ios 26.2 shared memory issues

[dr-matt]

We're encountering a new issue in our web app since installing ios 26.2 rc as well as the preceding beta versions. This is probably a webkit regression rather than an emscripten bug, but posting here in case others are experienced the same issue and hoping to get a little insight from the wasm / emscripten side.

Our app is multi-threaded and uses growable memory. It works fine in previous ios versions through the current 26.1, but now with 26.2 we are seeing immediate memory-related crashes once wasm processing begins. Rather than out-of-memory, the crashes are due to RuntimeError: Out of bounds memory access buried deep within a dependency library. It has something to do with new allocations - in some cases just a std::vector::resize - but it's been challenging to pinpoint thus far. Release and debug builds both exhibit the problem.

Switching to fixed-memory solves the issue, as does forcing single-threaded mode (pthreads still enabled in the build but limiting threads to 1 at the application level). But the combination of growable memory + multiple threads consistently triggers this crash.

We'll continue to try isolating the problem and come up with a MRE to share here. But in the meantime I wanted to report this to see if there's any corroboration that might help with debugging and/or filing the right bug report.

emcc -v:
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 4.0.21 (d6f42310010ddec0753a69bab01a4b33aae255b7)
clang version 22.0.0git (https:/github.com/llvm/llvm-project 60513b8d6ebacde46e8fbe4faf1319ac87e990e3)
Target: wasm32-unknown-emscripten
Thread model: posix

command line from cmake:
em++ -O3 -DNDEBUG -flto --bind --no-entry --gc-sections -sFILESYSTEM=0 -sEXPORT_ES6=1 -sENVIRONMENT=web,worker -v -pthread -sPTHREAD_POOL_SIZE_STRICT=0 -SCROSS_ORIGIN=1 -sMALLOC=mimalloc -sALLOW_MEMORY_GROWTH=1 --closure 1 -sASSERTIONS=1 -sNO_DISABLE_EXCEPTION_CATCHING --emit-tsd full.d.ts @CMakeFiles/full.simd.threads.growable.dir/objects1.rsp -o full.simd.threads.growable.js @CMakeFiles/full.simd.threads.growable.dir/linkLibs.rsp

bin/wasm-ld -o full.simd.threads.growable.wasm -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-cxx-exceptions -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr /tmp/tmph1lclr0xlibemscripten_js_symbols.so --import-memory --shared-memory --strip-debug -u__cxa_atexit --export=emscripten_stack_get_end --export=emscripten_stack_get_free --export=emscripten_stack_get_base --export=emscripten_stack_get_current --export=emscripten_stack_init --export=_emscripten_stack_alloc --export=__getTypeName --export=_embind_initialize_bindings --export=_emscripten_thread_free_data --export=_emscripten_thread_crashed --export=__cxa_can_catch --export=__cxa_increment_exception_refcount --export=__cxa_decrement_exception_refcount --export=setThrew --export=__cxa_free_exception --export=__wasm_call_ctors --export=_emscripten_tls_init --export=_emscripten_thread_init --export=_emscripten_stack_restore --export=emscripten_stack_set_limits --export=_emscripten_thread_exit --export=__get_exception_message --export=free --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_lib_deps --export-if-defined=__stop_em_lib_deps --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=main --export-if-defined=__main_argc_argv --export-if-defined=fflush --export-table -z stack-size=65536 --max-memory=2147483648 --initial-memory=16777216 --no-entry --no-stack-first --table-base=1 --global-base=1024 -lembind-mt-rtti <lots of objects here> -lGL-mt-getprocaddr -lal -lhtml5 -lstubs-debug -lnoexit -lc-mt-debug -lmimalloc-mt -lcompiler_rt-mt -lc++-debug-mt -lc++abi-debug-mt -lsockets-mt