Wrong output for single character files without newline

[Navin Kabra]

Consider this:

echo -n a > /tmp/a
echo -n b > /tmp/b
diff -B /tmp/a /tmp/b

Clearly, the two files are different, yet, diff seems to think that the
files are identical. I've managed to reproduce this problem on Ubuntu
14.04 with diffutils 3.3, on CloudLinux 5.10 with diffutils 2.8.1, and
also Ubuntu 10.04 with diffutils 2.8.1.

If I don't use the -B option, the problem goes away. If the files do end
with a newline, the problem goes away. If the files contain more than 1
character, the problem goes away. If combined with *some* of the other
options (e.g. -e or -y) the problem goes away.

[Eric Blake]

[adding bug-diffutils, as requested by diff --help]

Post by Navin Kabra
echo -n a > /tmp/a
echo -n b > /tmp/b
diff -B /tmp/a /tmp/b

'echo -n' is non-portable. Please get used to using 'printf' instead.

Post by Navin Kabra
Clearly, the two files are different, yet, diff seems to think that the
files are identical. I've managed to reproduce this problem on Ubuntu
14.04 with diffutils 3.3, on CloudLinux 5.10 with diffutils 2.8.1, and
also Ubuntu 10.04 with diffutils 2.8.1.
If I don't use the -B option, the problem goes away. If the files do end
with a newline, the problem goes away. If the files contain more than 1
character, the problem goes away. If combined with *some* of the other
options (e.g. -e or -y) the problem goes away.

Actually, I couldn't reproduce -y making the problem go away:

$ ./src/diff -By <(printf a) <(printf b)
a b
$ echo $?
0

Thanks for the extensive analysis; I can confirm that this bug is still
present in the latest diffutils.git sources, although I have not
personally hunted for the culprit line of code yet.