We had a frequent panic when the agent was using our edit file tool. The
root cause was that we were constructing a `BufferDiff` with
`BufferDiff::new`, then calling `set_base_text`, but not waiting for
that asynchronous operation to finish. This means there was a window of
time where the diff's base text was set to the initial value of
`""`--that's not a problem in itself, but it was possible for us to call
`PendingDiff::update` during that window, which calls
`BufferDiff::update_diff`, which calls
`BufferDiffSnapshot::new_with_base_buffer`, which takes two arguments
`base_text` and `base_text_snapshot` that are supposed to represent the
same text. We were getting the first of those arguments from the
`base_text` field of `PendingDiff`, which is set immediately to the
target base text without waiting for `BufferDiff::set_base_text` to run
to completion; and the second from the `BufferDiff` itself, which still
has the empty base text during that window.
As a result of that mismatch, we could end up adding `DeletedHunk` diff
transforms to the multibuffer for the diff card even though the
multibuffer's base text was empty, ultimately leading to a panic very
far away in rendering code.
I've fixed this by adding a new `BufferDiff` constructor for the case
where the buffer contents and the base text are (initially) the same,
like for the diff cards, and so we don't need an async diff calculation.
I also added a debug assertion to catch the basic issue here earlier,
when `BufferDiffSnapshot::new_with_base_buffer` is called with two base
texts that don't match.
Release Notes:
- N/A
---------
Co-authored-by: Conrad <conrad@zed.dev>
6b4c9119d8