Re: [PATCH v2 1/2] common/rc,xfs/207: Adding a common helper function to check xflag bits on a given file

["Darrick J. Wong" <djwong@kernel.org>]

On Sat, Nov 16, 2024 at 12:36:26AM +0530, Ritesh Harjani wrote:
> "Darrick J. Wong" <djwong@kernel.org> writes:
> 
> > On Fri, Nov 15, 2024 at 09:45:58AM +0530, Nirjhar Roy wrote:
> >> This patch defines a common helper function to test whether any of
> >> fsxattr xflags field is set or not. We will use this helper in the next
> >> patch for checking extsize (e) flag.
> >> 
> >> Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> >> Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
> >> Signed-off-by: Nirjhar Roy <nirjhar@linux.ibm.com>
> >> ---
> >>  common/rc     |  7 +++++++
> >>  tests/xfs/207 | 13 ++-----------
> >>  2 files changed, 9 insertions(+), 11 deletions(-)
> >> 
> >> diff --git a/common/rc b/common/rc
> >> index 2af26f23..fc18fc94 100644
> >> --- a/common/rc
> >> +++ b/common/rc
> >> @@ -41,6 +41,13 @@ _md5_checksum()
> >>  	md5sum $1 | cut -d ' ' -f1
> >>  }
> >>  
> >> +# Check whether a fsxattr xflags character ($2) field is set on a given file ($1).
> >> +# e.g, fsxattr.xflags =  0x80000800 [----------e-----X]
> >> +_test_fsx_xflags_field()
> >
> > How about we call this "_test_fsxattr_xflag" instead?
> >
> > fsx is already something else in fstests.
> >
> >> +{
> >> +    grep -q "fsxattr.xflags.*\[.*$2.*\]" <($XFS_IO_PROG -c "stat" "$1")
> >> +}
> >
> > Not sure why this lost the xfs_io | grep -q structure.  The return value
> > of the whole expression will always be the return value of the last
> > command in the pipeline.
> >
> 
> I guess it was suggested here [1]
> 
> [1]: https://lore.kernel.org/all/20241025025651.okneano7d324nl4e@dell-per750-06-vm-08.rhts.eng.pek2.redhat.com/

Ah.

> root-> grep -q "hello" <(echo "hello world"); echo $?
> 0
> 
> The cmd is not using the unnamed pipes ("|") any more. It's spawning the
> process () which does echo "hello world" and creating a named pipe or
> say temporary FD <() which is being read by grep now. So we still will
> have the correct return value. Slightly inefficitent compared to unnamed
> pipes though I agree. 

Well... it's subtle, being bash, right? :)

bash creates a pipe and a subprocess for the "echo hello world", then
hooks its stdout to the pipe, just like a regular "|" pipe.

But for "grep -q hello" things are different -- for the grep process,
the pipe is added as a new fd (e.g. /dev/fd/63), and then that path is
provided on the command line.  So what bash is doing is:

	grep -q "hello" /dev/fd/63

AFAICT for grep this makes no difference unless you want it to tell you
filenames:

$ grep -l hello <(echo hello world)
/dev/fd/63
$ echo hello world | grep -l hello
(standard input)

and I'm sure there's other weird implications that I'm not remembering.

> > (Correct?  I hate bash...)
> >
> 
> root-> ls -la <(echo "hello world");
> lr-x------ 1 root root 64 Nov 16 00:42 /dev/fd/63 -> 'pipe:[74211850]'
> 
> Did I make you hate it more? ;) 

Yep!

--D

> 
> -ritesh
> 
> > --D
> >
> >> +
> >>  # Write a byte into a range of a file
> >>  _pwrite_byte() {
> >>  	local pattern="$1"
> >> diff --git a/tests/xfs/207 b/tests/xfs/207
> >> index bbe21307..4f6826f3 100755
> >> --- a/tests/xfs/207
> >> +++ b/tests/xfs/207
> >> @@ -21,15 +21,6 @@ _require_cp_reflink
> >>  _require_xfs_io_command "fiemap"
> >>  _require_xfs_io_command "cowextsize"
> >>  
> >> -# Takes the fsxattr.xflags line,
> >> -# i.e. fsxattr.xflags = 0x0 [--------------C-]
> >> -# and tests whether a flag character is set
> >> -test_xflag()
> >> -{
> >> -    local flg=$1
> >> -    grep -q "\[.*${flg}.*\]" && echo "$flg flag set" || echo "$flg flag unset"
> >> -}
> >> -
> >>  echo "Format and mount"
> >>  _scratch_mkfs > $seqres.full 2>&1
> >>  _scratch_mount >> $seqres.full 2>&1
> >> @@ -65,14 +56,14 @@ echo "Set cowextsize and check flag"
> >>  $XFS_IO_PROG -c "cowextsize 1048576" $testdir/file3 | _filter_scratch
> >>  _scratch_cycle_mount
> >>  
> >> -$XFS_IO_PROG -c "stat" $testdir/file3 | grep 'fsxattr.xflags' | test_xflag "C"
> >> +_test_fsx_xflags_field "$testdir/file3" "C" && echo "C flag set" || echo "C flag unset"
> >>  $XFS_IO_PROG -c "cowextsize" $testdir/file3 | _filter_scratch
> >>  
> >>  echo "Unset cowextsize and check flag"
> >>  $XFS_IO_PROG -c "cowextsize 0" $testdir/file3 | _filter_scratch
> >>  _scratch_cycle_mount
> >>  
> >> -$XFS_IO_PROG -c "stat" $testdir/file3 | grep 'fsxattr.xflags' | test_xflag "C"
> >> +_test_fsx_xflags_field "$testdir/file3" "C" && echo "C flag set" || echo "C flag unset"
> >>  $XFS_IO_PROG -c "cowextsize" $testdir/file3 | _filter_scratch
> >>  
> >>  status=0
> >> -- 
> >> 2.43.5
> >> 
> >> 
>