Fix building Web Viewer on macOS #10828

ntjohnson1 · 2025-08-07T14:20:17Z

What

Basically mac has a fun combination where you can have clang/llvm components but not llvm-ar. Mac's ar pads bytes in way that make llvm barf. Our pixi env for c++ has the cxx-compiler package which has a more full fledged llvm install. On mac on activation set AR to point to llvm-ar. It works on my machine! I'm not sure if we should have a second person try or if we should setup a test for the web build on mac just for this weird edge case.

github-actions · 2025-08-07T14:20:45Z

Latest documentation preview deployed successfully.

Result	Commit	Link
✅	`ae04291`	https://landing-qv1io6xa2-rerun.vercel.app/docs

^{Note: This comment is updated whenever you push a commit.}

github-actions · 2025-08-07T14:20:56Z

Web viewer built successfully. If applicable, you should also test it:

I have tested the web viewer

Result	Commit	Link	Manifest
✅	`ae04291`	https://rerun.io/viewer/pr/10828	`+nightly` `+main`

^{Note: This comment is updated whenever you push a commit.}

emilk · 2025-08-07T14:51:51Z

Thanks for working on this! I'll give it a spin

emilk · 2025-08-07T14:53:57Z

I tested this by first running brew uninstall llvm to remove my old workaround for the issue. On main that indeed leads to LLVM error: section too large.

Checking out this branch and running pixi run rerun-web instead leads to a new exciting error:

❯ pr rerun-web        
✨ Pixi task (rerun-build-web in default): rustup target add wasm32-unknown-unknown && cargo run --quiet -p re_dev_tools -- build-web-viewer --no-default-features --features analytics,map_view --debug
info: component 'rust-std' for target 'wasm32-unknown-unknown' is up to date
Building web viewer…

Compiling Rust to wasm in /Users/emilk/code/rerun/rerun/target_wasm…
/Users/emilk/code/rerun/rerun> env -u CARGO_ENCODED_RUSTFLAGS -u RUSTFLAGS "cargo" "build" "--quiet" "--package=re_viewer" "--lib" "--target=wasm32-unknown-unknown" "--target-dir=/Users/emilk/code/rerun/rerun/target_wasm" "--no-default-features" "--features=analytics,map_view" "--config=.cargo/config.toml"
error: failed to run custom build command for `psm v0.1.26`

Caused by:
  process didn't exit successfully: `/Users/emilk/code/rerun/rerun/target_wasm/debug/build/psm-6f11cdb04821ff68/build-script-build` (exit status: 1)
  --- stdout
  cargo:rustc-check-cfg=cfg(switchable_stack,asm,link_asm)
  OPT_LEVEL = Some(2)
  OUT_DIR = Some(/Users/emilk/code/rerun/rerun/target_wasm/wasm32-unknown-unknown/debug/build/psm-a753c07d8387c98b/out)
  TARGET = Some(wasm32-unknown-unknown)
  HOST = Some(aarch64-apple-darwin)
  cargo:rerun-if-env-changed=CC_wasm32-unknown-unknown
  CC_wasm32-unknown-unknown = None
  cargo:rerun-if-env-changed=CC_wasm32_unknown_unknown
  CC_wasm32_unknown_unknown = None
  cargo:rerun-if-env-changed=TARGET_CC
  TARGET_CC = None
  cargo:rerun-if-env-changed=CC
  CC = None
  cargo:rerun-if-env-changed=CC_ENABLE_DEBUG_OUTPUT
  RUSTC_WRAPPER = None
  cargo:rerun-if-env-changed=CRATE_CC_NO_DEFAULTS
  CRATE_CC_NO_DEFAULTS = None
  DEBUG = Some(false)
  cargo:rerun-if-env-changed=CFLAGS
  CFLAGS = None
  cargo:rerun-if-env-changed=TARGET_CFLAGS
  TARGET_CFLAGS = None
  cargo:rerun-if-env-changed=CFLAGS_wasm32_unknown_unknown
  CFLAGS_wasm32_unknown_unknown = None
  cargo:rerun-if-env-changed=CFLAGS_wasm32-unknown-unknown
  CFLAGS_wasm32-unknown-unknown = None
  CARGO_ENCODED_RUSTFLAGS = Some(--cfg=web_sys_unstable_apis--cfg=getrandom_backend="wasm_js"--cfg=web_sys_unstable_apis--cfg=getrandom_backend="wasm_js")
  cargo:rustc-cfg=asm
  cargo:rustc-cfg=link_asm
  cargo:rustc-cfg=switchable_stack
  cargo:rerun-if-env-changed=AR_wasm32-unknown-unknown
  AR_wasm32-unknown-unknown = None
  cargo:rerun-if-env-changed=AR_wasm32_unknown_unknown
  AR_wasm32_unknown_unknown = None
  cargo:rerun-if-env-changed=TARGET_AR
  TARGET_AR = None
  cargo:rerun-if-env-changed=AR
  AR = Some(/.pixi/envs/cpp/bin/llvm-ar)
  cargo:rerun-if-env-changed=CC_KNOWN_WRAPPER_CUSTOM
  CC_KNOWN_WRAPPER_CUSTOM = None
  cargo:rerun-if-env-changed=ARFLAGS
  ARFLAGS = None
  cargo:rerun-if-env-changed=TARGET_ARFLAGS
  TARGET_ARFLAGS = None
  cargo:rerun-if-env-changed=ARFLAGS_wasm32_unknown_unknown
  ARFLAGS_wasm32_unknown_unknown = None
  cargo:rerun-if-env-changed=ARFLAGS_wasm32-unknown-unknown
  ARFLAGS_wasm32-unknown-unknown = None

  --- stderr


  error occurred in cc-rs: failed to find tool "/.pixi/envs/cpp/bin/llvm-ar": No such file or directory (os error 2)


Error: Failed to build Wasm

I do have .pixi/envs/cpp/bin/llvm-ar, but the above error has an extra slash 🤔

❯ pixi --version
pixi 0.46.0

Looks like ${PIXI_HOME} is "" on my machine O.o

Just removing ${PIXI_HOME}/ doesn't work either:

[target.osx-arm64.activation.env]
AR = ".pixi/envs/cpp/bin/llvm-ar"

->

error occurred in cc-rs: failed to find tool ".pixi/envs/cpp/bin/llvm-ar": No such file or directory (os error 2)

emilk · 2025-08-07T15:07:15Z

Related: Add ar notice to BUILD.md for Windows #10436

ntjohnson1 · 2025-08-07T15:16:00Z

UGH I thought I tested it when I changed from PWD to PIXI_HOME but I didn't (shame)

PWD doesn't work because pixi knows where home is so we don't need to run commands from project root.

pixi shell-hook shows all the magic variable pixi manages https://pixi.sh/dev/workspace/environment/
So PIXI_PROJECT_ROOT is the right fully defined home path. Will push in a sec.

emilk · 2025-08-07T15:19:53Z

That got us to a new place!

Compiling Rust to wasm in /Users/emilk/code/rerun/rerun/target_wasm…
/Users/emilk/code/rerun/rerun> env -u CARGO_ENCODED_RUSTFLAGS -u RUSTFLAGS "cargo" "build" "--quiet" "--package=re_viewer" "--lib" "--target=wasm32-unknown-unknown" "--target-dir=/Users/emilk/code/rerun/rerun/target_wasm" "--no-default-features" "--features=analytics,map_view" "--config=.cargo/config.toml"
rustc-LLVM ERROR: malformed uleb128, extends past end
error: could not compile `ring` (lib)

Caused by:
  process didn't exit successfully: `/Users/emilk/.rustup/toolchains/1.85.0-aarch64-apple-darwin/bin/rustc --crate-name ring --edition=2021 /Users/emilk/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/ring-0.17.14/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=173 --crate-type lib --emit=dep-info,metadata,link -C opt-level=2 -C embed-bitcode=no -C debug-assertions=on --cfg 'feature="alloc"' --cfg 'feature="default"' --cfg 'feature="dev_urandom_fallback"' --cfg 'feature="std"' --cfg 'feature="wasm32_unknown_unknown_js"' --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values("alloc", "default", "dev_urandom_fallback", "less-safe-getrandom-custom-or-rdrand", "less-safe-getrandom-espidf", "slow_tests", "std", "test_logging", "unstable-testing-arm-no-hw", "unstable-testing-arm-no-neon", "wasm32_unknown_unknown_js"))' -C metadata=2ed7839866b4c257 -C extra-filename=-1ab34e33765a28b1 --out-dir /Users/emilk/code/rerun/rerun/target_wasm/wasm32-unknown-unknown/debug/deps --target wasm32-unknown-unknown -C strip=debuginfo -L dependency=/Users/emilk/code/rerun/rerun/target_wasm/wasm32-unknown-unknown/debug/deps -L dependency=/Users/emilk/code/rerun/rerun/target_wasm/debug/deps --extern cfg_if=/Users/emilk/code/rerun/rerun/target_wasm/wasm32-unknown-unknown/debug/deps/libcfg_if-d842852db5826de0.rmeta --extern getrandom=/Users/emilk/code/rerun/rerun/target_wasm/wasm32-unknown-unknown/debug/deps/libgetrandom-a8d9ea6ac5b6fec0.rmeta --extern untrusted=/Users/emilk/code/rerun/rerun/target_wasm/wasm32-unknown-unknown/debug/deps/libuntrusted-cfe878d4eca71261.rmeta --cap-lints allow --cfg=web_sys_unstable_apis '--cfg=getrandom_backend="wasm_js"' --cfg=web_sys_unstable_apis '--cfg=getrandom_backend="wasm_js"' -L native=/Users/emilk/code/rerun/rerun/target_wasm/wasm32-unknown-unknown/debug/build/ring-38da87773bd67974/out -l static=ring_core_0_17_14_ -l static=ring_core_0_17_14__test` (exit status: 101)
Error: Failed to build Wasm

ntjohnson1 · 2025-08-07T15:25:43Z

That got us to a new place!

Umm
versions

$ cargo --version
cargo 1.87.0 (99624be96 2025-05-06)
$ rustup --version
rustup 1.28.2 (e4f3ad6f8 2025-04-28)
$ rustc --version
rustc 1.87.0 (17067e9ac 2025-05-09)
$ clang --version
clang --version
Apple clang version 17.0.0 (clang-1700.0.13.5)
Target: arm64-apple-darwin24.5.0
Thread model: posix

commands

$ rm -rf target_wasm/
$ rm -rf target/ # for fun
$ pixi run rerun-build-web
...snip
Finished /Users/nick/repos/rerun/web_viewer/re_viewer_bg.wasm

emilk · 2025-08-07T15:33:08Z

rm -rf target_wasm/ was the answer! It works!

emilk

Woohoo!

ntjohnson1 · 2025-08-07T15:37:31Z

Woohoo!

should we setup a test for the web build on mac just for this weird edge case?

emilk · 2025-08-08T10:35:09Z

should we setup a test for the web build on mac just for this weird edge case?

We already do, but not on every PR, unless we explicitly opt-in to the full test suite with @rerun-bot full-check

### Related Closes #9849 ### What Basically mac has a fun combination where you can have `clang`/`llvm` components but not `llvm-ar`. Mac's `ar` pads bytes in way that make llvm barf. Our pixi env for c++ has the `cxx-compiler` package which has a more full fledged llvm install. On mac on activation set `AR` to point to `llvm-ar`. It works on my machine! I'm not sure if we should have a second person try or if we should setup a test for the web build on mac just for this weird edge case.

Add env variable on activation on mac with todo to prune

c794c1a

ntjohnson1 added 🧑‍💻 dev experience developer experience (excluding CI) include in changelog 🍎 MacOS Issues specifically about MacOS support labels Aug 7, 2025

emilk self-assigned this Aug 7, 2025

emilk mentioned this pull request Aug 7, 2025

Add ar notice to BUILD.md for Windows #10436

Closed

Add the right pixi inserted env variable

ae04291

emilk approved these changes Aug 7, 2025

View reviewed changes

emilk changed the title ~~Fix Mac Web Viewer Build With Workaround~~ Fix building Web Viewer on macOS Aug 7, 2025

emilk merged commit 08b8464 into main Aug 7, 2025
48 checks passed

emilk deleted the nick/mac_wasm_build branch August 7, 2025 15:35

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Fix building Web Viewer on macOS #10828

Fix building Web Viewer on macOS #10828

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