Return standard output in popen_write

[claui]

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Expected behaviour

We expect the popen_write method to return the standard output of the child process.

This expectation is evident in how safe_popen_write is written:

  def self.safe_popen_write(*args, **options, &block)
    output = popen_write(*args, **options, &block)
    return output if $CHILD_STATUS.success?

    raise ErrorDuringExecution.new(args, status: $CHILD_STATUS, output: [[:stdout, output]])
  end

Actual behaviour

Contrary to what the above code suggests, no code has been written to actually obtain any output from the child process. Newly-written tests show that popen_write only returns a number ("4") instead of the expected standard output.

For example, given a file foo with the content Foo\n, one of the tests calls popen_write with cat foo - and an input of Bar. The expected output would be Foo\nBar\n but the actual output is the number 4 (which is what Ruby’s IO#write method returns).

Fix

By having popen_write transparently capture standard output, we restore the intended semantics.

The practical purpose of this PR is that when an embedded formula patch fails, it now displays the actual output of the patch binary (instead of the length of the patch).

Fixes #8244.

[MikeMcQuaid]

Nice work @claui!

[MikeMcQuaid]

No blocking comments, thanks @claui.

[MikeMcQuaid]

Thanks @claui!