fix: Don't set current thread when one is already there (#4681)

Author: bitsandfoxes

src/Sentry/Internal/MainSentryEventProcessor.cs

@@ -83,8 +83,8 @@ public SentryEvent Process(SentryEvent @event)
         @event.Release ??= Release;
         @event.Distribution ??= Distribution;
 
-        // if there's no exception with a stack trace, then get the current stack trace
-        if (@event.Exception?.StackTrace is null)
+        // If there's no current thread on the event and there is no exception with a stack trace, get the current stack trace
+        if (@event.Exception?.StackTrace is null && @event.SentryThreads?.Any(t => t.Current == true) != true)
         {
             var stackTrace = @event.SentryExceptions?.FirstOrDefault()?.Stacktrace
                             ?? SentryStackTraceFactoryAccessor().Create();

test/Sentry.Tests/Internals/MainSentryEventProcessorTests.cs

@@ -491,6 +491,34 @@ public void Process_AttachStacktraceTrueAndExistentThreadInEvent_AddsNewThread()
         Assert.Equal("second", evt.SentryThreads.Last().Name);
     }
 
+    [Fact]
+    public void Process_AttachStacktraceTrueAndCurrentThreadAlreadyExists_DoesNotAddAnotherThread()
+    {
+        var existingStackTrace = new SentryStackTrace();
+        _fixture.SentryOptions.AttachStacktrace = true;
+        var sut = _fixture.GetSut();
+
+        var evt = new SentryEvent
+        {
+            SentryThreads = new[]
+            {
+                new SentryThread
+                {
+                    Name = "existing-thread",
+                    Current = true,
+                    Stacktrace = existingStackTrace
+                }
+            }
+        };
+        _ = sut.Process(evt);
+
+        Assert.Single(evt.SentryThreads);
+        Assert.Equal("existing-thread", evt.SentryThreads.First().Name);
+        Assert.True(evt.SentryThreads.First().Current);
+        Assert.Same(existingStackTrace, evt.SentryThreads.First().Stacktrace);
+        _ = _fixture.SentryStackTraceFactory.DidNotReceive().Create();
+    }
+
     [Fact]
     public void Process_CultureInfoAndCultureInfoAreEqual_OnlyCultureInfoSet()
     {