fix(MongoDb): Wait for post-init startup readiness before replica set initiation

[HofmeisterAn]

What does this PR do?

The PR changes how the MongoDB replica set is initiated. Instead of initiating it immediately, it now waits until the final process is running.

The official MongoDB image starts mongod twice during the first-time initialization (bootstrap + final process). Waiting prevents the replica set from being initiated while this handover is in progress, which could otherwise cause the container startup to fail (container exits).

Why is it important?

We occasionally saw MongoDB tests fail because the container exited during startup with:

{"msg":"Error setting up listener","attr":{"error":{"code":9001,"codeName":"SocketException","errmsg":"Address already in use"}}}

Related issues

• Closes [Bug]: MongoDB stops while starting #1636

Summary by CodeRabbit

• Bug Fixes
  • Improved MongoDB replica set initialization reliability by adding a readiness check before starting the replica set setup, preventing potential startup conflicts.

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: be976518-48f6-4559-8b6a-d3c0db154e25

📥 Commits

Reviewing files that changed from the base of the PR and between 84f4f7c and 22708ed.

📒 Files selected for processing (1)

• src/Testcontainers.MongoDb/MongoDbBuilder.cs

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Walkthrough

The change introduces a readiness wait mechanism in MongoDB container initialization that pauses before attempting to initiate the replica set, preventing the replica set initialization from executing while the container is still stabilizing during startup.

Changes

Cohort / File(s)	Summary
MongoDB Replica Set Initialization src/Testcontainers.MongoDb/MongoDbBuilder.cs	Adds WaitIndicateReadiness prior to replica set initiation to prevent race conditions where the MongoDB container is still processing startup operations, guarding against socket binding failures.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

Suggested labels

test flakiness

Poem

🐰 A patient hop before the bind,
We wait for readiness to align,
No more racing through startup's race,
MongoDB now sets at steady pace!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: waiting for post-init startup readiness before replica set initiation, directly matching the code modification.
Description check	✅ Passed	The description includes mandatory sections (What/Why) and recommended sections (Related issues), providing clear context about the change and its rationale.
Linked Issues check	✅ Passed	The PR directly addresses the objectives of issue #1636 by implementing a readiness wait before replica set initiation to prevent socket binding failures during MongoDB startup handover.
Out of Scope Changes check	✅ Passed	All changes are in scope: adding a readiness wait in MongoDbBuilder.cs directly addresses the startup handover race condition identified in issue #1636.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch bugfix/mongodb-wait-before-init-replset

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