fix(MongoDb): Probe the server instead of counting log messages before initiating

src/Testcontainers.MongoDb/MongoDbBuilder.cs

@@ -198,7 +198,7 @@ private static async Task InitiateReplicaSetAsync(MongoDbContainer container, Mo
             return;
         }
 
-        var readiness = new WaitIndicateReadiness(configuration);
+        var readiness = new WaitReplicationEnabled();
 
         // This is a simple workaround to use the default options, which can be configured
         // with custom configurations as needed.
@@ -227,6 +227,32 @@ await WaitStrategy.WaitUntilAsync(initiate, options.Interval, options.Timeout, o
     }
 
     /// <inheritdoc cref="IWaitUntil" />
+    /// <inheritdoc cref="IWaitUntil" />
+    private sealed class WaitReplicationEnabled : IWaitUntil
+    {
+        // rs.status() only answers once the final mongod is serving. The official image forks a
+        // temporary mongod during first-time initialization to create the root user, and that
+        // process is not started with --replSet, so it never reports NotYetInitialized. This
+        // preserves the handover guarantee from #1636 without counting log messages, whose
+        // number depends on log content the module does not control: #1732.
+        private const string ScriptContent = "try{rs.status();quit(0);}catch(e){quit(e.codeName===\"NotYetInitialized\"?0:1);}";
+
+        /// <inheritdoc />
+        public Task<bool> UntilAsync(IContainer container)
+        {
+            return UntilAsync(container as MongoDbContainer);
+        }
+
+        /// <inheritdoc cref="IWaitUntil.UntilAsync" />
+        private static async Task<bool> UntilAsync(MongoDbContainer container)
+        {
+            var execResult = await container.ExecScriptAsync(ScriptContent)
+                .ConfigureAwait(false);
+
+            return 0L.Equals(execResult.ExitCode);
+        }
+    }
+
     private sealed class WaitIndicateReadiness : IWaitUntil
     {
         private static readonly string[] LineEndings = { "\r\n", "\n" };

tests/Testcontainers.MongoDb.Tests/MongoDbReplicaSetReadinessTest.cs

@@ -0,0 +1,59 @@
+using System.Text;
+using System.Threading;
+using DotNet.Testcontainers.Configurations;
+
+namespace Testcontainers.MongoDb;
+
+/// <summary>
+/// The readiness check that runs before the replica set is initiated counts occurrences of a log
+/// message and compares that count for equality. Any other log line carrying the same text pushes
+/// the count past the expected value, and it can then never match
+/// (https://github.com/testcontainers/testcontainers-dotnet/issues/1732).
+/// </summary>
+public sealed class MongoDbReplicaSetReadinessTest : IAsyncLifetime
+{
+    private const string ExtraMarkerScriptFilePath = "/docker-entrypoint-initdb.d/00-extra-marker.sh";
+
+    private readonly MongoDbContainer _mongoDbContainer = new MongoDbBuilder(TestSession.GetImageFromDockerfile())
+        .WithReplicaSet()
+        .WithResourceMapping(
+            Encoding.Default.GetBytes("#!/bin/bash\necho 'Waiting for connections'\necho 'Waiting for connections'\n"),
+            ExtraMarkerScriptFilePath,
+            fileMode: Unix.FileMode755)
+        .Build();
+
+    public ValueTask InitializeAsync()
+    {
+        return ValueTask.CompletedTask;
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        await _mongoDbContainer.DisposeAsync()
+            .ConfigureAwait(false);
+
+        GC.SuppressFinalize(this);
+    }
+
+    [Fact]
+    [Trait(nameof(DockerCli.DockerPlatform), nameof(DockerCli.DockerPlatform.Linux))]
+    public async Task StartsWhenTheLogContainsAdditionalReadinessMessages()
+    {
+        // Given
+        // The default wait strategy timeout is one hour, so bound the wait to keep a regression
+        // from stalling the test run instead of failing it.
+        using var cts = new CancellationTokenSource(TimeSpan.FromMinutes(3));
+
+        // When
+        await _mongoDbContainer.StartAsync(cts.Token)
+            .ConfigureAwait(true);
+
+        // Then
+        const string scriptContent = "rs.status().ok;";
+
+        var execResult = await _mongoDbContainer.ExecScriptAsync(scriptContent, cts.Token)
+            .ConfigureAwait(true);
+
+        Assert.True(0L.Equals(execResult.ExitCode), execResult.Stderr);
+    }
+}