WSL 2 Ubuntu `mix compile` issue

[ken-kost]

Build Fix Report: WSL 2 libgbm Linker Failure

System

• Project: Emerge — an Elixir GUI framework backed by a Rust NIF (emerge_skia) using Skia for rendering
• Environment: WSL 2, Ubuntu 22.04 (Jammy) on x86_64
• Build toolchain: Elixir/Mix + Rustler (NIF bridge) + Cargo

---

Problem

mix compile failed at the Rust link step:

rust-lld: error: unable to find library -lgbm

The crate unconditionally declared gbm = "0.18" and drm = "0.14" as Cargo dependencies. These crates link against the system libgbm (Mesa's Generic Buffer Manager), which is only needed for the DRM backend — bare-metal framebuffer rendering with no window manager. That backend is irrelevant on WSL 2, which uses WSLg (a Weston compositor over RDP to Windows DirectX).

---

Root Cause of System State

libgbm1 v25.0.5 was installed from the kisak-mesa PPA. The only available libgbm-dev package requires libgbm1 = 23.2.1 exactly (strict pin). The kisak-mesa PPA for Ubuntu 22.04 was subsequently discontinued, leaving the system with a newer runtime than any matching dev package — an unresolvable apt conflict.

---

What Was Attempted (and Why It Failed)

Attempt	Failure reason
apt install libgbm-dev	Version conflict: installed libgbm1 is v25, dev package requires v23
apt install --allow-downgrades libgbm1=23.2.1	Cascading conflicts: entire Mesa 25 stack (libegl-mesa0, etc.) is entangled
add-apt-repository --remove ppa:kisak/kisak-mesa	Removes source list only; installed packages remain at v25
apt install ppa-purge && ppa-purge ppa:kisak/kisak-mesa	PPA discontinued for Jammy — no package list to diff against; ppa-purge reports "could not find package list" even after re-adding the PPA source
Re-add PPA, then ppa-purge	Same failure — PPA metadata for Jammy is gone

Conclusion: No apt-level fix was viable. The Mesa install is permanently broken for this purpose.

---

Why a Code Fix Was the Right Path

libgbm is not needed for:

• The Wayland backend (winit + glutin + EGL, used on WSL 2 via WSLg)
• The raster backend (offscreen CPU rendering)
• Any tests

It is needed exclusively for the DRM backend (src/backend/drm.rs, src/drm_input.rs), which requires direct GPU framebuffer access — impossible in WSL 2.

Rustler supports a features: option that passes Cargo feature flags at compile time, configurable from Elixir application config.

---

What Was Changed

native/emerge_skia/Cargo.toml

• Added [features] section with drm = ["dep:drm", "dep:gbm", "dep:evdev"]
• Made drm, gbm, evdev optional dependencies
• Kept libc as a required dep (also used in src/video.rs)

native/emerge_skia/src/backend/mod.rs

• Gated pub mod drm behind #[cfg(feature = "drm")]

native/emerge_skia/src/lib.rs

• Gated mod drm_input and its use imports
• Gated BackendKind::Drm enum variant
• Gated the BackendKind::Drm match arm in stop() (the Drop impl)
• Gated the BackendKind::Drm match arm in start_with_config()
• Gated "drm" string → BackendKind::Drm parsing; returns a clear error if DRM feature is not compiled
• Added #[cfg_attr(not(feature = "drm"), allow(dead_code))] on drm_card, drm_hw_cursor, drm_input_log fields
• Annotated bounded::<RenderMsg> for type inference (previously inferred from the DRM branch)
• Used _cursor_rx + conditional rebind to suppress unused variable warning

lib/emerge_skia/native.ex

• Added features: Application.compile_env(:emerge, :emerge_skia_features, []) — reads Cargo features from Elixir app config, defaulting to none

---

Result

• mix compile succeeds with no linker errors
• mix test: 179 tests, 0 failures
• DRM backend code remains in the repo, intact
• To enable DRM on a system with libgbm-dev: config :emerge, emerge_skia_features: ["drm"]

[ken-kost]

 mix run demo.exs    
    warning: the variable "id_len" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
    │
 98 │     <<id_bin::binary-size(id_len), rest::binary>> = rest
    │                           ~
    │
    └─ lib/emerge/engine/serialization.ex:98:27: Emerge.Engine.Serialization.decode_nodes/3

     warning: the variable "attr_len" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
     │
 101 │     <<attr_bin::binary-size(attr_len), rest::binary>> = rest
     │                             ~
     │
     └─ lib/emerge/engine/serialization.ex:101:29: Emerge.Engine.Serialization.decode_nodes/3

     warning: the variable "len" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
     │
 169 │     <<id_bin::binary-size(len), rest::binary>> = rest
     │                           ~
     │
     └─ lib/emerge/engine/serialization.ex:169:27: Emerge.Engine.Serialization.decode_ids/3

     warning: the variable "id_len" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
     │
 136 │     <<id_bin::binary-size(id_len), rest::binary>> = rest
     │                           ~
     │
     └─ lib/emerge/engine/patch.ex:136:27: Emerge.Engine.Patch.decode_patches/2

     warning: the variable "attr_len" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
     │
 139 │     <<attr_bin::binary-size(attr_len), rest::binary>> = rest
     │                             ~
     │
     └─ lib/emerge/engine/patch.ex:139:29: Emerge.Engine.Patch.decode_patches/2

     warning: the variable "id_len" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
     │
 145 │     <<id_bin::binary-size(id_len), rest::binary>> = rest
     │                           ~
     │
     └─ lib/emerge/engine/patch.ex:145:27: Emerge.Engine.Patch.decode_patches/2

     warning: the variable "parent_len" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
     │
 153 │     <<parent_bin::binary-size(parent_len), rest::binary>> = rest
     │                               ~
     │
     └─ lib/emerge/engine/patch.ex:153:31: Emerge.Engine.Patch.decode_patches/2

     warning: the variable "len" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
     │
 156 │     <<subtree_bin::binary-size(len), rest::binary>> = rest
     │                                ~
     │
     └─ lib/emerge/engine/patch.ex:156:32: Emerge.Engine.Patch.decode_patches/2

     warning: the variable "id_len" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
     │
 162 │     <<id_bin::binary-size(id_len), rest::binary>> = rest
     │                           ~
     │
     └─ lib/emerge/engine/patch.ex:162:27: Emerge.Engine.Patch.decode_patches/2

     warning: the variable "host_len" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
     │
 168 │     <<host_bin::binary-size(host_len), rest::binary>> = rest
     │                             ~
     │
     └─ lib/emerge/engine/patch.ex:168:29: Emerge.Engine.Patch.decode_patches/2

     warning: the variable "len" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
     │
 171 │     <<subtree_bin::binary-size(len), rest::binary>> = rest
     │                                ~
     │
     └─ lib/emerge/engine/patch.ex:171:32: Emerge.Engine.Patch.decode_patches/2

     warning: the variable "len" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
     │
 184 │     <<id_bin::binary-size(len), rest::binary>> = rest
     │                           ~
     │
     └─ lib/emerge/engine/patch.ex:184:27: Emerge.Engine.Patch.decode_child_ids/3

     warning: incompatible types given to RustlerPrecompiled.target/3:

         RustlerPrecompiled.target(%{}, targets, nif_versions)

     given types:

         %{os_type: not_set()}, dynamic(), dynamic()

     but expected one of:

         %{..., os_type: term()}, term(), term()

     where "nif_versions" was given the type:

         # type: dynamic()
         # from: lib/emerge_skia/build_config.ex:235:53
         nif_versions

     where "targets" was given the type:

         # type: dynamic()
         # from: lib/emerge_skia/build_config.ex:235:44
         targets

     typing violation found at:
     │
 236 │     RustlerPrecompiled.target(%{}, targets, nif_versions)
     │                        ~
     │
     └─ lib/emerge_skia/build_config.ex:236:24: EmergeSkia.BuildConfig.default_precompiled_target_resolver/2

     warning: the variable "len" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
     │
 243 │     <<attrs_bin::binary-size(len), rest::binary>> = rest
     │                              ~
     │
     └─ lib/emerge/engine/attr_codec.ex:243:30: Emerge.Engine.AttrCodec.decode_state_style/1

     warning: the variable "len" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
     │
 270 │     <<payload::binary-size(len), rest::binary>> = rest
     │                            ~
     │
     └─ lib/emerge/engine/attr_codec.ex:270:28: Emerge.Engine.AttrCodec.decode_animation/2

     warning: the variable "len" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
     │
 295 │     <<attrs_bin::binary-size(len), rest::binary>> = rest
     │                              ~
     │
     └─ lib/emerge/engine/attr_codec.ex:295:30: Emerge.Engine.AttrCodec.decode_animation_keyframes/3

     warning: the variable "len" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
     │
 336 │     <<value::binary-size(len), rest::binary>> = rest
     │                          ~
     │
     └─ lib/emerge/engine/attr_codec.ex:336:26: Emerge.Engine.AttrCodec.decode_string/1

     warning: the variable "len" is accessed inside size(...) of a bitstring but it was defined outside of the match. You must precede it with the pin operator
     │
 346 │     <<value::binary-size(len), rest::binary>> = rest
     │                          ~
     │
     └─ lib/emerge/engine/attr_codec.ex:346:26: Emerge.Engine.AttrCodec.decode_atom/1

Starting EmergeSkia demo... backend=wayland
Configured demo logical source root /home/ken/emerge/_build/dev/lib/emerge/priv/demo_images
Configured demo font asset demo_fonts/Lobster-Regular.ttf

[Damirados]

It should consistently start now.