supportRelativeURL bombs when Node "path" object is not present

[mcclure]

I am attempting to use source-map-support with a platform where v8 is embedded directly into an application, which is not Node or a browser (it's a plugin for the Unreal game engine). I do not know if this is a use case you support, but source-map-support does have many options that seem intended for making it work in unusual environments.

The Unreal environment does not have full fs or path objects (both are available through require, but return empty objects). It does have a dedicated function for loading a file into a string from disk. I am currently running this:

    require('source-map-support').install({overrideRetrieveSourceMap: true, retrieveSourceMap: function(source:string) {
        if (!source.startsWith("file:///")) { return null }
        let name = decodeURIComponent(source.substr(8)) + ".map"
        let result = JavascriptLibrary.ReadStringFromFile(null, name)
        if (!result) { return null }
        return { url: source, map: result }
      }
    })

This works except for the supportRelativeURL function, which calls both path.dirname and path.resolve without checking. In my testing, source-map-support fails 100% because supportRelativeURL gets called by mapSourcePosition. (mapSourcePosition actually calls supportRelativeURL twice. In my testing, the second one is the only one that is triggering, comment "Only return the original position if a matching line was found…". As a side note though the first call is interesting, since in cases where it does trigger, it seems to only be about maintaining the fileContentsCache. However if the user is using overrideRetrieveSourceMap— which I am— the fileContentsCache is unused, so this clause is useless work…).

If I replace the contents of supportRelativeURL with return url, source-map-support overall works great.

I am not completely sure how to proceed here. I could potentially add the missing path methods to the environment (although path.resolve concerns me since that involves per-OS support to do properly). It does seem too bad though that source-map-support almost allows me to configure it into no dependence on the filesystem, then falls down in this one function.

I think my expected behavior is that source-map-support should check for the presence of the methods on path it needs, and be resistant to path being null, since after all it already goes to the bother of doing this with fs. If this is not practical, it might help for people in unusual situations to document in the README that the path, path.dirname and path.resolve objects are required (how is it requiring "path" works in the browser, anyway?).

[mcclure]

As an update, I patched UnrealJS to have the fs and path methods that node-source-map-support uses, so I am no longer blocked on this. However, fixing the issue with the path object might still be desirable for anyone else out there using a non-Node v8 embed.

[LinusU]

Great writeup! While I don't have time to look at this myself, I would love to accept a PR that fixes this 👍

[jmakeig]

This is exactly my situation too. I’m working in MarkLogic, which embeds V8. It loads JavaScript from the database and doesn’t have the Node fs or path modules available. I’d want to be able to override the built-in file resolution, just as above, to look in the database using MarkLogic APIs. I’ll dig in and see if I can come up with a patch based on the thorough investigation by @mcclure.