libdiff: Detect and recover from file truncation.

[des]

Summary

If a memory-mapped file is truncated before we get to the end, the
atomizer may catch SIGBUS. Detect that, reduce the input length to
what we were actually able to read, and set a flag so the caller can
take further action (e.g. warn the user and / or start over).

Sponsored by: Klara, Inc.

[markj]

contrib/libdiff/lib/diff_atomize_text.c

190–191

Why exactly is this change needed? I can't see the problem with continuing to use pos.

[des]

contrib/libdiff/lib/diff_atomize_text.c

190–191

pos is volatile, line_start can be cached / optimized.

[markj]

contrib/libdiff/lib/diff_atomize_text.c

190–191

But then why do we still have .at = pos?

[des]

contrib/libdiff/lib/diff_atomize_text.c

190–191

Because I didn't spot it...

[des]

line_start