--- title: >- Building gomobile Using Nix description: >- Harder than I thought it would be! series: nebula tags: tech --- When I last left off with the nebula project I wanted to [nix][nix]-ify the build process for Cryptic's [mobile_nebula][mobile_nebula] fork. While I've made progress on the overall build, one particular bit of it really held me up, so I'm writing about that part here. I'll finish the full build at a later time. ## gomobile [gomobile][gomobile] is a toolkit for the go programming language to allow for running go code on Android and iOS devices. `mobile_nebula` uses `gomobile` to build a simple wrapper around the nebula client that the mobile app can then hook into. This means that in order to nix-ify the entire `mobile_nebula` project I first need to nix-ify `gomobile`, and since there isn't (at time of writing) an existing package for `gomobile` in the nixpkgs repo, I had to roll my own. I started with a simple `buildGoModule` nix expression: ``` pkgs.buildGoModule { pname = "gomobile"; version = "unstable-2020-12-17"; src = pkgs.fetchFromGitHub { owner = "golang"; repo = "mobile"; rev = "e6ae53a27f4fd7cfa2943f2ae47b96cba8eb01c9"; sha256 = "03dzis3xkj0abcm4k95w2zd4l9ygn0rhkj56bzxbcpwa7idqhd62"; }; vendorSha256 = "1n1338vqkc1n8cy94501n7jn3qbr28q9d9zxnq2b4rxsqjfc9l94"; } ``` The basic idea here is that `buildGoModule` will acquire a specific revision of the `gomobile` source code from github, then attempt to build it. However, `gomobile` is a special beast in that it requires a number of C/C++ libraries in order to be built. I discovered this upon running this expression, when I received this error: ``` ./work.h:12:10: fatal error: GLES3/gl3.h: No such file or directory 12 | #include // install on Ubuntu with: sudo apt-get install libegl1-mesa-dev libgles2-mesa-dev libx11-dev ``` This stumped me for a bit, as I couldn't figure out a) the "right" place to source the `GLES3` header file from, and b) how to properly hook that into the `buildGoModule` expression. My initial attempts involved trying to include versions of the header file from my `androidsdk` nix package which I had already gotten (mostly) working, but the version which ships there appears to expect to be using clang. `cgo` (go's compiler which is used for C/C++ interop) only supports gcc, so that strategy failed. I didn't like having to import the header file from `androidsdk` anyway, as it meant that my `gomobile` would only work within the context of the `mobile_nebula` project, rather than being a standalone utility. ## nix-index At this point I flailed around some more trying to figure out where to get this header file from. Eventually I stumbled on the [nix-index][nix-index] project, which implements something similar to the `locate` utility on linux: you give it a file pattern, and it searches your active nix channels for any packages which provide a file matching that pattern. Since nix is amazing it's not actually necessary to install `nix-index`, I simply start up a shell with the package available using `nix-shell -p nix-index`. On first run I needed to populate the index by running the `nix-index` command, which took some time, but after that finding packages which provide the file I need is as easy as: ``` > nix-shell -p nix-index [nix-shell:/tmp]$ nix-locate GLES3/gl3.h (zulip.out) 82,674 r /nix/store/wbfw7w2ixdp317wip77d4ji834v1k1b9-libglvnd-1.3.2-dev/include/GLES3/gl3.h libglvnd.dev 82,674 r /nix/store/pghxzmnmxdcarg5bj3js9csz0h85g08m-libglvnd-1.3.2-dev/include/GLES3/gl3.h emscripten.out 82,666 r /nix/store/x3c4y2h5rn1jawybk48r6glzs1jl029s-emscripten-2.0.1/share/emscripten/system/include/GLES3/gl3.h ``` So my mystery file is provided by a few packages, but `libglvnd.dev` stood out to me as it's also the pacman package which provides the same file in my real operating system: ``` > yay -Qo /usr/include/GLES3/gl3.h /usr/include/GLES3/gl3.h is owned by libglvnd 1.3.2-1 ``` This gave me some confidence that this was the right track. ## cgo My next fight was with `cgo` itself. Go's build process provides a few different entry points for C/C++ compiler/linker flags, including both environment variables and command-line arguments. But I wasn't using `go build` directly, instead I was working through nix's `buildGoModule` wrapper. This added a huge layer of confusion as all of nixpkgs is pretty terribly documented, so you really have to just divine behavior from the [source][buildGoModule-source] (good luck). After lots of debugging (hint: `NIX_DEBUG=1`) I determined that all which is actually needed is to set the `CGO_CFLAGS` variable within the `buildGoModule` arguments. This would translate to the `CGO_CFLAGS` environment variable being set during all internal commands, and whatever `go build` commands get used would pick up my compiler flags from that. My new nix expression looked like this: ``` pkgs.buildGoModule { pname = "gomobile"; version = "unstable-2020-12-17"; src = pkgs.fetchFromGitHub { owner = "golang"; repo = "mobile"; rev = "e6ae53a27f4fd7cfa2943f2ae47b96cba8eb01c9"; sha256 = "03dzis3xkj0abcm4k95w2zd4l9ygn0rhkj56bzxbcpwa7idqhd62"; }; vendorSha256 = "1n1338vqkc1n8cy94501n7jn3qbr28q9d9zxnq2b4rxsqjfc9l94"; CGO_CFLAGS = [ "-I ${pkgs.libglvnd.dev}/include" ]; } ``` Running this produced a new error. Progress! The new error was: ``` /nix/store/p792j5f44l3f0xi7ai5jllwnxqwnka88-binutils-2.31.1/bin/ld: cannot find -lGLESv2 collect2: error: ld returned 1 exit status ``` So pretty similar to the previous issue, but this time the linker wasn't finding a library file rather than the compiler not finding a header file. Once again I used `nix-index`'s `nix-locate` command to find that this library file is provided by the `libglvnd` package (as opposed to `libglvnd.dev`, which provided the header file). Adding `libglvnd` to the `CGO_CFLAGS` did not work, as it turns out that flags for the linker `cgo` uses get passed in via `CGO_LDFLAGS` (makes sense). After adding this new variable I got yet another error; this time `X11/Xlib.h` was not able to be found. I repeated the process of `nix-locate`/add to `CGO_*FLAGS` a few more times until all dependencies were accounted for. The new nix expression looked like this: ``` pkgs.buildGoModule { pname = "gomobile"; version = "unstable-2020-12-17"; src = pkgs.fetchFromGitHub { owner = "golang"; repo = "mobile"; rev = "e6ae53a27f4fd7cfa2943f2ae47b96cba8eb01c9"; sha256 = "03dzis3xkj0abcm4k95w2zd4l9ygn0rhkj56bzxbcpwa7idqhd62"; }; vendorSha256 = "1n1338vqkc1n8cy94501n7jn3qbr28q9d9zxnq2b4rxsqjfc9l94"; CGO_CFLAGS = [ "-I ${pkgs.libglvnd.dev}/include" "-I ${pkgs.xlibs.libX11.dev}/include" "-I ${pkgs.xlibs.xorgproto}/include" "-I ${pkgs.openal}/include" ]; CGO_LDFLAGS = [ "-L ${pkgs.libglvnd}/lib" "-L ${pkgs.xlibs.libX11}/lib" "-L ${pkgs.openal}/lib" ]; } ``` ## Tests The `CGO_*FLAGS` variables took care of all compiler/linker errors, but there was one issue left: `buildGoModule` apparently runs the project's tests after the build phase. `gomobile`'s tests were actually mostly passing, but some failed due to trying to copy files around, which nix was having none of. After some more [buildGoModule source][buildGoModule-source] divination I found that if I passed an empty `checkPhase` argument it would skip the check phase, and therefore skip running these tests. ## Fin! The final nix expression looks like so: ``` pkgs.buildGoModule { pname = "gomobile"; version = "unstable-2020-12-17"; src = pkgs.fetchFromGitHub { owner = "golang"; repo = "mobile"; rev = "e6ae53a27f4fd7cfa2943f2ae47b96cba8eb01c9"; sha256 = "03dzis3xkj0abcm4k95w2zd4l9ygn0rhkj56bzxbcpwa7idqhd62"; }; vendorSha256 = "1n1338vqkc1n8cy94501n7jn3qbr28q9d9zxnq2b4rxsqjfc9l94"; CGO_CFLAGS = [ "-I ${pkgs.libglvnd.dev}/include" "-I ${pkgs.xlibs.libX11.dev}/include" "-I ${pkgs.xlibs.xorgproto}/include" "-I ${pkgs.openal}/include" ]; CGO_LDFLAGS = [ "-L ${pkgs.libglvnd}/lib" "-L ${pkgs.xlibs.libX11}/lib" "-L ${pkgs.openal}/lib" ]; checkPhase = ""; } ``` Once I complete the nix-ification of `mobile_nebula` I'll submit a PR to the nixpkgs upstream with this, so that others can have `gomobile` available as well! [nix]: https://nixos.org/manual/nix/stable/ [mobile_nebula]: https://github.com/cryptic-io/mobile_nebula [gomobile]: https://github.com/golang/mobile [nix-index]: https://github.com/bennofs/nix-index [buildGoModule-source]: https://github.com/NixOS/nixpkgs/blob/26117ed4b78020252e49fe75f562378063471f71/pkgs/development/go-modules/generic/default.nix