using TransactionWriter

* Refactor TransactionWriter in current state server

* Refactor TransactionWriter in federation sender

* Refactor TransactionWriter in key server

* Refactor TransactionWriter in media API

* Refactor TransactionWriter in server key API

* Refactor TransactionWriter in sync API

* Refactor TransactionWriter in user API

* Fix deadlocking Sync API tests

* Un-deadlock device database

* Fix appservice API

* Rename TransactionWriters to Writers

* Move writers up a layer in sync API

* Document sqlutil.Writer interface

* Add note to Writer documentation

4 files changed, 14 insertions, 10 deletions

diff --git a/appservice/storage/postgres/storage.go b/appservice/storage/postgres/storage.go
index 9fda87ae..95215816 100644
--- a/appservice/storage/postgres/storage.go
+++ b/appservice/storage/postgres/storage.go
@@ -32,6 +32,7 @@ type Database struct {
 	events eventsStatements
 	txnID  txnStatements
 	db     *sql.DB
+	writer sqlutil.Writer
 }
 
 // NewDatabase opens a new database
@@ -41,10 +42,11 @@ func NewDatabase(dbProperties *config.DatabaseOptions) (*Database, error) {
 	if result.db, err = sqlutil.Open(dbProperties); err != nil {
 		return nil, err
 	}
+	result.writer = sqlutil.NewDummyWriter()
 	if err = result.prepare(); err != nil {
 		return nil, err
 	}
-	if err = result.PartitionOffsetStatements.Prepare(result.db, "appservice"); err != nil {
+	if err = result.PartitionOffsetStatements.Prepare(result.db, result.writer, "appservice"); err != nil {
 		return nil, err
 	}
 	return &result, nil
diff --git a/appservice/storage/sqlite3/appservice_events_table.go b/appservice/storage/sqlite3/appservice_events_table.go
index 5cc07ed3..5dfb72f6 100644
--- a/appservice/storage/sqlite3/appservice_events_table.go
+++ b/appservice/storage/sqlite3/appservice_events_table.go
@@ -67,7 +67,7 @@ const (
 
 type eventsStatements struct {
 	db                                     *sql.DB
-	writer                                 sqlutil.TransactionWriter
+	writer                                 sqlutil.Writer
 	selectEventsByApplicationServiceIDStmt *sql.Stmt
 	countEventsByApplicationServiceIDStmt  *sql.Stmt
 	insertEventStmt                        *sql.Stmt
@@ -75,9 +75,9 @@ type eventsStatements struct {
 	deleteEventsBeforeAndIncludingIDStmt   *sql.Stmt
 }
 
-func (s *eventsStatements) prepare(db *sql.DB) (err error) {
+func (s *eventsStatements) prepare(db *sql.DB, writer sqlutil.Writer) (err error) {
 	s.db = db
-	s.writer = sqlutil.NewTransactionWriter()
+	s.writer = writer
 	_, err = db.Exec(appserviceEventsSchema)
 	if err != nil {
 		return
diff --git a/appservice/storage/sqlite3/storage.go b/appservice/storage/sqlite3/storage.go
index 59af9016..916845ab 100644
--- a/appservice/storage/sqlite3/storage.go
+++ b/appservice/storage/sqlite3/storage.go
@@ -32,6 +32,7 @@ type Database struct {
 	events eventsStatements
 	txnID  txnStatements
 	db     *sql.DB
+	writer sqlutil.Writer
 }
 
 // NewDatabase opens a new database
@@ -41,21 +42,22 @@ func NewDatabase(dbProperties *config.DatabaseOptions) (*Database, error) {
 	if result.db, err = sqlutil.Open(dbProperties); err != nil {
 		return nil, err
 	}
+	result.writer = sqlutil.NewExclusiveWriter()
 	if err = result.prepare(); err != nil {
 		return nil, err
 	}
-	if err = result.PartitionOffsetStatements.Prepare(result.db, "appservice"); err != nil {
+	if err = result.PartitionOffsetStatements.Prepare(result.db, result.writer, "appservice"); err != nil {
 		return nil, err
 	}
 	return &result, nil
 }
 
 func (d *Database) prepare() error {
-	if err := d.events.prepare(d.db); err != nil {
+	if err := d.events.prepare(d.db, d.writer); err != nil {
 		return err
 	}
 
-	return d.txnID.prepare(d.db)
+	return d.txnID.prepare(d.db, d.writer)
 }
 
 // StoreEvent takes in a gomatrixserverlib.HeaderedEvent and stores it in the database
diff --git a/appservice/storage/sqlite3/txn_id_counter_table.go b/appservice/storage/sqlite3/txn_id_counter_table.go
index 0ae0feee..b2940e35 100644
--- a/appservice/storage/sqlite3/txn_id_counter_table.go
+++ b/appservice/storage/sqlite3/txn_id_counter_table.go
@@ -38,13 +38,13 @@ const selectTxnIDSQL = `
 
 type txnStatements struct {
 	db              *sql.DB
-	writer          sqlutil.TransactionWriter
+	writer          sqlutil.Writer
 	selectTxnIDStmt *sql.Stmt
 }
 
-func (s *txnStatements) prepare(db *sql.DB) (err error) {
+func (s *txnStatements) prepare(db *sql.DB, writer sqlutil.Writer) (err error) {
 	s.db = db
-	s.writer = sqlutil.NewTransactionWriter()
+	s.writer = writer
 	_, err = db.Exec(txnIDSchema)
 	if err != nil {
 		return