Avoid leaking a MeterListener per Cache in DEBUG builds

docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@@ -1,5 +1,6 @@
 ### Fixed
 
+* Stop leaking a `System.Diagnostics.Metrics.MeterListener` per `Cache` in DEBUG builds. Each cache created a `CacheMetricsListener` (which starts a `MeterListener` registered in the process-global metrics registry) and never disposed it, so listeners accumulated for the lifetime of the process. Because every cache hit/miss/add published to all registered listeners, the per-operation cost grew linearly with the number of leaked listeners, so repeated checks (and Debug FCS test runs) slowed down over time. The per-cache `DebugDisplay` totals are now tracked directly instead of via a `MeterListener`. ([PR #19995](https://github.com/dotnet/fsharp/pull/19995))
 * Semantic classification no longer marks recursive object self-references (`as this`, `let rec` self-refs) as mutable. ([Issue #5229](https://github.com/dotnet/fsharp/issues/5229))
 * Fix `MethodAccessException` under `--realsig+` when a closure (inner `let rec`, `task`/`async` state machine, or quotation splice) inside a member defined in an intrinsic type augmentation (`type C with member ...`) accesses a `private` member of `C`. The synthesized closure is now nested inside the declaring type instead of beside it in the module class. ([Issue #19933](https://github.com/dotnet/fsharp/issues/19933), [PR #19955](https://github.com/dotnet/fsharp/pull/19955))
 * Preserve source range for type errors on empty-bodied computation expressions (e.g. `foo {}`) in pipelines, function arguments, and type-annotated contexts, instead of reporting `unknown(1,1)`. ([Issue #19550](https://github.com/dotnet/fsharp/issues/19550), [PR #19849](https://github.com/dotnet/fsharp/pull/19849))

src/Compiler/Utilities/Caches.fs

@@ -32,12 +32,6 @@ module CacheMetrics =
         tags.Add("cacheId", box cacheId)
         tags
 
-    let Add (tags: inref<TagList>) = adds.Add(1L, &tags)
-    let Update (tags: inref<TagList>) = updates.Add(1L, &tags)
-    let Hit (tags: inref<TagList>) = hits.Add(1L, &tags)
-    let Miss (tags: inref<TagList>) = misses.Add(1L, &tags)
-    let Eviction (tags: inref<TagList>) = evictions.Add(1L, &tags)
-    let EvictionFail (tags: inref<TagList>) = evictionFails.Add(1L, &tags)
     let Created (tags: inref<TagList>) = creations.Add(1L, &tags)
     let Disposed (tags: inref<TagList>) = disposals.Add(1L, &tags)
 
@@ -279,6 +273,23 @@ type Cache<'Key, 'Value when 'Key: not null> internal (options: CacheOptions<'Ke
 
     let tags = CacheMetrics.mkTags name
 
+    // Per-cache metric totals used by DebugDisplay. These are incremented directly (see recordMetric)
+    // rather than via a per-cache CacheMetrics.CacheMetricsListener: a listener starts a
+    // System.Diagnostics.Metrics.MeterListener which registers in the process-global metrics registry,
+    // so one per Cache would never be disposed (leak) and would make every measurement O(number of
+    // caches) as listeners accumulate.
+#if DEBUG
+    let debugStats = CacheMetrics.Stats()
+#endif
+
+    // Publish one measurement to the global Meter counter (for --times / StatsToString) and, in DEBUG,
+    // record it in this cache's own totals for DebugDisplay.
+    let recordMetric (counter: Counter<int64>) =
+        counter.Add(1L, &tags)
+#if DEBUG
+        debugStats.Incr counter.Name 1L
+#endif
+
     // Track disposal state (0 = not disposed, 1 = disposed)
     let mutable disposed = 0
 
@@ -310,10 +321,10 @@ type Cache<'Key, 'Value when 'Key: not null> internal (options: CacheOptions<'Ke
 
                 match store.TryRemove(first.Value.Key) with
                 | true, _ ->
-                    CacheMetrics.Eviction &tags
+                    recordMetric CacheMetrics.evictions
                     evicted.Trigger()
                 | _ ->
-                    CacheMetrics.EvictionFail &tags
+                    recordMetric CacheMetrics.evictionFails
                     evictionFailed.Trigger()
                     deadKeysCount <- deadKeysCount + 1
 
@@ -361,10 +372,6 @@ type Cache<'Key, 'Value when 'Key: not null> internal (options: CacheOptions<'Ke
 
             post, dispose
 
-#if DEBUG
-    let debugListener = new CacheMetrics.CacheMetricsListener(tags)
-#endif
-
     do CacheMetrics.Created &tags
 
     member val Evicted = evicted.Publish
@@ -373,12 +380,12 @@ type Cache<'Key, 'Value when 'Key: not null> internal (options: CacheOptions<'Ke
     member _.TryGetValue(key: 'Key, value: outref<'Value>) =
         match store.TryGetValue(key) with
         | true, entity ->
-            CacheMetrics.Hit &tags
+            recordMetric CacheMetrics.hits
             post (EvictionQueueMessage.Update entity)
             value <- entity.Value
             true
         | _ ->
-            CacheMetrics.Miss &tags
+            recordMetric CacheMetrics.misses
             value <- Unchecked.defaultof<'Value>
             false
 
@@ -388,7 +395,7 @@ type Cache<'Key, 'Value when 'Key: not null> internal (options: CacheOptions<'Ke
         let added = store.TryAdd(key, entity)
 
         if added then
-            CacheMetrics.Add &tags
+            recordMetric CacheMetrics.adds
             post (EvictionQueueMessage.Add(entity, store))
 
         added
@@ -405,11 +412,11 @@ type Cache<'Key, 'Value when 'Key: not null> internal (options: CacheOptions<'Ke
 
         if wasMiss then
             post (EvictionQueueMessage.Add(result, store))
-            CacheMetrics.Add &tags
-            CacheMetrics.Miss &tags
+            recordMetric CacheMetrics.adds
+            recordMetric CacheMetrics.misses
         else
             post (EvictionQueueMessage.Update result)
-            CacheMetrics.Hit &tags
+            recordMetric CacheMetrics.hits
 
         result.Value
 
@@ -424,10 +431,10 @@ type Cache<'Key, 'Value when 'Key: not null> internal (options: CacheOptions<'Ke
 
         // Returned value tells us if the entity was added or updated.
         if Object.ReferenceEquals(addValue, result) then
-            CacheMetrics.Add &tags
+            recordMetric CacheMetrics.adds
             post (EvictionQueueMessage.Add(addValue, store))
         else
-            CacheMetrics.Update &tags
+            recordMetric CacheMetrics.updates
             post (EvictionQueueMessage.Update result)
 
     member _.CreateMetricsListener() =
@@ -447,5 +454,5 @@ type Cache<'Key, 'Value when 'Key: not null> internal (options: CacheOptions<'Ke
     override this.Finalize() = this.Dispose()
 
 #if DEBUG
-    member _.DebugDisplay() = debugListener.ToString()
+    member _.DebugDisplay() = debugStats.ToString()
 #endif