tests/heredoctest.py

rust: speed up zstd decompression by re-using the decompression context

Admittedly, zstd is already pretty fast, but this change makes it
a bit faster yet: it saves ~5% of time it takes to read our large repo.

The actual motivating use case is treemanifest: in treemanifest
we end up reading *lots* of small directories, and many of them
need decompression, and there the saving for [rhg files] is >10%.
(which also seems unreasonable, we should probably keep things uncompressed more)

import sys


def flush():
    sys.stdout.flush()
    sys.stderr.flush()


globalvars = {}
lines = sys.stdin.readlines()
while lines:
    l = lines.pop(0)
    if l.startswith('SALT'):
        print(l[:-1])
    elif l.startswith('>>> '):
        snippet = l[4:]
        while lines and lines[0].startswith('... '):
            l = lines.pop(0)
            snippet += l[4:]
        c = compile(snippet, '<heredoc>', 'single')
        try:
            flush()
            exec(c, globalvars)
            flush()
        except Exception as inst:
            flush()
            print(repr(inst))