Add file based Azure Arc detection

[Avery-Dunn]

Fixes the issue described in #846, and is similar to the fix in MSAL .NET: AzureAD/microsoft-authentication-library-for-dotnet#4856

This PR adds file-based detection of Azure Arc, in addition to the current detection based on environment variables. If the files are found but the identity endpoint variable is not set, we default to a known endpoint.

[neha-bhargava]

The changes looks good, were you able to test it?

[bgavrilMS]

Needs at least a test for whatever OS the CI is using.

[Avery-Dunn]

Needs at least a test for whatever OS the CI is using.

@bgavrilMS : I originally didn't add tests because it is very difficult to modify static/final variables in a test and our mocking framework doesn't allow mocks of low-level Java features like file manipulation, and the PR over in .NET didn't have them so I figured y'all ran into similar issues too.

However, in the latest commit I removed the 'final' modifier from the final paths and added some package-private setters, allowing the tests to create temp directories to test the new behavior. We have some similar used-for-tests-only setters for a few other fields that otherwise could be private, and this is basically the only way to have meaningful tests for this file detection.

[Robbie-Microsoft]

Suggested change

	private static final String FALLBACK_IDENTITY_ENDPOINT = "http://127.0.0.1:40342/metadata/identity/oauth2/token";
	private static final String FALLBACK_AZURE_ARC_IDENTITY_ENDPOINT = "http://127.0.0.1:40342/metadata/identity/oauth2/token";

[Robbie-Microsoft]

it's a nit, you can choose to ignore

[Robbie-Microsoft]

Don't you also need to check that imdsEndpoint doesn't exist before using FALLBACK_IDENTITY_ENDPOINT?

[Robbie-Microsoft]

and if neither exists, also set imdsEndpoint = "N/A: himds executable exists";

[Robbie-Microsoft]

nit: should you define a custom object instead? This is what we do in msal-node:

type FilePathMap = {
    win32: string;
    linux: string;
};

export const SUPPORTED_AZURE_ARC_PLATFORMS: FilePathMap = {
    win32: `${process.env["ProgramData"]}\\AzureConnectedMachineAgent\\Tokens\\`,
    linux: "/var/opt/azcmagent/tokens/",
};

export const AZURE_ARC_FILE_DETECTION: FilePathMap = {
    win32: `${process.env["ProgramFiles"]}\\AzureConnectedMachineAgent\\himds.exe`,
    linux: "/opt/azcmagent/bin/himds",
};

[Robbie-Microsoft]

Can "windows" and "linux" be saved as constants in a constants file? Surely they are used elsewhere, or will be? Otherwise, if you implement my suggestion above, you could get the file path via:

// get the expected Windows or Linux file path of the himds executable
const fileDetectionPath: string =
    AZURE_ARC_FILE_DETECTION[process.platform as keyof FilePathMap];

[Robbie-Microsoft]

You might find msal-node's implementation of this feature interesting. It can be found in this file.

[Avery-Dunn]

Closing, will come back to this if there's a customer requesting this support.