add file-backed ReadonlySharedMemory#1498
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Wrap host-mapping lifetime management in dedicated RAII guards. On Linux `Mmap` owns the `mmap` reservation and calls `munmap` on drop. On Windows `MappedView` owns the mapped view and calls `UnmapViewOfFile`, and `FileMapping` owns the file-mapping HANDLE and calls `CloseHandle`. `HostMapping` becomes a thin aggregate of these guards instead of carrying raw `(ptr, size, handle)` fields with a manual `Drop` impl, and exposes `ptr()`, `size()`, and `file_mapping_handle()` accessors that the `SharedMemory` trait defaults use. Signed-off-by: Ludvig Liljenberg <4257730+ludfjig@users.noreply.github.com>
Signed-off-by: Ludvig Liljenberg <4257730+ludfjig@users.noreply.github.com>
Signed-off-by: Ludvig Liljenberg <4257730+ludfjig@users.noreply.github.com>
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draft, will rebase once #1485 is merged
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ReadonlySharedMemory::from_file(file, guest_mapped_size), a file-backed counterpart to from_bytes. It reserves [guard][blob][guard] with the file mapped over the middle slot: Linux uses a PROT_NONE anonymous reservation with MAP_FIXED, Windows uses a VirtualAlloc2 placeholder split into three with MapViewOfFile3(MEM_REPLACE_PLACEHOLDER). The Windows path uses the WindowsMapping::FileBacked variant added in #1485 so surrogate-process plumbing can tell the two backings apart.ReadonlySharedMemory::from_bytesalso changes from Option to a required usize.This is currently unused but will be used in the future.