fix(bundle-source): assert that the entrypoint exists at all

[warner]

closes #1206

[warner]

@kriskowal this is "draft" because the patch is insufficient in at least the following ways:

• the fs.existsSync() call should probably be coming from powers, if something suitable exists therein
• existsSync would be a funny power to provided when read is the actually useful one
• that call does not understand e.g. ../demo/bigint as meaning ../demo/bigint.js

but I'm hoping you might be able to point me in the right direction so I can make it better. What's the right sort of way to build the assertion? Would it be easier to build in bundle{ZipBase64,NestedEvaluateAndGetExports), rather than up front in bundleSource?

(incidentally, patching my agoric-sdk copy with this code was enough to help figure out where I had a typo in my bundleSource() invocation, which was my most immediate goal)

[mhofman]

un-requested feedback: I still would prefer avoiding Sync APIs where possible. In this case stat and checking for an ENOENT error is the alternative

[kriskowal]

I’m sure there’s a more correct way to ensure that the file exists within compartment-mapper. exists is one of those “considered harmful” syscalls because it isn’t proof against race conditions on a non-transactional filesystem. existsSync is “considered harmful” because it limits portability to the web (perhaps not a concern specifically for bundleSource which has a higher hill to climb before it is portable).

The mystery is how it’s possible for compartment-mapper to construct a bundle at all if it cannot find the entrypoint module. This is probably due to the .catch(() => undefined) in the loop that searches for a file to satisfy an import specifier in the archiver’s importHook, and probably some missing logic to construct an appropriate error if the loop falls through. But, we see that error pretty frequently in other cases, so it must be a problem specific to the entrypoint module, specific to makeArchive, that allows it to produce an empty zip instead of throwing.