GNU logs - #61300, boring messages

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Subject: bug#61300: wc -c doesn't advance stdin position when it's a regular file
Resent-From: Stephane Chazelas <stephane@HIDDEN>
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From: Stephane Chazelas <stephane@HIDDEN>
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"wc -c" without filename arguments is meant to read stdin til
EOF and report the number of bytes it has read.

When stdin is on a regular file, GNU wc has that optimisation
whereby it skips the reading, does a pos = lseek(0,0,SEEK_CUR)
to find out its current position within the file, fstat(0) and
reports st_size - pos (assuming st_size > pos).

However, it does not move the position to the end of the file.
That means for instance that:

$ echo test > file
$ { wc -c; wc -c; } < file
5
5

Instead of 5, then 0:

$ { wc -c; cat; } < file
5
test

So the optimisation is incomplete.

It also reports the size of the file even if it could not possibly read it
because it's not open in read mode:

{ wc -c; } 0>> file
5

IMO, it should only do the optimisation if
- fcntl(F_GETFL) to check that the file is opened in O_RDONLY or O_RDWR
- current checks for /proc /sys-like filesystems
- pos > st_size
- lseek(0,st_size,SEEK_POS) is successful.

(that leaves a race window above where it could move the cursor
backward, but I would think that can be ignored as if something
else reads at the same time, there's not much we can expect
anyway).

-- 
Stephane

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X-Loop: help-debbugs@HIDDEN
From: help-debbugs@HIDDEN (GNU bug Tracking System)
To: Stephane Chazelas <stephane@HIDDEN>
Subject: bug#61300: Acknowledgement (wc -c doesn't advance stdin position
 when it's a regular file)
Message-ID: <handler.61300.B.167562165730635.ack <at> debbugs.gnu.org>
References: <20230205182728.5i2oi23purlzp6jj@HIDDEN>
X-Gnu-PR-Message: ack 61300
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Date: Sun, 05 Feb 2023 18:28:02 +0000

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