Automatically (or semi-automatically) map TMPDIR into VM

[rix0rrr]

What is the problem you're trying to solve?.

CDK will sometimes put directories with Dockerfiles into a user's temporary directory, and ask finch to build them. Right now, since the user's temporary directory isn't mounted into the finch VM, they get a confusing error like:

time="2023-03-16T09:43:07Z" level=fatal msg="lstat /private/var/folders/zx/d5wln9n10sn0tcj1v9798f1c0000gr/T/jsii-kernel-9VYgrO/node_modules/@aws-cdk/aws-lambda-python-alpha/lib/Containerfile: no such file or directory"

It's confusing on many levels:

• There is indeed no Containerfile, but there is a Dockerfile
• It's not obvious to a user that this path is inside the finch VM, instead of their own file system

Describe the feature you'd like

Right now we're telling users to put this into their finch.yaml:

additional_directories:
  - path: /var/folders
  - path: /private/var/folders

It would be nice if finch could do this automatically.

If not, I wonder if there are performance implications of mapping everything under /var/folders. If so, could finch maybe support a symbolic name for the temporary directory instead?

additional_directories:
  - path: TMPDIR

And either map that automatically always, or give the user the ability to add the above.

Additional context

Our issue: aws/aws-cdk#24458

[rix0rrr]

Slight complication in the fact that:

$ echo $TMPDIR  
/var/folders/zx/d5wln9n10sn0tcj1v9798f1c0000gr/T/

But actually

/var -> /private/var

And the actual directory (that should probably be mapped) is:

/private/var/folders/zx/d5wln9n10sn0tcj1v9798f1c0000gr/T/

But now, unless the VM also has the /var -> /private/var symlink, it matters whether we use the unresolved $TMPDIR or the resolved $TMPDIR, because they will contain different path names and only one of them will be mapped...

Can you add a symlink for /var/folders -> /private/var/folders into the VM?

[vsiravar]

Released in v0.8.0