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fix(fspy-shm): map the Windows IPC view above Cygwin's fixed layout - #952
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The 4 GiB view the injected DLL maps before msys-2.0.dll starts lands on Cygwin's fixed-address shared regions, so MSYS2 programs in tracked tasks fail with Win32 error 487. Ask MapViewOfFile3 for an address at or above Cygwin's mmap arena, falling back to MapViewOfFile where it is unavailable. Fixes voidzero-dev#830.
Replace `map_view_of_file_above`, which returned `Option<Result<_>>` and fixed read-write access and offset 0, with `get_map_view_of_file3()` returning `Option<MapViewOfFile3>` and `MapViewOfFile3::call`, a thin wrapper that takes protection, offset, size, and an optional lowest address. `None` now only means the function is missing; whether to fall back stays in `fspy_shm`. The value is `Copy`, so callers can keep it without hidden global state in `fspy_nostd`. Document why the lookup is loader-lock safe, use `GetCurrentProcess()` instead of a hand-written pseudo-handle, and test the wrapper in `fspy_nostd` against a paging-file mapping. Add the changelog entry and describe the view placement in the `fspy_shm` README. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01978g7Vcc3w5QWZTswuUsoN
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Fixes #830.
Cause
The injected DLL maps the whole 4 GiB IPC view during
DLL_PROCESS_ATTACH, beforemsys-2.0.dllinitializes, and lets the system pick the address. A Cygwin/MSYS2 process keeps a fixed layout (winsup/cygwin/local_includes/memory_layout.h): the executable at0x100400000, the DLL at0x180040000, shared regions at0x1a0000000-0x200000000, then DLL arenas, thread stacks, the cygheap, and an mmap arena up to0x700000000000. Up to about 4032 MiB the view still fits in the hole below the executable. A 4 GiB view doesn't, so it lands abovesh.exe, over the shared regions, andmsys-2.0.dll's fixed-addressMapViewOfFileExforshared.5fails with error 487. The threshold measurements are in voidzero-dev/vite-plus#3121.Change
ShmHandle::mapon Windows now asksMapViewOfFile3for an address at or above0x7000_0000_0000(Cygwin'sMMAP_STORAGE_HIGH), which leaves the view clear of every range Cygwin places at a fixed address. User space goes up to0x7FFF_FFFE_FFFF, so there is about 16 TiB of room.MapViewOfFile3is looked up inkernelbase.dllat run time, not imported, so the DLL still loads on Windows 10 before 1803 (Server 2016). There, or if the call fails,mapfalls back to the plainMapViewOfFileit used before.Tests
On Windows 11 (10.0.26200) with Git for Windows 2.56.0.windows.4 (bash 5.3.15):
four_gib_mapping_lies_above_the_cygwin_fixed_layoutchecks that both the creating and the opening view land above0x7000_0000_0000.cargo test -p fspy_shm -p fspy_nostdpasses, and so doescargo clippy -p fspy_nostd -p fspy_shm --all-targets -- -D warnings.vt run: a cached task (node probe.mjs) whose script spawns Git Bash'ssh.exe -c, withvt cache cleanbefore each run:b3e73102(main), 3/3 runs fail withbash.exe: *** fatal error - MapViewOfFileEx 'shared.5'(0x12C, size 60344, m 0, created 0), Win32 error 487. Terminating.shprints its version.AI-assisted (Claude Opus 5.5): I reviewed the change and ran the Windows tests above.