Backport "reader check" changes from Damus iOS

[danieldaquino]

This backports the following commit from the Damus iOS repo: damus-io/damus@fe482a9

The rationale of this change is to reduce the chance of stale reader slots by following this recommendation from lmdb.h docs:

 *	@section caveats_sec Caveats
 *	Troubleshooting the lock file, plus semaphores on BSD systems:
 *
 *	- A broken lockfile can cause sync issues.
 *	  Stale reader transactions left behind by an aborted program
 *	  cause further writes to grow the database quickly, and
 *	  stale locks can block further operation.
 *
 *	  Fix: Check for stale readers periodically, using the
 *	  #mdb_reader_check function or the \ref mdb_stat_1 "mdb_stat" tool.
 *	  Stale writers will be cleared automatically on some systems:
 *	  - Windows - automatic
 *	  - Linux, systems using POSIX mutexes with Robust option - automatic
 *	  - not on BSD, systems using POSIX semaphores.
 *	  Otherwise just make all programs using the database close it;
 *	  the lockfile is always reset on first open of the environment.

Summary by CodeRabbit

• Bug Fixes
  • Enhanced database initialization process with a new startup check that detects and clears stale connection slots, improving startup reliability and preventing potential issues from residual database locks.

[danieldaquino]

CC @jb55, please let me know if you have any concerns. Thanks!

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: faaae869-0817-4fef-b692-53ecb18c8dbe

📥 Commits

Reviewing files that changed from the base of the PR and between 2d459e8 and a15402e.

📒 Files selected for processing (1)

• src/nostrdb.c

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📝 Walkthrough

Walkthrough

This PR adds startup-time detection and cleanup of stale LMDB reader slots. A new helper function (ndb_lmdb_reader_check) wraps the LMDB library's reader-check facility, then integrates it into environment initialization. If reader-slot cleanup fails during startup, initialization aborts and the environment is cleaned up.

Changes

LMDB Reader Slot Initialization

Layer / File(s)	Summary
Reader Check Helper Function src/nostrdb.c	Introduces ndb_lmdb_reader_check(MDB_env *env) that calls mdb_reader_check, logs the result or errors, and returns success/failure.
Integration into Initialization src/nostrdb.c	After mdb_env_open succeeds in ndb_init_lmdb, the code invokes the reader-slot check; on failure, closes the environment and aborts initialization.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related issues

• Backport "reader check" change from Damus iOS #127: Both PRs implement LMDB reader-check startup behavior for detecting and clearing stale reader slots, part of the same initialization improvement effort.

Poem

🐰 A startup dance with slots so stale,
We check and clean when readers fail,
With LMDB's gentle care,
No ghost readers linger there! ✨

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

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately reflects the main change: backporting a reader check feature from Damus iOS to nostrdb, which is the primary objective of the PR.
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.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[jb55]

thanks! lgtm. strfry also has this, i didn't realize how important it was.