[gabepesco]

Description

#5859 fixed #5858 by giving U2M OAuth a single shared connection. That stops the browser login racing (#5646), but every query then goes through one connection, and the connector serializes requests on it. So concurrent_tasks has no effect for U2M users.

This PR keeps one connection per thread and shares the login instead:

• One login for all connections. SharedU2MCredentialsProvider is passed to every connection as credentials_provider. It creates the connector's own OAuth provider once, under a lock, so the browser flow runs a single time per workspace in each process.
• One token refresh at a time. Header reads are serialized. Databricks can issue single-use refresh tokens, so a separate token copy per connection would break on the first refresh.
• Recovery after a failed login. If a token read fails (refresh token expired or revoked), the login is discarded and the next request logs in again. Threads waiting on a login that fails get its error instead of each opening another browser window. A network failure during a refresh keeps the login, so the next request retries the refresh without a browser flow.
• Fallback for older connectors. Connectors without the internals this relies on (< 4.1) keep fix: Set DatabricksConnectionConfig to only use a shared_connection with U2M OAuth #5859's single shared connection, so shared_connection is now True only in that case.

PAT and M2M auth are unchanged. No state or schema changes, so no migration is needed.

Known gap: auth_type is still passed to the connector because its kernel backend rejects a custom credentials_provider. On warehouses where connector 4.6+ falls back to that backend, each connection would run its own login. I haven't been able to verify this path.

Related: #5646, #5746, #5858, #5859

Test Plan

• New unit tests in tests/utils/test_databricks.py. They cover a single browser flow across threads, serialized token reads, retry after a failed login, waiting threads not retrying a failed login, and re-login after a failed token read, and keeping the login after a network failure during refresh. Two tests run the real connector with only OAuthManager.get_tokens mocked; one builds per-connection auth exactly as connect() does, with both auth_type and the shared provider. Each test fails when the fix it covers is reverted.
• Updated tests/core/test_connection_config.py: U2M now uses ThreadLocalConnectionPool with the shared provider, and the old-connector fallback uses the shared connection.
• Live check against a Databricks workspace with U2M (databricks-oauth), running 4 threads with 15 small queries each. There was one browser login and 4 distinct connections, and it finished in 6.5s vs 13.9s with the shared connection (2.1x).
• Not verified live: token refresh after the ~1h access-token expiry (covered by mocked tests only), and the kernel backend path noted above.

Checklist

• I have run make style and fixed any issues (ruff and mypy run directly; the pre-commit hooks need bash, which I don't have on Windows)
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test) (related suites pass; I didn't run the full fast-test suite locally)
• My commits are signed off (git commit -s) per the DCO

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features
  • Databricks U2M OAuth can reuse sign-in credentials across connections when supported by the installed connector, reducing repeated browser sign-ins.
• Improvements
  • When shared sign-in is unavailable, Databricks connections retain the existing shared-connection behavior.
  • Temporary sign-in and credential-refresh failures can be retried. Network errors do not trigger a new browser sign-in.
  • Credentials are reused across equivalent workspace hostnames and authentication types.

[coderabbitai]

Walkthrough

Databricks U2M OAuth connections now use a shared credentials provider when the connector supports shared U2M login. The provider reuses authentication state and serializes login and header reads. Unsupported connectors retain shared-connection behavior.

Changes: Databricks U2M OAuth

Layer / File(s)	Summary
Shared provider lifecycle and validation sqlmesh/utils/databricks.py, tests/utils/test_databricks.py	A registry reuses providers by normalized workspace hostname and auth type. Each provider creates its auth provider lazily, serializes login and header reads, and handles login and token-refresh errors. Tests cover concurrent calls, retries, provider reuse, and connector integration.
Connection selection and provider wiring sqlmesh/core/config/connection.py, tests/core/test_connection_config.py	Configuration selects a shared credentials provider and thread-local connection pool when the connector supports shared U2M login. Otherwise, it uses shared-connection behavior. Tests cover provider reuse and fallback.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: mday-io

Merge Risk: Minimal · up to b4021

Databricks U2M OAuth connections now share one login across per-thread connections when the connector supports it, and fall back to a shared connection otherwise. No actionable merge-blocking risk is established. Live token refresh after the access token expires was not verified by the author.

Security architecture risk: Low · up to b4021

Serializing login and token refresh supports concurrent queries without duplicating refresh-token state. Remaining risks concern identity sharing across separate configurations and connector backends that may not honor the shared provider. A cross-user authorization bypass has not been established.

Retained concerns

• Low · security · inferred: Credential ownership expands across configurations and adapters: every U2M consumer using the same normalized workspace and auth type receives the same login. If distinct user contexts can coexist in a supported process, later contexts would inherit the first login's authority. The implementation explicitly assumes one interactive user per process, and no supported cross-user caller was established; this is an unresolved identity-isolation concern, not a verified authorization bypass.
• Low · reliability · inferred: The fallback decision checks for an internal connector module, not whether the selected backend honors credentials_provider. The source acknowledges a kernel backend that uses its own U2M flow. If that backend is selected while shared_connection is false, per-thread connections would regain independent browser logins and competing OAuth flows, weakening authentication failure containment relative to the base. Actual fallback reachability remains unverified; this does not establish a CSRF-control bypass.

Security review details

Security Blast Radius

• inferred — The maximum demonstrated sharing scope is every matching U2M configuration and adapter in one process for one normalized workspace and auth type, including different warehouse HTTP paths. Their requests use the shared login's existing authority. Distinct workspace or auth-type keys remain separate; cross-user exploitation or cross-process exposure was not established.

Security Findings and Attack Paths

• inferred — The deferred authorization candidate requires distinct user contexts to reach the same workspace/auth-type provider in one supported process. Such a later context would receive the existing login rather than independently authenticate. That prerequisite remains unproven: the supplied security assessment retains no findings, and inspected web settings select cached contexts from process configuration rather than a user identity.

Trust Boundaries and Controls

• observed — The provider delegates browser OAuth to the connector's own authentication implementation. SQLMesh adds synchronization around login and header generation, protecting the shared refresh-token state from simultaneous reads or refreshes. This synchronization does not itself provide user isolation between configurations.

Resilience and Maintainability Implications

• observed — The provider deliberately owns an HTTP client that outlives individual connections. Adapter close disposes pooled connections, while the new module retains providers without a close or eviction operation. This establishes process-lifetime credential ownership, not an adapter-scoped logout boundary; the external client's cleanup contract remains unverified.

Hardening Proposals

• proposed — Make the single-user-per-process assumption an explicit supported boundary. If separate identities must coexist, use an authentication-session ownership key and an explicit reset lifecycle rather than workspace-only reuse; define credential and HTTP-client cleanup without invalidating unrelated active consumers.
• proposed — Base concurrency eligibility on the effective backend's shared-provider contract, retaining shared connections where that contract is unsupported. Validate backend fallback and authentication recovery together so an upgrade cannot silently restore independent browser flows.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 46.15% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documenta…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title clearly describes concurrent Databricks U2M OAuth queries with a shared login.
Description check		The description includes the required sections and explains the changes, tests, and known gaps. It notes that the full fast-test suite was not run.
Linked Issues check		[#5858] is closed and provides historical context only. No active directly linked issue coding requirements apply.
Out of Scope Changes check		The connection configuration, shared credentials provider, fallback, and tests support the PR's U2M concurrency objective. The known kernel-backend limitation is disclosed in the PR description. The s…

Full details: Out of Scope Changes check

Explanation

The connection configuration, shared credentials provider, fallback, and tests support the PR's U2M concurrency objective. The known kernel-backend limitation is disclosed in the PR description. The summarized changes do not show unrelated work.

Full details: Docstring Coverage

Explanation

Docstring coverage is 46.15% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 13 functions across 4 files.

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

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• sqlmesh/core/config/connection.py

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