Support nix by not calling `git clone` during build about mold

[rui314]

@nitsky I made a last-minute change for 0.9 so that mimalloc is no longer a submodule but just a subdirectory. i.e. mold now contains a copy of mimalloc in its subdirectory. mold's makefile no longer tries to do git submodule update. This should make your life easier.

[nitsky]

@piegamesde I submitted a nixpkgs PR: NixOS/nixpkgs#128889.

[rui314]

mold is statically-linked to mimalloc because as you guessed Ubuntu doesn't have a package of mimalloc. I want mold to use mimalloc as the default malloc as it is measurably faster than even TBBMalloc. We could provide several options to users as to how to build mold -- using mimalloc in /usr/lib, using mimalloc as a static library, or use TBBMalloc as a last resort. However, that might be a little too complicated.

How about adding mimalloc as a git submodule to the project just like I did before? This way, make doesn't require any network access, and only you have to do is to add --recursive option to the initial git clone. Does that work with nix?

[nitsky]

Submodules will work fine with nix. Would you like to implement the change yourself, or get a PR?

[rui314]

Sounds good. Since adding a submodule is easy, I'll do that for you.

[nitsky]

@rui314 I was successful building mold with nix, but only after applying a patch to remove the git submodule update --init --recursive in the Makefile. Git operations like this are not allowed in nix's sandboxed builds. Would you consider updating the Makefile to eliminate that command or make it optional?

[rui314]

If you run the first clone command as git clone --recursive, didn't it create mimalloc/CMakeLists.txt from the beginning? That git submodule update --init --recursive runs only when mimalloc/CMakeLists.txt is missing. So, I thought that as long as you run the initial cloning with --recursive, that make rule wouldn't be triggered.

[nitsky]

You are right, I made a mistake in my nix configuration, sorry for the trouble. I am now able to package mold with nix. You can close this issue.

[piegamesde]

@nitsky Mind sharing your code? See also NixOS/nixpkgs#128706

[nitsky]

@piegamesde I'll submit a PR to nixpkgs.