Possible race conditions when compiling and running

I'm seeing this in logs for `/compile` requests:

A web handler threw an exception. Details:
data/haskell/build/Pxx/Pxxxxxxxxxxxxxxxxxxxxxx.basever: openFile: does not exist (No such file or directory)

This indicates that some part of the system thinks the build is done, but there's no basever file. I'm not sure how that could happen. Maybe a race condition of some kind?

I also see this for `/runMsg`:

A web handler threw an exception. Details:
data/haskell/build/Pxx/Pxxxxxxxxxxxxxxxxxxxxxx.err.txt: getFileStatus: does not exist (No such file or directory)

Similar situation: the system thinks that the build is complete, but there's no build output file, which should always be created as part of a build.

まず、Web ハンドラー `/compile` と `/runMsg` を追跡し、どのようにビルドが完了したと判断しているかを確認します。`data/haskell/build/` にある `basever` と `.err.txt` の欠落ファイルの処理を調べます。Race condition を理解し、これらのリクエストがビルド成果物の欠落を報告しなくなれば完了です。

索引モデルが issue の本文から書いたものです。