From 23c0908fd18cf3475e1c569a6af1183208d038c9 Mon Sep 17 00:00:00 2001
From: Ryan Miguel <rmiguel@barracuda.com>
Date: Fri, 21 Oct 2022 10:48:08 -0700
Subject: [PATCH 1/2] Return NOT_JSON if decode fails and defer
 set_timeline_upper_limit call until after check_valid_filter. Fixes #13661.
 Signed-off-by: Ryan Miguel <miguel.ryanj@gmail.com>.

---
 changelog.d/14262.misc      | 1 +
 synapse/rest/client/sync.py | 8 ++++----
 2 files changed, 5 insertions(+), 4 deletions(-)
 create mode 100644 changelog.d/14262.misc

diff --git a/changelog.d/14262.misc b/changelog.d/14262.misc
new file mode 100644
index 000000000000..a0c11c5a232e
--- /dev/null
+++ b/changelog.d/14262.misc
@@ -0,0 +1 @@
+Return M_NOT_JSON rather than M_UNKNOWN when decode fails.
diff --git a/synapse/rest/client/sync.py b/synapse/rest/client/sync.py
index 8a16459105f3..f2013faeb206 100644
--- a/synapse/rest/client/sync.py
+++ b/synapse/rest/client/sync.py
@@ -146,12 +146,12 @@ async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
         elif filter_id.startswith("{"):
             try:
                 filter_object = json_decoder.decode(filter_id)
-                set_timeline_upper_limit(
-                    filter_object, self.hs.config.server.filter_timeline_limit
-                )
             except Exception:
-                raise SynapseError(400, "Invalid filter JSON")
+                raise SynapseError(400, "Invalid filter JSON", errcode=Codes.NOT_JSON)
             self.filtering.check_valid_filter(filter_object)
+            set_timeline_upper_limit(
+                filter_object, self.hs.config.server.filter_timeline_limit
+            )
             filter_collection = FilterCollection(self.hs, filter_object)
         else:
             try:

From 291e446208a0bde2c427b705f2dcc2ff2983f679 Mon Sep 17 00:00:00 2001
From: David Robertson <david.m.robertson1@gmail.com>
Date: Mon, 24 Oct 2022 16:54:50 +0100
Subject: [PATCH 2/2] Reword changelog

---
 changelog.d/14262.misc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/changelog.d/14262.misc b/changelog.d/14262.misc
index a0c11c5a232e..c1d23bc67de9 100644
--- a/changelog.d/14262.misc
+++ b/changelog.d/14262.misc
@@ -1 +1 @@
-Return M_NOT_JSON rather than M_UNKNOWN when decode fails.
+Provide a specific error code when a `/sync` request provides a filter which doesn't represent a JSON object.