Move all Git commands behind a PlatformScm interface

[NiasSt90]

Describe the proposed change(s).

To allow to support new platforms (like Gerrit #4109) which are not based on Git or use Git in a different way we need to abstract the current use of Git behind a new Interface PlatformScm.

All (most?) current exported functions from the util/git module defines the size/scope of the Interface (perhaps with some renaming).
The util/git module will be the default implementation for the existing platforms.

export interface Platform {
....
  //commitFiles?(config: CommitFilesConfig): Promise<CommitSha | null>;
  scmApi():PlatformScm;
}

export interface PlatformScm {
  initRepo(args: StorageConfig): Promise<void>;
  setAuthor(author: string | undefined): void;
  resetToCommit(commit: string): Promise<void>;
  branchExists(branchName: string): boolean;
  checkoutBranch(branchName: string): Promise<CommitSha>;
  isBranchBehindBase(branchName: string, baseBranch: string): Promise<boolean>
  isBranchModified(branchName: string): Promise<boolean>;
  commitFiles?(config: CommitFilesConfig): Promise<CommitSha | null>;
  //...TODO
}

[rarkins]

If our platforms were class based it would make this a lot easier. e.g. the base class could default to passing directly through to git if the function is not overridden in the platform. Either way I would prefer not to add 8+ functions to the existing platforms which aren't relevant, so we should try to wrap/centralize it

[NiasSt90]

Perhaps we could separate the SCM implementation similar to the the current (platform) api implementation (platform/api.ts).

Not behind the platform interface it exists besides the platform interface. Then no need to change any existing platform module (i.e. export the util/git module as default scm()).

const scm = new Map<string, PlatformScm>();
export default scm;

scm.set('azure', gitScm);
...
scm.set('gerrit', gerrit);
scm.set('mercurial', mercurial);

Then the option platformCommit=true should switch from gitScm to githubScm (example names).

[rarkins]

Yes, good idea

[NiasSt90]

I've created a proof of concept (commit) and would like to get some feedback about the details.

The commit is on-top of the current Gerrit-PR only to show(validate) the usage of partial overriding the (default) PlatformSCM interface.
It will be separated if the proof-of-concept was done.

Currently it's allowed to partially override functions.
If not overridden the defaultGitScm implementation will be called (current util/git module).

[rarkins]

This looks ok to me. @viceice @JamieMagee what do you think about this approach vs putting everything behind platform.?

[viceice]

the general idea looks ok, but i've some annotations. So best is to create a draft PR where we can add those.

[JamieMagee]

Yeah, I like this idea (and going towards interfaces + classes for all other modules like datasources). It would also help any implementation of #18094

[NiasSt90]

I've added the Draft-PR now without the Gerrit-part (stripped off) and six functions already moved behind the interface.

It looks like on Github there is currently no option to create (or move) a pull-request on-top of another.
What is the best way to continue with the Gerrit-PullRequest?
Wait until this one here is finished and merged?

[renovate-release]

🎉 This issue has been resolved in version 34.150.0 🎉

The release is available on:

• GitHub release
• 34.150.0

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