coverage report not produced with --passWithNoTests flag and no tests

[Soviut]

🐛 Bug Report

We use Coveralls to centralize our test coverage on all our services. New services start with no tests and we use the --passWithNoTests flag for our coverage report step in CircleCI. Which looks like this

jest --coverage --passWithNoTests --runInBand && cat ./coverage/lcov.info | coveralls

We've noticed Coveralls failing because the ./coverage/lcov.info doesn't get created and coveralls fails to parse

No tests found, exiting with code 0
----------|----------|----------|----------|----------|-------------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
----------|----------|----------|----------|----------|-------------------|
All files |        0 |        0 |        0 |        0 |                   |
----------|----------|----------|----------|----------|-------------------|
[error] "2020-01-09T15:58:11.526Z"  'error from lcovParse: ' 'Failed to parse string'
[error] "2020-01-09T15:58:11.527Z"  'input: ' ''
[error] "2020-01-09T15:58:11.528Z"  'error from convertLcovToCoveralls'
/home/circleci/project/node_modules/coveralls/bin/coveralls.js:19
      throw err;
      ^
Failed to parse string

To Reproduce

Steps to reproduce the behavior:

Run the following

jest --coverage --passWithNoTests --runInBand && cat ./coverage/lcov.info

No /coverage/lcov.info is produced.

However, with a dummy/stub test it does work and produces the file.

Expected behavior

A valid coverage file would be produced with no tests if the --passWithNoTests flag is set.

[SimenB]

Is this a bug in istanbul? @coreyfarrell, seems like a report should be generated even if it's empty? A text report is generated, I'd think an lcov report would also be?

[coreyfarrell]

If I run the following commands from an empty directory:

npm init -y
npm i -D nyc
mkdir .nyc_output
npx nyc report -r lcovonly

This creates a 0 byte coverage/lcov.info file. With no data I'm not sure what the lcov.info is expected to contain? I suspect that from jest coverage/lcov.info is being created, just it's empty and coveralls does not deal with this situation. Maybe coveralls should give a better error like "No coverage received"?

I did some quick experimentation with genhtml (the standard gcc utility for converting lcov.info to html reports). It reported the error genhtml: ERROR: no valid records found in tracefile lcov.info. I'm not sure what can be done with this situation, even manually I was unable to create an lcov file with no data that was accepted by genhtml.

[SimenB]

Right, so in theory coveralls should accept an empty file?

[coreyfarrell]

Probably. At minimum it shouldn't say "parse error" it should detect the special case of an empty file. Not sure if it should report to the coveralls server in this case, I'd think yes so that coveralls site can report that no coverage was received rather than just being unaware of that CI run.

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