simplex-chat-python crashes on Windows: msvcrt.free on GHC-heap pointer (chat_migrate_init access violation) — propose exporting chat_free
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- 68/100
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调研方向
Start with src/Simplex/Chat/Mobile/Shared.hs, src/Simplex/Chat/Mobile.hs, and the binding call sites in packages/simplex-chat-python/src/simplex_chat/core.py and packages/simplex-chat-nodejs/cpp/simplex.cc. Reproduce the Windows failure with the documented ChatApi.init command, then verify that both bindings use the library-owned deallocator and that the command completes without the access violation.
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描述
Bug: simplex-chat-python crashes on Windows — msvcrt.free on a GHC-heap pointer
Summary
On Windows, simplex-chat-python (simplex_chat/core.py::_read_and_free) frees the result strings returned by libsimplex via msvcrt.free. But libsimplex is GHC/mingw-compiled, and those strings are allocated with Foreign.Marshal.Alloc.mallocBytes — which allocates from the GHC runtime's own heap, not msvcrt's heap. Freeing a GHC-heap pointer through msvcrt.free is a cross-CRT free, and it crashes on the very first FFI call:
OSError: exception: access violation reading 0xFFFFFFFFFFFFFFFF
This makes the Python package unusable on Windows out of the box.
Reproduce
pip install simplex-chat
python -c "import asyncio; from simplex_chat import ChatApi, SqliteDb; asyncio.run(ChatApi.init(SqliteDb(file_prefix='x')))"
Crash occurs inside chat_migrate_init → _read_and_free → _native.libc().free(...).
Root cause (source-level)
- Result strings are built in
src/Simplex/Chat/Mobile/Shared.hs:37and:45:
and returned from thebuf <- mallocBytes (len + 1) -- GHC runtime heapforeign export ccallfunctions insrc/Simplex/Chat/Mobile.hs. - The Python binding frees them with the C runtime's
free(msvcrt on win32) inpackages/simplex-chat-python/src/simplex_chat/core.py::_read_and_free. - On POSIX this happens to work because glibc's
malloc/freeis the shared allocator both sides resolve to. On Windows the two heaps are distinct, so the free is invalid.
Why the Node.js binding doesn't crash
packages/simplex-chat-nodejs/cpp/simplex.cc:163 also calls plain free(c_res). It gets away with it only because the binding runs inside a Node/V8 process whose CRT matches what libsimplex was linked against in that build. That's a coincidence of the host process, not a correct contract — the same latent mismatch exists there.
Proposed fix
Have libsimplex export its own deallocator so every caller frees with the matching allocator:
foreign export ccall "chat_free" cChatFree :: CString -> IO ()
cChatFree :: CString -> IO ()
cChatFree = Foreign.Marshal.Alloc.free
Then each binding calls chat_free (instead of libc().free / free) on the returned pointer. This is the standard pattern for C libraries that hand out heap-allocated buffers (cf. SDL_free, OPENSSL_free, sqlite3_free) and removes the cross-CRT assumption entirely on all platforms.
I'm happy to open a PR implementing chat_free plus the Python (_read_and_free) and Node (HandleCResult) call-site changes if you'd like — let me know your preferred name/signature.
Workaround (applied locally)
On win32, skip the free and only copy the string. The per-call JSON result strings are small (KBs), so the bounded leak is far safer than a hard crash; POSIX keeps the original free path.
Environment
- simplex-chat (Python) 6.5.6, LIBS_VERSION 6.5.6
- Windows 10/11 x86_64
- Python 3.11+
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