fix(sdk): Use the server name from the user id as a fallback URL for fetching the well-known info by jmartinesp · Pull Request #5996 · matrix-org/matrix-rust-sdk

jmartinesp · 2026-01-05T11:55:58Z

When using Client::fetch_client_well_known, use the server name in the user id as a fallback value if the server name is missing. If it's not there or this first attempt fails, use the homeserver url instead as we were doing so far.

Fixes #5877 (or workarounds it).

Public API changes documented in changelogs (optional)

Signed-off-by:

codecov · 2026-01-05T12:10:54Z

Codecov Report

❌ Patch coverage is 91.30435% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.97%. Comparing base (cd9f433) to head (81705e8).
⚠️ Report is 26 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/client/mod.rs	91.30%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5996      +/-   ##
==========================================
+ Coverage   88.59%   88.97%   +0.38%     
==========================================
  Files         364      359       -5     
  Lines      104341    99318    -5023     
  Branches   104341    99318    -5023     
==========================================
- Hits        92438    88370    -4068     
+ Misses       7537     6956     -581     
+ Partials     4366     3992     -374

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codspeed-hq · 2026-01-05T12:16:36Z

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing fix/use-server-name-from-user-id-to-fetch-well-known-if-everything-else-fails (81705e8) with main (cc622a5)}

Summary

✅ 50 untouched benchmarks

bmarty

Thanks. One question!

bmarty · 2026-01-05T12:52:34Z

+            .to_owned();
+
+        // First try using the server name or homeserver url
+        match self.fetch_client_well_known_with_url(server_url.to_string()).await {


Does it make sense to try with the server name first? We know that this can fail, and when this is failing, this is precisely why we need a .well-known file (to discover the homeserver API endpoint).

Maybe this is useful for the first attempt, when login in, and the userId is not known yet? In this case server_url is not resolved yet to the CS endpoint?

The .well-known URL should always be computed from the userId domain part, and this should never fail if the file exists.

That's the alternative I mentioned in the PR description. I kept the previous structure adding this 2nd attempt, but maybe it's better to always try first with the server name, explicit or implicit, then the homeserver url as the fallback 🤔 .

That said, if we extract the server name from the user id and we have no other url to check... which scheme should we add for the constructed URL? Should we enforce https, or just default to plain http, just in case the server does not use HTTPS?

I'd go for https only

I'd agree with that, but I think I'll try to reuse the scheme in the homeserver url, since otherwise we won't be able to test this behaviour (the network mocking layer in the tests doesn't work with HTTPS if I'm not mistaken).

Hywan

Looks good to me. I thought we were already doing that… anyway. Thanks for the tests too!

Can you explain improve the comments to explain why we are doing this? I believe the explanations must live in the documentation of the method itself rather than an inline comment.

Finally, the CHANGELOG.md expects a new entry 😃.

Hywan · 2026-01-07T09:04:40Z

+        // Use the server name, either an explicit one or an implicit one taken from
+        // the user id
+        let server_url = self
            .server()
-            .unwrap_or(
-                // Sometimes people configure their well-known directly on the homeserver so use
-                // this as a fallback when the server name is unknown.
-                &self.homeserver(),
-            )
-            .to_string();
+            .map(|server| server.to_string())
+            .or_else(|| self.user_id().map(|id| format!("{}://{}", scheme, id.server_name())));


I believe this part deserves a bit more explanations about why we are doing this, otherwise it looks good!

jmartinesp · 2026-01-07T09:24:39Z

Done, thanks: I added some more inline comments as well as doc comments. I also added the changelog entry, I usually do that once the approach used in the PR has been validated, so I don't have to also change it every time the logic in the PR does 😅 .

andybalaam · 2026-01-07T09:38:12Z

For reference, the spec for this behaviour is at https://spec.matrix.org/v1.17/client-server-api/#well-known-uris I think. If you haven't, it would be worth reading through and checking we comply with it.

I looked it up because I thought we weren't allowed to guess hostname from Matrix ID, but it looks like in this case we are.

Hywan

Great!

…er name from the `Client::user_id` as a possible fallback too

jmartinesp marked this pull request as ready for review January 5, 2026 12:18

jmartinesp requested a review from a team as a code owner January 5, 2026 12:18

jmartinesp requested review from poljar and removed request for a team January 5, 2026 12:18

bmarty reviewed Jan 5, 2026

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jmartinesp force-pushed the fix/use-server-name-from-user-id-to-fetch-well-known-if-everything-else-fails branch from d64ecfc to 0ad1350 Compare January 5, 2026 14:27

zecakeh reviewed Jan 5, 2026

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Comment thread crates/matrix-sdk/src/client/mod.rs Outdated

Hywan requested changes Jan 7, 2026

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jmartinesp requested a review from Hywan January 7, 2026 09:24

Hywan approved these changes Jan 7, 2026

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jmartinesp added 3 commits January 7, 2026 11:07

fix(sdk): When using fetch_client_well_known_with_url, use the serv…

50ebacb

…er name from the `Client::user_id` as a possible fallback too

doc: Add more doc comments to Client::fetch_client_well_known

93d9355

doc: Add changelog entry

81705e8

jmartinesp force-pushed the fix/use-server-name-from-user-id-to-fetch-well-known-if-everything-else-fails branch from 5e5700f to 81705e8 Compare January 7, 2026 10:07

jmartinesp merged commit 48d1d1f into main Jan 7, 2026
52 checks passed

jmartinesp deleted the fix/use-server-name-from-user-id-to-fetch-well-known-if-everything-else-fails branch January 7, 2026 10:25

pixlwave mentioned this pull request Jan 30, 2026

Replace the TtlStoreValue type on cached Versions/WellKnown with background refreshes. #6090

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