encoding: fix trim() to be O(n) instead of O(n^2)
`encoding.trim()` iterated over the possible lengths smaller than the
input and created a slice for each. It then calculated the column
width of the result, which is of course O(n), so the overall algorithm
was O(n). This patch rewrites it to iterate over the unicode
characters, keeping track of the length so far. Also, the old
algorithm started from the end of the string, which made it much worse
when the input is large and the limit is small (such as the typical 72
we pass to it).
You can time it by running something like this:
```
time python3 -c 'from mercurial.utils import stringutil; print(stringutil.ellipsis(b"0123456789" * 1000, 5))'
```
That drops from 4.05 s to 83 ms with this patch (and most of that is
of course startup time).
Differential Revision: https://phab.mercurial-scm.org/D12089
A script that implements uppercasing all letters in a file.
$ UPPERCASEPY="$TESTTMP/uppercase.py"
$ cat > $UPPERCASEPY <<EOF
> import sys
> from mercurial.utils.procutil import setbinary
> setbinary(sys.stdin)
> setbinary(sys.stdout)
> sys.stdout.write(sys.stdin.read().upper())
> EOF
$ TESTLINES="foo\nbar\nbaz\n"
$ printf $TESTLINES | "$PYTHON" $UPPERCASEPY
FOO
BAR
BAZ
This file attempts to test our workarounds for pickle's lack of
support for short reads.
$ cat >> $HGRCPATH <<EOF
> [extensions]
> fix =
> [fix]
> uppercase-whole-file:command="$PYTHON" $UPPERCASEPY
> uppercase-whole-file:pattern=set:**
> EOF
$ hg init repo
$ cd repo
# Create a file that's large enough that it seems to not fit in
# pickle's buffer, making it use the code path that expects our
# _blockingreader's read() method to return bytes.
$ echo "some stuff" > file
$ for i in $($TESTDIR/seq.py 13); do
> cat file file > tmp
> mv -f tmp file
> done
$ hg commit -Am "add large file"
adding file
Check that we don't get a crash
$ hg fix -r .
saved backup bundle to $TESTTMP/repo/.hg/strip-backup/*-fix.hg (glob)