Warn about WriteLinesToFile default behavior which is Append

[KirillOsenkov]

WriteLinesToFile appends by default unless you specify Overwrite. This is highly non-intuitive and probably almost never what the user intends.

Since we can't change the behavior, I propose to introduce a new optional parameter Append, that defaults to false, and if it's not specified and Overwrite is not specified, warn about it (that you should specify one or the other).

Not sure if a warning or a message, perhaps build check?

[KirillOsenkov]

It is especially insidious, because it works fine the first time, it's only after an incremental build you start seeing the file contents double.

Ask me how I know (I broke the build of all PowerBI)

[KirillOsenkov]

Perhaps we should have a property that controls the default behavior like WriteLinesToFileOverwriteByDefault and set it to false. Users can set it to true and WriteLinesToFile default behavior will change across the entire repo.

[KalleOlaviNiemitalo]

AFAIK tasks cannot directly read properties, only parameters. So when a developer implements a custom target that runs the WriteLinesToFile task, I don't see how the task would know the value of WriteLinesToFileOverwriteByDefault.

[KirillOsenkov]

hmm that's a good point

[MichalPavlik]

@baronfel, what do you think?