Recent git added a hack to explicitly avoid cygwin's pread as unsafe: http://git.661346.n2.nabble.com/PATCH-v2-1-1-index-pack-Disable-threading-on-cygwin-td7562195.html
Comments like this aren't very re-assuring either: http://sourceware.org/ml/cygwin/2011-06/msg00057.html or this big hairy comment in the source of fhandle_disk_file: Using this handle for pread/pwrite would break atomicity, because the read/write operation would have to be followed by a seek back to the old file position. What we do is to open another handle to the file on the first call to either pread or pwrite. This is used for any subsequent pread/pwrite. Thus the current file position of the "normal" file handle is not touched. FIXME: Note that this is just a hack. The problem with this approach is that a change to the file permissions might disallow to open the file with the required permissions to read or write. This appears to be a border case, but that's exactly what git does. It creates the file for reading and writing and after writing it, it chmods the file to read-only. Then it calls pread on the file to examine the content. This works, but if git would use the original handle to pwrite to the file, it would be broken with our approach. One way to implement this is to open the pread/pwrite handle right at file open time. We would simply maintain two handles, which wouldn't be much of a problem given how we do that for other fhandler types as well. However, ultimately fhandler_disk_file should become a derived class of fhandler_base_overlapped. Each raw_read or raw_write would fetch the actual file position, read/write from there, and then set the file position again. Fortunately, while the file position is not maintained by the I/O manager, it can be fetched and set to a new value by all processes holding a handle to that file object. Pread and pwrite differ from raw_read and raw_write just by not touching the current file pos. What still remains to make pread a first-class thread-safe implementation that obeys POSIX, so that I don't have to cripple my next build of git to avoid threaded pread? -- Eric Blake ebl...@redhat.com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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