Re: [PATCH v5 15/15] read-cache: make sure file handles are not inherited by child processes

> From: Lars Schneider <larsxschneider@gmail.com>
> 
> Consider the case of a file that requires filtering and is present in branch A
> but not in branch B. If A is the current HEAD and we checkout B then the
> following happens:
> 
> 1. ce_compare_data() opens the file
> 2.   index_fd() detects that the file requires to run a clean filter and
>      calls index_stream_convert_blob()
> 4.     index_stream_convert_blob() calls convert_to_git_filter_fd()
> 5.       convert_to_git_filter_fd() calls apply_filter() which creates a new
>          long running filter process (in case it is the first file of this kind
>          to be filtered)
> 6.       The new filter process inherits all file handles. This is the default
>          on Linux/OSX and is explicitly defined in the `CreateProcessW` call
>          in `mingw.c` on Windows.
> 7. ce_compare_data() closes the file
> 8. Git unlinks the file as it is not present in B
> 
> The unlink operation does not work on Windows because the filter process has
> still an open handle to the file. Apparently that is no problem on Linux/OSX.
> Probably because "[...] the two file descriptors share open file status flags"
> (see fork(2)).