[Prometheus] Refactor locks during collection (#6635)

Author: martincostello

src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusCollectionManager.cs

@@ -46,56 +46,59 @@ public Task<CollectionResponse> EnterCollect(bool openMetricsRequested)
     {
         this.EnterGlobalLock();
 
-        // If we are within {ScrapeResponseCacheDurationMilliseconds} of the
-        // last successful collect, return the previous view.
-        var previousDataViewGeneratedAtUtc = openMetricsRequested
-            ? this.previousOpenMetricsDataViewGeneratedAtUtc
-            : this.previousPlainTextDataViewGeneratedAtUtc;
-
-        if (previousDataViewGeneratedAtUtc.HasValue
-            && this.scrapeResponseCacheDurationMilliseconds > 0
-            && previousDataViewGeneratedAtUtc.Value.AddMilliseconds(this.scrapeResponseCacheDurationMilliseconds) >= DateTime.UtcNow)
+        DateTime? previousDataViewGeneratedAtUtc;
+
+        try
         {
-            Interlocked.Increment(ref this.readerCount);
-            this.ExitGlobalLock();
+            // If we are within {ScrapeResponseCacheDurationMilliseconds} of the
+            // last successful collect, return the previous view.
+            previousDataViewGeneratedAtUtc = openMetricsRequested
+                ? this.previousOpenMetricsDataViewGeneratedAtUtc
+                : this.previousPlainTextDataViewGeneratedAtUtc;
+
+            if (previousDataViewGeneratedAtUtc.HasValue
+                && this.scrapeResponseCacheDurationMilliseconds > 0
+                && previousDataViewGeneratedAtUtc.Value.AddMilliseconds(this.scrapeResponseCacheDurationMilliseconds) >= DateTime.UtcNow)
+            {
 #if NET
-            return new ValueTask<CollectionResponse>(new CollectionResponse(this.previousOpenMetricsDataView, this.previousPlainTextDataView, previousDataViewGeneratedAtUtc.Value, fromCache: true));
+                return new ValueTask<CollectionResponse>(new CollectionResponse(this.previousOpenMetricsDataView, this.previousPlainTextDataView, previousDataViewGeneratedAtUtc.Value, fromCache: true));
 #else
-            return Task.FromResult(new CollectionResponse(this.previousOpenMetricsDataView, this.previousPlainTextDataView, previousDataViewGeneratedAtUtc.Value, fromCache: true));
+                return Task.FromResult(new CollectionResponse(this.previousOpenMetricsDataView, this.previousPlainTextDataView, previousDataViewGeneratedAtUtc.Value, fromCache: true));
 #endif
-        }
+            }
 
-        // If a collection is already running, return a task to wait on the result.
-        if (this.collectionRunning)
-        {
-            this.collectionTcs ??= new TaskCompletionSource<CollectionResponse>(TaskCreationOptions.RunContinuationsAsynchronously);
+            // If a collection is already running, return a task to wait on the result.
+            if (this.collectionRunning)
+            {
+                this.collectionTcs ??= new TaskCompletionSource<CollectionResponse>(TaskCreationOptions.RunContinuationsAsynchronously);
 
-            Interlocked.Increment(ref this.readerCount);
-            this.ExitGlobalLock();
 #if NET
-            return new ValueTask<CollectionResponse>(this.collectionTcs.Task);
+                return new ValueTask<CollectionResponse>(this.collectionTcs.Task);
 #else
-            return this.collectionTcs.Task;
+                return this.collectionTcs.Task;
 #endif
-        }
+            }
 
-        this.WaitForReadersToComplete();
+            this.WaitForReadersToComplete();
 
-        // Start a collection on the current thread.
-        this.collectionRunning = true;
+            // Start a collection on the current thread.
+            this.collectionRunning = true;
 
-        if (openMetricsRequested)
-        {
-            this.previousOpenMetricsDataViewGeneratedAtUtc = null;
+            if (openMetricsRequested)
+            {
+                this.previousOpenMetricsDataViewGeneratedAtUtc = null;
+            }
+            else
+            {
+                this.previousPlainTextDataViewGeneratedAtUtc = null;
+            }
         }
-        else
+        finally
         {
-            this.previousPlainTextDataViewGeneratedAtUtc = null;
+            Interlocked.Increment(ref this.readerCount);
+            this.ExitGlobalLock();
         }
 
-        Interlocked.Increment(ref this.readerCount);
-        this.ExitGlobalLock();
-
         CollectionResponse response;
         var result = this.ExecuteCollect(openMetricsRequested);
         if (result)
@@ -122,16 +125,21 @@ public Task<CollectionResponse> EnterCollect(bool openMetricsRequested)
 
         this.EnterGlobalLock();
 
-        this.collectionRunning = false;
+        try
+        {
+            this.collectionRunning = false;
 
-        if (this.collectionTcs != null)
+            if (this.collectionTcs != null)
+            {
+                this.collectionTcs.SetResult(response);
+                this.collectionTcs = null;
+            }
+        }
+        finally
         {
-            this.collectionTcs.SetResult(response);
-            this.collectionTcs = null;
+            this.ExitGlobalLock();
         }
 
-        this.ExitGlobalLock();
-
 #if NET
         return new ValueTask<CollectionResponse>(response);
 #else