ndb: replace force unwraps with safe optionals in NdbTxn

[alltheseas]

Summary

Replace force unwraps (as!) with safe optional binding (as?) in NdbTxn and SafeNdbTxn transaction inheritance logic. This prevents crashes when thread-local storage is in an unexpected state.

Changes:

• Move ref_count check into the if let conditional binding
• Use already-bound txn_generation instead of re-reading with force unwrap
• Simplify ref count increment to single expression

The fix ensures transaction inheritance gracefully falls back to creating a new transaction if thread-local state is incomplete, rather than crashing on missing values.

Checklist

Standard PR Checklist

• I have read (or I am familiar with) the Contribution Guidelines
• I have tested the changes in this PR
• I have profiled the changes to ensure there are no performance regressions, or I do not need to profile the changes.
  • If not needed, provide reason: Changes only affect transaction initialization logic, no performance impact.
• I have opened or referred to an existing github issue related to this change.
  • Related to crash analysis from TestFlight v1.16 (Build 1277)
• My PR is either small, or I have split it into smaller logical commits that are easier to review
• I have added the signoff line to all my commits. See Signing off your work
• I have added appropriate changelog entries for the changes in this PR. See Adding changelog entries
• I have added appropriate Closes: or Fixes: tags in the commit messages wherever applicable, or made sure those are not needed. See Submitting patches

Test report

Device: macOS Simulator (iPhone 17 Pro)

iOS: 18.0

Damus: master + this PR

Setup: Unit test suite via xcodebuild test

Steps:

1. Run xcodebuild test -project damus.xcodeproj -scheme damus -destination 'platform=iOS Simulator,name=iPhone 17 Pro' -only-testing:damusTests/NdbTests
2. Verify test_inherited_transactions passes - confirms transaction inheritance with safe optional binding works

Results:

• PASS
• Executed 14 tests, with 0 failures (0 unexpected) in 1.415 seconds
• All existing tests continue to pass

Testing directions for reviewers

1. Unit tests: Run NdbTests target - test_inherited_transactions specifically tests the transaction inheritance path
2. Code review: Verify the force unwraps at lines 47-48 (old) are replaced with safe optional binding in the if let chain

Other notes

The original code had:

if let active_txn = Thread.current.threadDictionary["ndb_txn"] as? ndb_txn,
   let txn_generation = Thread.current.threadDictionary["txn_generation"] as? Int,
   txn_generation == ndb.generation
{
    self.generation = Thread.current.threadDictionary["txn_generation"] as! Int  // Force unwrap!
    let ref_count = Thread.current.threadDictionary["ndb_txn_ref_count"] as! Int  // Force unwrap!
    ...
}

The fix moves ref_count into the conditional binding and reuses the already-bound txn_generation:

if let active_txn = Thread.current.threadDictionary["ndb_txn"] as? ndb_txn,
   let txn_generation = Thread.current.threadDictionary["txn_generation"] as? Int,
   let ref_count = Thread.current.threadDictionary["ndb_txn_ref_count"] as? Int,
   txn_generation == ndb.generation
{
    self.generation = txn_generation  // Already bound, no force unwrap
    Thread.current.threadDictionary["ndb_txn_ref_count"] = ref_count + 1
    ...
}

Signed-off-by: alltheseas

Summary by CodeRabbit

• Refactor
  • Enhanced internal transaction handling stability with improved error-handling patterns.

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[coderabbitai]

📝 Walkthrough

Walkthrough

Changes to transaction initialization in NdbTxn.swift replace force-unwrapping with optional binding for ref_count retrieval and align generation assignment across both NdbTxn.init and SafeNdbTxn.new, maintaining existing control flow while improving safety.

Changes

Cohort / File(s)	Summary
Transaction Initialization Safety nostrdb/NdbTxn.swift	Replaced force-unwrapping with optional binding for ref_count; uses txn_generation from outer scope for generation assignment. Pattern applied consistently in both NdbTxn.init and SafeNdbTxn.new. Ref_count incrementation and storage consolidated. No API changes.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

Poem

🐰 With optional binding bright,
We banish force-unwrap's plight,
Safe transactions now take flight,
Generation flows just right! 🌟

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: replacing force unwraps with safe optional binding in NdbTxn, which is the primary focus of the changeset.
Description check	✅ Passed	The PR description is comprehensive and follows the template structure, including a clear summary, completed standard PR checklist with justifications, detailed test report with steps and results, and thorough technical explanation with code examples.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[alltheseas]

Closing in favor of consolidated Linux-style topic branches. This work will be audited against AGENTS.md requirements and cherry-picked into ordered topic branches. See #3624 for tracking.