Emscripten upgrade breaks Envoy #1273
Description
There are at least two issues upgrading Envoy to v3.1.44 (seen in envoyproxy/envoy#29118), and I suspect both are related to Bazel toolchain resolution since #1036.
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Inability to build cmake targets, such as zlib. The error is
CMAKE_C_COMPILER not set(details).When I try to add an emcmake toolchain (draft), I run into toolchain resolution issues (details). The issue is that the regular cmake toolchain uses
exec_compatible_withqualifiers, rather thantarget_compatible_withwhich would skip this choice for wasm targets.I suspect there are 2 possible roads to fix cmake support:
- use regular cmake, but configure it properly to use the emcc compiler
- use a custom toolchain to invoke emcmake, fix above toolchain resolution issues
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Improper cpp toolchain resolution. Envoy uses Rust to build Wasm plugins, and Rust registers a dummy cc toolchain. For reasons I don't understand, the Rust dummy toolchain is selected ahead of the real emcc toolchain, causing compilation failures like
DUMMY_GCC_TOOL failed(it's not intended to actually build anything). It seems like emsdk needs a way to force its toolchain after the emscripten transition.
P.S. The above issues pop up when setting incompatible_use_toolchain_transition (or flags like --incompatible_enable_cc_toolchain_resolution), which many projects and targets already rely on.