From da1a5616eb03dddb7cb31d4c7e33f1b8dd552d62 Mon Sep 17 00:00:00 2001
From: Luke Barnard <luke@matrix.org>
Date: Tue, 1 May 2018 14:04:13 +0100
Subject: [PATCH 1/3] Prevent error responses wedging group request concurrency
 limit

Fixes https://github.com/vector-im/riot-web/issues/6592
---
 src/stores/GroupStore.js | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/src/stores/GroupStore.js b/src/stores/GroupStore.js
index 23ce5314ec..f4a66f432f 100644
--- a/src/stores/GroupStore.js
+++ b/src/stores/GroupStore.js
@@ -62,6 +62,11 @@ function limitConcurrency(fn) {
         }
     })
     .then(fn)
+    .catch((err) => {
+        ongoingRequestCount--;
+        checkBacklog();
+        throw err;
+    })
     .then((result) => {
         ongoingRequestCount--;
         checkBacklog();

From 71c1198d12b5028147f69e6117f592e28afbaf36 Mon Sep 17 00:00:00 2001
From: Luke Barnard <luke@matrix.org>
Date: Tue, 1 May 2018 18:01:25 +0100
Subject: [PATCH 2/3] Rewrite limitConcurrency to fix error catching

Make sure that we only catch errors that are a result of
calling fn()
---
 src/stores/GroupStore.js | 44 ++++++++++++++++++++--------------------
 1 file changed, 22 insertions(+), 22 deletions(-)

diff --git a/src/stores/GroupStore.js b/src/stores/GroupStore.js
index f4a66f432f..0e2e8e6b86 100644
--- a/src/stores/GroupStore.js
+++ b/src/stores/GroupStore.js
@@ -48,30 +48,30 @@ function checkBacklog() {
 
 // Limit the maximum number of ongoing promises returned by fn to LIMIT and
 // use a FIFO queue to handle the backlog.
-function limitConcurrency(fn) {
-    return new Promise((resolve, reject) => {
-        const item = () => {
-            ongoingRequestCount++;
-            resolve();
-        };
-        if (ongoingRequestCount >= LIMIT) {
-            // Enqueue this request for later execution
-            backlogQueue.push(item);
-        } else {
-            item();
-        }
-    })
-    .then(fn)
-    .catch((err) => {
+async function limitConcurrency(fn) {
+    if (ongoingRequestCount >= LIMIT) {
+        // Enqueue this request for later execution
+        await new Promise((resolve, reject) => {
+            backlogQueue.push(resolve);
+        });
+    }
+
+    let result;
+    let error;
+
+    ongoingRequestCount++;
+    try {
+        result = await fn();
+    } catch (err) {
+        error = err;
+    } finally {
         ongoingRequestCount--;
         checkBacklog();
-        throw err;
-    })
-    .then((result) => {
-        ongoingRequestCount--;
-        checkBacklog();
-        return result;
-    });
+    }
+
+    if (error) throw error;
+
+    return result;
 }
 
 /**

From 2dfb3146b00f8d5264dc72b416754c6e7966c3fe Mon Sep 17 00:00:00 2001
From: Luke Barnard <luke@matrix.org>
Date: Wed, 2 May 2018 10:39:15 +0100
Subject: [PATCH 3/3] Simplify concurrent request error handling

---
 src/stores/GroupStore.js | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

diff --git a/src/stores/GroupStore.js b/src/stores/GroupStore.js
index 0e2e8e6b86..e7cea5667e 100644
--- a/src/stores/GroupStore.js
+++ b/src/stores/GroupStore.js
@@ -56,22 +56,16 @@ async function limitConcurrency(fn) {
         });
     }
 
-    let result;
-    let error;
-
     ongoingRequestCount++;
     try {
-        result = await fn();
+        return await fn();
     } catch (err) {
-        error = err;
+        // We explicitly do not handle the error here, but let it propogate.
+        throw err;
     } finally {
         ongoingRequestCount--;
         checkBacklog();
     }
-
-    if (error) throw error;
-
-    return result;
 }
 
 /**