docs: add OAuth implementation sprint plan

3-task breakdown covering foundation + HTTP client changes,
M2M provider, and U2M provider with full test coverage.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: vikrantpuppala

rust/docs/designs/oauth-sprint-plan.md

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+
+# Sprint Plan: OAuth U2M + M2M Implementation
+
+**Sprint dates:** 2026-03-07 to 2026-03-20
+**Sprint goal:** Implement complete OAuth 2.0 authentication (U2M + M2M) for the Rust ADBC driver as described in the [OAuth design doc](oauth-u2m-m2m-design.md).
+
+---
+
+## Story
+
+**Title:** Implement OAuth U2M and M2M authentication
+
+**Description:**
+Add OAuth 2.0 support to the Rust ADBC driver, covering:
+- M2M (Client Credentials) flow for service principal authentication
+- U2M (Authorization Code + PKCE) flow for interactive browser-based login
+- Shared infrastructure: OIDC discovery, token lifecycle management, file-based token caching
+- Integration with `Database` config and `DatabricksHttpClient` (two-phase init via `OnceLock`)
+- ODBC-aligned numeric config values (`AuthMech`/`Auth_Flow` scheme)
+
+**Acceptance Criteria:**
+- [ ] M2M flow works end-to-end: `Database` config -> `new_connection()` -> `get_auth_header()` returns valid Bearer token from client credentials exchange
+- [ ] U2M flow works end-to-end: browser launch -> callback server captures code -> PKCE token exchange -> cached token reused on subsequent connections
+- [ ] Token refresh state machine works: FRESH (no-op) -> STALE (background refresh) -> EXPIRED (blocking refresh)
+- [ ] File-based token cache persists U2M tokens across connections with correct permissions (0o600)
+- [ ] `DatabricksHttpClient` supports two-phase init: created without auth, auth set later via `OnceLock`
+- [ ] All config options from the design doc are parseable via `set_option()`
+- [ ] Invalid config combinations fail with clear error messages at `new_connection()`
+- [ ] `cargo fmt`, `cargo clippy -- -D warnings`, `cargo test` all pass
+
+---
+
+## Sub-Tasks
+
+### Task 1: Foundation + HTTP Client Changes
+
+**Scope:** Shared OAuth infrastructure and modifications to existing code to support two-phase auth initialization.
+
+**Files to create:**
+- `src/auth/oauth/mod.rs` — module root, re-exports
+- `src/auth/oauth/token.rs` — `OAuthToken` struct with `is_expired()`, `is_stale()`, JSON serialization
+- `src/auth/oauth/oidc.rs` — `OidcEndpoints` struct, `discover()` function hitting `{host}/oidc/.well-known/oauth-authorization-server`
+- `src/auth/oauth/cache.rs` — `TokenCache` with `load()`/`save()`, SHA-256 hashed filenames, 0o600 permissions
+- `src/auth/oauth/token_store.rs` — `TokenStore` with `RwLock<Option<OAuthToken>>`, `AtomicBool` for refresh coordination, FRESH/STALE/EXPIRED state machine
+
+**Files to modify:**
+- `Cargo.toml` — add `oauth2 = "5"`, `sha2 = "0.10"`, `open = "5"`, `dirs = "5"`
+- `src/client/http.rs` — change `auth_provider` from `Arc<dyn AuthProvider>` to `OnceLock<Arc<dyn AuthProvider>>`, add `set_auth_provider()`, update `execute()` to read from `OnceLock`
+- `src/database.rs` — add `AuthMechanism`/`AuthFlow` enums (with `TryFrom<i64>`), new config fields (`auth_mechanism`, `auth_flow`, `auth_client_id`, `auth_client_secret`, `auth_scopes`, `auth_token_endpoint`, `auth_redirect_port`), `set_option()`/`get_option_string()` for all new keys
+- `src/auth/mod.rs` — add `pub mod oauth;`, remove old `pub use oauth::OAuthCredentials`, delete `src/auth/oauth.rs` (replaced by directory module)
+
+**Tests:**
+- `token.rs`: `test_token_fresh_not_expired`, `test_token_stale_threshold`, `test_token_expired_within_buffer`, `test_token_serialization_roundtrip`
+- `oidc.rs`: `test_discover_workspace_endpoints`, `test_discover_invalid_response`, `test_discover_http_error`
+- `cache.rs`: `test_cache_key_deterministic`, `test_cache_save_load_roundtrip`, `test_cache_missing_file`, `test_cache_file_permissions`, `test_cache_corrupted_file`
+- `token_store.rs`: `test_store_fresh_token_no_refresh`, `test_store_expired_triggers_blocking_refresh`, `test_store_concurrent_refresh_single_fetch`, `test_store_stale_returns_current_token`
+- `http.rs`: `test_execute_without_auth_works_before_auth_set`, `test_execute_fails_before_auth_set`, `test_execute_succeeds_after_auth_set`, `test_set_auth_provider_twice_panics_or_errors`
+- `database.rs`: `test_set_auth_mechanism_valid`, `test_set_auth_mechanism_invalid`, `test_set_auth_flow_valid`, `test_set_auth_flow_invalid`, `test_new_connection_missing_mechanism`, `test_new_connection_oauth_missing_flow`, `test_new_connection_pat_missing_token`
+
+**Definition of done:** All foundation modules compile, unit tests pass, `DatabricksHttpClient` two-phase init works, database config parsing works for all auth options.
+
+---
+
+### Task 2: M2M Provider (Client Credentials)
+
+**Scope:** Implement the M2M OAuth flow using `oauth2::BasicClient` for client credentials exchange.
+
+**Files to create:**
+- `src/auth/oauth/m2m.rs` — `ClientCredentialsProvider` implementing `AuthProvider`
+
+**Files to modify:**
+- `src/auth/oauth/mod.rs` — add `pub mod m2m;`, re-export `ClientCredentialsProvider`
+- `src/auth/mod.rs` — re-export `ClientCredentialsProvider`
+- `src/database.rs` — wire `AuthFlow::ClientCredentials` match arm in `new_connection()` to create `ClientCredentialsProvider`
+
+**Implementation details:**
+- Construct `oauth2::BasicClient` from OIDC-discovered endpoints
+- Implement `oauth2` HTTP adapter that routes through `DatabricksHttpClient::execute_without_auth()`
+- Use `TokenStore` for token lifecycle (no disk cache for M2M)
+- `get_auth_header()` → `TokenStore::get_or_refresh()` → `client.exchange_client_credentials()` if needed
+
+**Tests:**
+- `m2m.rs`: `test_m2m_token_exchange`, `test_m2m_auto_refresh`, `test_m2m_oidc_discovery`
+- Wiremock integration (`tests/`): `test_m2m_full_flow_discovery_and_token_exchange`, `test_m2m_token_refresh_on_expiry`, `test_m2m_discovery_failure_propagates`
+- Database validation: `test_new_connection_client_credentials_missing_secret`
+
+**Definition of done:** M2M flow works end-to-end from `Database` config through `new_connection()` to `get_auth_header()`. Wiremock tests verify the full OIDC discovery -> token exchange -> refresh cycle.
+
+---
+
+### Task 3: U2M Provider (Authorization Code + PKCE)
+
+**Scope:** Implement the U2M OAuth flow with browser-based login, PKCE, callback server, and token caching.
+
+**Files to create:**
+- `src/auth/oauth/callback.rs` — `CallbackServer` with localhost HTTP listener, state validation, timeout
+- `src/auth/oauth/u2m.rs` — `AuthorizationCodeProvider` implementing `AuthProvider`
+
+**Files to modify:**
+- `src/auth/oauth/mod.rs` — add `pub mod callback;`, `pub mod u2m;`, re-export `AuthorizationCodeProvider`
+- `src/auth/mod.rs` — re-export `AuthorizationCodeProvider`
+- `src/database.rs` — wire `AuthFlow::Browser` match arm in `new_connection()` to create `AuthorizationCodeProvider`
+
+**Implementation details:**
+- On creation: try loading cached token via `TokenCache::load()`
+- If cached token has valid `refresh_token`: store in `TokenStore`, refresh on first `get_auth_header()` if stale/expired
+- If no cache: generate PKCE via `PkceCodeChallenge::new_random_sha256()`, build auth URL via `client.authorize_url()`, start `CallbackServer`, launch browser via `open::that()`, wait for callback, exchange code via `client.exchange_code().set_pkce_verifier()`
+- Save token to cache after every successful acquisition/refresh
+- Refresh uses `client.exchange_refresh_token()` through `DatabricksHttpClient::execute_without_auth()`
+- If refresh token is expired, fall back to full browser flow
+
+**Tests:**
+- `callback.rs`: `test_callback_captures_code`, `test_callback_validates_state`, `test_callback_timeout`
+- `u2m.rs`: `test_u2m_refresh_token_flow`, `test_u2m_cache_hit`, `test_u2m_cache_miss_with_expired_refresh`
+- Wiremock integration: `test_u2m_refresh_token_full_flow`
+- Database validation: `test_new_connection_token_passthrough_missing_token`
+- E2E (ignored): `test_m2m_end_to_end`, `test_u2m_end_to_end`
+
+**Definition of done:** U2M flow works end-to-end. Token cache persists across connections. Refresh path works via wiremock tests. Browser flow verified manually via `#[ignore]` E2E test.