Modernise and fix the nix flake by TheArcaneBrony · Pull Request #19185 · element-hq/synapse

TheArcaneBrony · 2025-11-14T13:46:10Z

Summarised changes:

Update nix flake inputs
Updated devenv from 0.6.3 to 1.10
Bumped rust from 1.83 to 1.88 (needed to compile synapse)
Moved postgres to pg 17 (current nixpkgs stable default) at a non-default port (prevents conflicting when the dev machine already has postgres running)
Added a shorthand script in scripts-dev to run the tests with multithreading
Source the venv to stop poetry from wiping itself
Make cargo use git CLI for git modules (don't remember why this was needed)
Make sure pg_config is available if we need to build psychopg2 from source for some reason
Disabled diffs for flake.lock showing up by default via gitattributes (generated file)

Pull Request Checklist

Pull request is based on the develop branch
Pull request includes a changelog file. The entry should:
- Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
- Use markdown where necessary, mostly for code blocks.
- End with either a period (.) or an exclamation mark (!).
- Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
Code style is correct (run the linters)

CLAassistant · 2025-11-14T13:46:16Z

All committers have signed the CLA.

TheArcaneBrony · 2025-11-15T06:53:51Z

Identified a missing piece: i'm unable to run the linter, not sure what all i'm missing or how to get that working, since the cargo thingy seems to throw a fit when i run lint.sh if i nix shell it

anoadragon453

Thanks for taking the time to do this! A few notes below.

anoadragon453 · 2025-11-17T12:19:14Z

                  # NOTE: We currently need to set the Rust version unnecessarily high
                  # in order to work around https://github.com/matrix-org/synapse/issues/15939
-                  (rust-bin.stable."1.82.0".default.override {
+                  (rust-bin.stable."1.88.0".default.override {


This should be at least 1.89, so that we can compile ruff: https://github.com/astral-sh/ruff/blob/1a86e1347200c1db319b9f9960799c1b192bb3fd/Cargo.toml#L8

Question: is there any risk in putting the version too high, or should it be kept at the minimum to ensure compatibility?

No risk. Typically the higher the better with Rust in the dev environment.

Might aswell bump it to 1.91.1 then, I suppose

I have tested it with 1.94.1 and it worked.

anoadragon453 · 2025-11-17T12:19:56Z

+                # Workaround to make cargo fetch git repositories with the git CLI
+                env.CARGO_NET_GIT_FETCH_WITH_CLI = "true";


It'd be ideal to justify why this is needed, in case it has unintended side-effects.

anoadragon453 · 2025-11-17T12:20:15Z

+                  # Ensure the venv is activated explicitly, in order not to confuse poetry
+                  . .venv/bin/activate 


What do you mean by "confuse poetry"? What happens if this isn't here?

I'd been doing this manually for ages - if I skip that step, for some reason, any poetry command causes the .venv to get cleared, causing synapse to fail to start due to missing dependencies.
I could probably clarify that in the comment

That would be useful to do so, yeah.

anoadragon453 · 2025-11-17T12:20:30Z

                  debian-devscripts # (`dch` for manipulating the Debian changelog)
                  libnotify # (the release script uses `notify-send` to tell you when CI jobs are done)
+
+                  # For building psychopg2 from source, if needed


Suggested change

# For building psychopg2 from source, if needed

# For building psycopg2 from source, if needed

lol

anoadragon453 · 2025-11-17T12:21:37Z

@@ -0,0 +1 @@
+flake.lock -diff


Suggested change

flake.lock -diff

# flake.lock is only meant to be read by machines.

flake.lock -diff

anoadragon453 · 2025-11-17T12:22:34Z

@@ -0,0 +1,2 @@
+#! /usr/bin/env sh
+poetry run trial -j`nproc` tests 


I'm not sure if such a short command really justifies a script?

I keep forgetting and having to hunt it down in the documentation, so it's just there as an aid.
Can be dropped if you want :)

yeahhh, this just doesn't quite sit right with me.

If you like, you could add a single-line script to the nix flake instead? https://devenv.sh/scripts/

RamReso

I like it, the dev environment works well with you changes. I would like to see these changes merged, because without them the flake doesn't work.

RamReso · 2026-04-02T09:11:33Z

                # corresponding Nix packages on https://search.nixos.org/packages.
                #
                # This was done until `./install-deps.pl --dryrun` produced no output.
                env.PERL5LIB = "${with pkgs.perl538Packages; makePerlPath [


perl538Packages no longer exists. Using perl5Packages works for me.

RamReso · 2026-04-02T09:12:34Z

                  # NOTE: We currently need to set the Rust version unnecessarily high
                  # in order to work around https://github.com/matrix-org/synapse/issues/15939
-                  (rust-bin.stable."1.82.0".default.override {
+                  (rust-bin.stable."1.88.0".default.override {


I have tested it with 1.94.1 and it worked.

RamReso · 2026-04-02T09:13:32Z

    systems.url = "github:nix-systems/default";
    # A development environment manager built on Nix. See https://devenv.sh.
-    devenv.url = "github:cachix/devenv/v0.6.3";
+    devenv.url = "github:cachix/devenv/v1.10";


using v2.0.6 works as well.

Signed-off-by: Rory& <root@rory.gay>

TheArcaneBrony requested a review from a team as a code owner November 14, 2025 13:46

anoadragon453 requested changes Nov 17, 2025

View reviewed changes

anoadragon453 changed the title ~~Update devenv~~ Modernise and fix the nix flake Nov 19, 2025

RamReso reviewed Apr 2, 2026

View reviewed changes

TheArcaneBrony added 4 commits May 25, 2026 15:29

nix: update flake lock, disable diff by default for flake.lock

7579247

Signed-off-by: Rory& <root@rory.gay>

Update devenv from 0.6.3 to 1.10

c7348b8

Signed-off-by: Rory& <root@rory.gay>

Add shortcut script to run tests

cfd9224

Signed-off-by: Rory& <root@rory.gay>

Add changelog

69073d2

Signed-off-by: Rory& <root@rory.gay>

TheArcaneBrony force-pushed the update-devenv branch from 584701e to 69073d2 Compare May 25, 2026 13:29

		# Workaround to make cargo fetch git repositories with the git CLI
		env.CARGO_NET_GIT_FETCH_WITH_CLI = "true";

		# Ensure the venv is activated explicitly, in order not to confuse poetry
		. .venv/bin/activate

	# For building psychopg2 from source, if needed
	# For building psycopg2 from source, if needed

	flake.lock -diff
	# flake.lock is only meant to be read by machines.
	flake.lock -diff

		@@ -0,0 +1,2 @@
		#! /usr/bin/env sh
		poetry run trial -j`nproc` tests

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TheArcaneBrony commented Nov 14, 2025 •

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