repl: eval empty lines, fixing debugger repeat

[lrowe]

Pull Request check-list

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• Does make -j8 test (UNIX) or vcbuild test nosign (Windows) pass with
  this change (including linting)?
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Affected core subsystem(s)

repl, debugger

Description of change

Tweak the better empty line handling introduced in #2163 so that empty lines
are still passed to the eval function. This is required for the debugger to
repeat the last command on an empty line.

Fixes: #6010

[lrowe]

All tests pass other than parallel/test-tick-processor which was failing on master on my machine.

Debugger repetition is tested in https://github.com/nodejs/node/blob/master/test/debugger/test-debugger-repl-term.js#L18-L23. I'm not sure how that is passing while a manual test fails. However the debugger tests are not run by default.

With this fix the debugger tests no longer timeout, but there are multiple failures, presumably they have not been kept up to date.

[bnoordhuis]

Yes, the debugger tests aren't run regularly; they're pretty flaky due to the nature of the debugger.

I don't think this change is acceptable as-is because it changes all REPL behavior, not just the debugger's. The fix should probably be somewhere in lib/_debugger.js.

If you can get test/debugger/test-debugger-repl-term.js to work reliably, please move it over to test/parallel - they're run regularly.

[lrowe]

This behaviour of the REPL with it's defaultEval should be unchanged. In order to prevent "undefined from printing if the string is empty", b54f153 avoids calling eval. With this patch eval is called but an undefined result for an empty string is not printed.

I don't think it's possible to fix this from within the debugger since the REPLServers's on('line', ... event listener no longer calls the eval function (which the debugger provides) for empty lines. It's not possible for the debugger to override the on('line', ... event listener since it is a closure within REPLServer which sets variables declared in the REPLServer scope.

I'll have a go at fixing the debugger tests. I wonder if the reliability problems were due to running multiple debugger tests in parallel when they all use the same port.

[lrowe]

The debugger tests all pass now. I need to work out how to run them in parallel to check reliability - I expect failures due to multiple tests using the same port.

[Trott]

I believe the issue this is addressing has been fixed in 1df84f4. The issue should be resolved in Node.js 6.0.0 and newer. I'm going to close this, but If I'm mistaken/misunderstanding, please re-open.