Fix issue with transitive/recursive building of PIC libraries #25707
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kripken
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Nov 3, 2025
This issue was introduces in emscripten-core#25522 when `-sRELOCATABLE` was removed from `get_base_cflags. This is a somewhat tricky to reproduce / understand issue with transitive library building: 1. User attempts to compile program using SDL2 + SDL2_GFX. 2. `emcc` attempts to builds all needed libraries in reverse dependency order. And ends up with the follows sequence: 1. libsdl2-pic 2. libsdl2_gfx-pic 3. `emcc` builds libsdl2-pic 4. `emcc` builds libsdl2_gfx-pic. The command that it constructs when building each source file looks like `emcc -fPIC gfx.c -sUSE_SDL=2`. However because neither `-sMAIN_MODULE` nor `-sRELOCATABLE` were specified this `emcc` command will not try to build the non-PIC version of libsdl2. At this point the compiler will crash with: `AssertionError: attempt to lock the cache while a parent process is holding the lock (sysroot/lib/wasm32-emscripten/libSDL2.a)` This issue is hard to reproduce because if libsdl2 is already built it will not trigger the issue.
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This issue was introduces in #25522 when
-sRELOCATABLEwas removed from `get_base_cflags.This is a somewhat tricky to reproduce / understand issue with transitive library building:
User attempts to compile program using SDL2 + SDL2_GFX.
emccattempts to builds all needed libraries in reverse dependency order. And ends up with the follows sequence:libsdl2-pic
libsdl2_gfx-pic
emccbuilds libsdl2-picemccbuilds libsdl2_gfx-pic. The command that it constructs when building each source file looks likeemcc -fPIC gfx.c -sUSE_SDL=2. However because neither-sMAIN_MODULEnor-sRELOCATABLEwere specified thisemcccommand will not try to build the non-PIC version of libsdl2. At this point the compiler will crash with:AssertionError: attempt to lock the cache while a parent process is holding the lock (sysroot/lib/wasm32-emscripten/libSDL2.a)This issue is hard to reproduce because if libsdl2 is already built it will not trigger the issue.