TokenCredential.supports_caching #19940
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Part of #19308. Today, BearerTokenCredentialPolicy caches the last access token it acquired and calls
get_token()only when this cached token will soon expire. This isn't safe with on-behalf-of tokens because they contain a user assertion identifying the user on whose behalf the application accesses resources. When an application changes its intended user, a client must not continue using a token for the prior user. Our design for on-behalf-of authentication makes the credential responsible for tracking the application's user assertion changes, which is to say it requires BearerTokenCredentialPolicy to callget_token()every time it authorizes a request. So, this PR addsTokenCredential.supports_caching(). BearerTokenCredentialPolicy calls this method to learn whether a given TokenCredential maintains its own token cache. When this returns True, BearerTokenCredentialPolicy defers to the credential's cache, callingget_token()every time it wants to authorize a request. The policy doesn't expect all credentials to implement this method, and assumes it's safe to cache tokens from credentials which do not.