## Summary of changes
This PR introduces three new abstractions:
1. `Resolver`: Resolves which reader and writer to use for a template.
2. `Writer`: Writes a template project to disk. Prompts the user if
necessary.
3. `Reader`: Reads a template specification from disk, built into the
CLI or from GitHub.
Introducing these abstractions helps decouple reading a template from
writing it. When I tried adding telemetry for the `bundle init` command,
I noticed that the code in `cmd/init.go` was getting convoluted and hard
to test. A future change could have accidentally logged PII when a user
initialised a custom template.
Hedging against that risk is important here because we use a generic
untyped `map<string, string>` representation in the backend to log
telemetry for the `databricks bundle init`. Otherwise, we risk
accidentally breaking our compliance with our centralization
requirements.
### Details
After this PR there are two classes of templates that can be
initialized:
1. A `databricks` template: This could be a builtin template or a
template outside the CLI like mlops-stacks, which is still owned and
managed by Databricks. These templates log their telemetry arguments and
template name.
2. A `custom` template: These are templates created by and managed by
the end user. In these templates we do not log the template name and
args. Instead a generic placeholder string of "custom" is logged in our
telemetry system.
NOTE: The functionality of the `databricks bundle init` command remains
the same after this PR. Only the internal abstractions used are changed.
## Tests
New unit tests. Existing golden and unit tests. Also a fair bit of
manual testing.
## Changes
While working on the v2 of #1744, I found that:
* Template initialization first copies built-in templates to a temporary
directory before initializing them
* Reading a template's contents goes through a `filer.Filer` but is
hardcoded to a local one
This change updates the interface for reading templates to be `fs.FS`.
This is compatible with the `embed.FS` type for the built-in templates,
so they no longer have to be copied to a temporary directory before
being used.
The alternative is to use a `filer.Filer` throughout, but this would
have required even more plumbing, and we don't need to _read_ templates,
including notebooks, from the workspace filesystem (yet?).
As part of making `template.Materialize` take an `fs.FS` argument, the
logic to match a given argument to a particular built-in template in the
`init` command has moved to sit next to its implementation.
## Tests
Existing tests pass.
shreyas-goenka
Pieter Noordhuis