Files open O_WRONLY not supported in fallocate emulation.

In the posix_fallocate description in the manual we list various
drawbacks with the emulation, including the fact that a file opened
with O_APPEND fails with EBADF. Similarly a file opened with O_WRONLY
fails with EBADF. We must be able to emulate a compare-and-swap via
pread/compare/pwrite in order to make the emulation as safe as possible.
It is not acceptable to ignore the read failure because it could result
in significant data loss across all of the blocks. There is no other way
to make this work without a true atomic CAS and SIGBUS handler (which
is looking more attractive as a way to remove the race condition).

This patch adds O_WRONLY to the manual as another bullet to clarify the
limits of the emulation.

Manual looks good in PDF.

1 files changed, 4 insertions, 0 deletions

diff --git a/manual/filesys.texi b/manual/filesys.texi
index ed4f5fdd85..40648853bd 100644
--- a/manual/filesys.texi
+++ b/manual/filesys.texi
@@ -3300,6 +3300,10 @@ underlying file in the to-be-allocated area.  Non-null bytes could be
 overwritten with null bytes.
 
 @item
+If @var{fd} has been opened with the @code{O_WRONLY} flag, the function
+will fail with an @code{errno} value of @code{EBADF}.
+
+@item
 If @var{fd} has been opened with the @code{O_APPEND} flag, the function
 will fail with an @code{errno} value of @code{EBADF}.