Fix long line perf of compilation-shell-minor-mode

[t0yv0]

To reproduce the issue of Emacs freezing on long lines, do the following:

M-x shell
M-x compilation-shell-minor-mode
$ python3 -c 'print("x"*1024*8)'

Depending on the machine configuration, make your lines longer. 1024*8 is enough to freeze my MacBook Pro 2019 visibly but I need more characters to slow down an XPS-13 running Ubuntu.

In the real world the issue comes up when I'm enjoying my compilation-shell-minor-mode buffer but accidentally cat or print some data with very long lines. Then Emacs freezes and I experience one of my very rare moments of unhappiness with the editor.

Using M-x profiler-start, profiler-stop, profiler-report the culprit is compilation-parse-errors. The function loops over many (about 50) patterns of potential error output and scans the buffer for them repeatedly.

The PR edits the function to use a limited version of re-search-forward. Instead of finding the pattern everywhere, it only looks for the pattern in the first 1024 chars of every line. It seems plausible that real-world compiler warnings would sit close to the beginning of lines.

With the patch the experience (in combination with global-so-long-mode) becomes tolerable; while not lightning fast, Emacs is a lot more responsive in this situation and I as a user retain control to fix the problem, such as comint-clear-buffer etc.

[daviderestivo]

Emacs does not use GitHub for PRs. This is just a mirror for users convenience. I recommend you send a bug report to bug-gnu-emacs@gnu.org.

[t0yv0]

I did - thank you!