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From Branko Čibej <br...@apache.org>
Subject Re: svn commit: r1702305 - /subversion/trunk/subversion/libsvn_subr/stream.c
Date Fri, 11 Sep 2015 09:01:55 GMT
On 10.09.2015 23:50, Evgeny Kotkov wrote:
> Stefan Fuhrmann <stefan2@apache.org> writes:
>
> +   * Special files like STDIN will have no flags set at all. In that case,
> +   * we can't filter and must allow any operation - which may then fail at
> +   * the APR level further down the stack.
> +   */
> +  apr_uint32_t flags = apr_file_flags_get(file);
> +  svn_boolean_t supports_read = (flags == 0) || (flags & APR_READ);
> +  svn_boolean_t supports_write = (flags == 0) || (flags & APR_WRITE);
>
> Files corresponding to the standard I/O streams actually have the appropriate
> APR_READ / APR_WRITE flags set starting from APR 1.3 — see, for example,
> apr_file_open_flags_stderr() in [1].  Hence, the (flags == 0) check is going
> to return false for them.
>
>> Note that this check is not perfect for arbitrary APR file handles
>> (may enable more functions than the handle actually supports) but
>> works correctly for our STD* streams and the files opened through
>> our svn_io_* API.
> The actual problem, to my mind, is that relying on flags to determine if the
> file allows reading or writing, is fragile.  There are examples of apr_file_t's
> that don't have the corresponding flags, but still allow reading and writing,
> and svn_stream_from_aprfile2() is going to break for them.
>
> One example would be apr_file_pipe_create() on Unix that sets APR_INHERIT
> flag on the created pipe [2].  Another example is creating a pipe on Windows
> that currently initializes flags to zero [3].

I've reviewed the JavaHL code again and it indeed appears that this is
caused by the difference in implementations of apr_file_pipe_create_ex()
on Windows and elsewhere. The bindings code itself is not
platform-specific; see TunnelContext in
subversion/bindings/javahl/native/OperationContext.cpp. Those pipes get
wrapped into streams deep in the RA layer.

One could argue that apr_file_pipe_create is broken since it doesn't set
the appropriate flags on the input and output handles, but that doesn't
really help make this particular instance in the bindings code work.

-- Brane

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