- Simplify readFile interface to only support reading full files

- Remove the choice of reading in a blocking way,
  since we want to prevent hanging

- Remove the dry run read, since it was not useful

- Split the implementation between two overloads,
  where one supports returning the whole file contents in one go,
  while the other calls back a predicate everytime some data is read,
  up to a fixed block size (as with the previous implementation)

Smjert deleted the stefano/improvement/readfile-nonblock branch

zwass mentioned this pull request

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A refactor of the `readFile` function (in #8410) caused all parsing of shortcut files to fail. The test had a bug that caused the relevant checks to be skipped.

Fixes #8599