configure.py: Write last key in each section

[lambdageek]

The loop that writes the keys in each section of bootstrap.toml accumulates all the commented lines before a given key and emits them when it reaches the next key in the section. This ends up dropping lines accumulated for the last key

Fixes #143605

[rustbot]

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[Kobzol]

r? kobzol

@lolbinarycat does it look right? :)

[lolbinarycat]

I only changed parse_example_config so I don't think this is right.

[lolbinarycat]

Oh! I believe this is kinda an existing bug that just so happened to work before because the last block in a section was always the comments for the next section, while now those comments are considered technically part of the first item in a block.

Still, I'm not sure if any other user of write_uncommented is relying on this behavior, and evidently we don't have enough tests to catch a lot of edge cases...

It would arguably be safer to modify parse_example_config instead to add back the empty sections, then document this quirk on write_uncommented, but that's not exactly a satisfying solution...

In any case it would be nice to give write_uncommented a docstring if possible.

@Kobzol I'll leave it up to you to make the final call, should we do this the "right" way (what the PR does now), or the way that is less likely to cause further regressions (making parse_example_config produce dummy blocks)?

[Kobzol]

Thanks for taking a look. I examined the function in detail and determined that it is too magical to be understood 😆 I tried to rewrite it like this, which hopefully makes it clearer:

def write_uncommented(target, f):
     """Writes each block in 'target' that is not composed entirely of comments to 'f'.
    A block is a sequence of non-empty lines separated by empty lines.
    """
    block = []

    def flush(last):
        # If the block is entiry made of comments, ignore it
        entire_block_comments = all(ln.startswith("#") or ln == "" for ln in block)
        if not entire_block_comments and len(block) > 0:
            for line in block:
                f.write(line + "\n")
            # Required to output a newline before the start of a new section
            if last:
                f.write("\n")
        block.clear()

    for line in target:
        block.append(line)
        if len(line) == 0:
            flush(last=False)

    flush(last=True)
    return f

What do you think of this?