feat: support read-only Casbin DB for passive-region stacks

[sauraww]

Problem

The Casbin Diesel adapter issues CREATE TABLE IF NOT EXISTS on every
adapter construction. Postgres rejects that on a standby, where every
transaction is forced read-only regardless of configuration, so pods in
a passive region panic at startup and crash-loop. Seeding the root
admin fails the same way, since save_policy() writes to the DB.

Solution

Add CASBIN_READ_ONLY to opt a stack into read-only operation: build the
adapter via new_read_only (no DDL), skip the root-admin seed, and
reject policy mutations up front rather than after the in-memory model
has already been changed.

Currently pointing the diesel-adapter patch at the fork carrying new_read_only.

upstream PR is:
apache/casbin-rust-diesel-adapter#107

Summary by CodeRabbit

• New Features
  • Added a read-only authorization mode for passive stacks using a replicated policy database. When enabled, the service loads existing policies without adding the root administrator policy or allowing policy changes.
  • Read-only mode is off by default and can be enabled through configuration.

[semanticdiff-com]

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Walkthrough

The Casbin policy engine now supports a CASBIN_READ_ONLY setting. When enabled, it uses a read-only Diesel adapter, skips root-admin policy creation, and rejects mutation access.

Changes

Casbin read-only mode

Layer / File(s)	Summary
Configure the adapter and initialize the engine .env.example, Cargo.toml, crates/service_utils/.../casbin.rs	Adds the CASBIN_READ_ONLY example and patches diesel-adapter to a Git revision. The engine selects the read-only adapter when enabled and skips root-admin policy creation in that mode.
Reject policy mutations crates/service_utils/.../casbin.rs	When read-only mode is enabled, enforcer_mut returns an error before acquiring the enforcer lock.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant Environment
  participant CasbinPolicyEngine
  participant DieselAdapter
  Environment->>CasbinPolicyEngine: Provide CASBIN_READ_ONLY setting
  CasbinPolicyEngine->>DieselAdapter: Create read-only adapter when enabled
  CasbinPolicyEngine->>CasbinPolicyEngine: Skip root-admin policy creation when read-only
  CasbinPolicyEngine->>CasbinPolicyEngine: Reject enforcer_mut when read-only

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Suggested reviewers: ayushjain17, datron

Merge Risk: 🟡 Moderate · up to 4e1b1

An invalid read-only setting can prevent a stack from starting. Return a configuration error instead of panicking before merging.

Security Architecture Review

Security architecture risk: 🟡 Moderate · up to 4e1b1

Read-only mode blocks Casbin policy writes, but a successful organization or workspace change can still leave the corresponding permissions unchanged if the application database accepts the change. Deployment behavior determines whether passive stacks can encounter this condition.

Retained concerns

• Medium · security · inferred: On a stack where application metadata remains writable, organization creation, workspace creation, or workspace-admin updates can succeed after their Casbin policy callback is rejected. In particular, an admin update can leave the former admin's policy grant in place while reporting the metadata change as successful; the inspected handlers do not retry or compensate for that rejection.

Security review details

Security Blast Radius

• inferred — If metadata writes are permitted on a read-only Casbin stack, the affected authorization state includes organization-admin and workspace-admin grants. An admin-update mismatch can preserve the former administrator's access; repeated authorized operations could affect multiple organizations or workspaces. Actual passive-region reachability is unverified.

Security Findings and Attack Paths

• inferred — An authorized workspace-admin update supplies the new email and commits metadata before its policy callback. Where that write succeeds in read-only mode, the callback fails, its error is swallowed, and the prior policy grant can remain usable. No anonymous mutation route or direct bypass of enforcer_mut was established.

Trust Boundaries and Controls

• observed — The inspected application mutation routes are marked authorized, and the read-only gate precedes the Casbin callback and its write lock. These controls limit direct policy mutation but do not make a prior application-database commit and its subsequent policy notification atomic.

Resilience and Maintainability Implications

• inferred — Recovering the writable mode does not itself establish repair of metadata changes whose callbacks were rejected: the inspected handlers invoke those callbacks as part of the original request, with no compensating step there. A separate reconciliation mechanism has not been established.

Hardening Proposals

• proposed — Define whether passive stacks may accept admin-changing application writes. If they may, route policy-changing transitions through a writable policy owner or provide durable, idempotent reconciliation before treating the metadata transition as complete.
• proposed — Validate passive-stack configuration and establish an operational bound or degraded-mode policy for stale authorization when replica refresh fails; verify the patched adapter's read-only DDL and write contract before relying on it.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 1 files. (2 skipped: 2…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: support for read-only Casbin databases in passive-region stacks.

Full details: Docstring Coverage

Explanation

Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 1 files. (2 skipped: 2 unsupported.)

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The read-only path will not mutate
The adapter loads, the roots stay still
No lock is taken against its will
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Actionable comments posted: 1

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Inline comments:
Review comments at @crates/service_utils/src/middlewares/auth_z/casbin.rs:
- Line 392: Update the CASBIN_READ_ONLY parsing in CasbinPolicyEngine::new to
use an error-returning parse path and propagate invalid values as configuration
errors instead of panicking or defaulting to writable mode.

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📒 Files selected for processing (3)

• .env.example
• Cargo.toml
• crates/service_utils/src/middlewares/auth_z/casbin.rs

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