From ad8461491b35c739683971deff32a3203d0d7e5d Mon Sep 17 00:00:00 2001
From: Alex Auvolat <alex@adnab.me>
Date: Fri, 13 Mar 2020 11:29:03 +0100
Subject: [PATCH] Avoid occasionnal deadlocks (1 in 256)

---
 account.go | 9 ++++-----
 server.go  | 3 ++-
 util.go    | 6 +++---
 3 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/account.go b/account.go
index b0313a4..0b480d6 100644
--- a/account.go
+++ b/account.go
@@ -453,17 +453,16 @@ func (a *Account) eventInternal(event *Event) error {
 
 		var cache_key string
 		if event.Id != "" {
+			// Use mx_room_id as a lock slot key, because this section is
+			// concurrent with the part that sends events out in server.go,
+			// and at that point we don't know the event's ID yet
+			// since it will be attributed by the backend during the call to send
 			dbLockSlot(mx_room_id)
 			defer dbUnlockSlot(mx_room_id)
 
 			// If the event has an ID, make sure it is processed only once
 			cache_key = fmt.Sprintf("%s/event_seen/%s/%s",
 				a.Protocol, mx_room_id, event.Id)
-			slot_key := dbKvSlotKey(cache_key)
-
-			dbLockSlot(slot_key)
-			defer dbUnlockSlot(slot_key)
-
 			if dbKvGet(cache_key) == "yes" {
 				// false: cache key was not modified, meaning we
 				// already saw the event
diff --git a/server.go b/server.go
index 1654971..1868078 100644
--- a/server.go
+++ b/server.go
@@ -166,6 +166,7 @@ func handleTxnEvent(e *mxlib.Event) error {
 				ev.Author = acct.Conn.User()
 				ev.Room = room.RoomID
 
+				// use room id as lock slot key, see account.go in eventInternal
 				dbLockSlot(e.RoomId)
 				defer dbUnlockSlot(e.RoomId)
 
@@ -173,7 +174,7 @@ func handleTxnEvent(e *mxlib.Event) error {
 				if err == nil && created_ev_id != "" {
 					cache_key := fmt.Sprintf("%s/event_seen/%s/%s",
 						room.Protocol, e.RoomId, created_ev_id)
-					dbKvPut(cache_key, "yes")
+					dbKvPutLocked(cache_key, "yes")
 				}
 				return err
 			} else {
diff --git a/util.go b/util.go
index 4e23bb1..cd16a59 100644
--- a/util.go
+++ b/util.go
@@ -27,15 +27,15 @@ func ezbrSystemRoom(user_mx_id string) (string, error) {
 	}
 
 	widget_kv_key := "ezbr_widget_on:" + mx_room_id
-	if config.WebURL != "" && dbKvTestAndSet(widget_kv_key, "yes") {
+	if config.WebURL != "" && dbKvGet(widget_kv_key) != "yes" {
 		widget := map[string]interface{}{
 			"type": "m.easybridge",
 			"url":  config.WebURL,
 			"name": "Easybridge account configuration dashboard",
 		}
 		err = mx.PutStateAs(mx_room_id, "im.vector.modular.widgets", "ezbr_widget", widget, ezbrMxId())
-		if err != nil {
-			dbKvPut(widget_kv_key, "")
+		if err == nil {
+			dbKvPut(widget_kv_key, "yes")
 		}
 	}