Calling symlinkOrCopySync on a path with existing file 💥

[twokul]

https://embercommunity.slack.com/archives/dev-ember-cli/p1481157690003118

input node includes files foo.js and foo.hbs

• using a broccoli-filter that takes .hbs -> .js
• 💥

This is because we unconditionally call symlinkOrCopySync from srcPath to destPath. But since we copy forward all files, the output path already includes foo.js when we attempt to symlink the output of transpiling foo.hbs. I think step one would be to setup a failing test in broccoli-filter with ^ as a minimal reproduction.

The solution likely either includes:

• tracking when we have already written to a given dest path and just call fs.unlinkSync(destPath) before calling symlinkOrCopy.sync
• use try/catch and auto-retry upon EEXIST error after unlinking

cc @rwjblue

[joliss]

It seems OK to me to throw an error here. It's not clear to me that we should simply overwrite foo.js.

[rwjblue]

If we do want to error, we should emit an actually useful error message (and potentially consider making an { overwrite: true } style option).