check the *actual* length to allow more stack usage *and* allow smaller pool rentals

src/csharp/Grpc.Core/Grpc.Core.csproj

@@ -100,6 +100,8 @@
 
   <ItemGroup>
     <PackageReference Include="System.Interactive.Async" Version="3.2.0" />
+    <!-- System.Buffers *may* come in transitively, but: we can *always* use ArrayPool -->
+    <PackageReference Include="System.Buffers" Version="4.5.0" />
   </ItemGroup>
 
   <ItemGroup Condition=" '$(TargetFramework)' == 'net45' ">

src/csharp/Grpc.Core/Internal/CallSafeHandle.cs

@@ -135,8 +135,16 @@ namespace Grpc.Core.Internal
             {
                 var ctx = completionQueue.CompletionRegistry.RegisterBatchCompletion(CompletionHandler_ISendStatusFromServerCompletionCallback, callback);
                 var optionalPayloadLength = optionalPayload != null ? new UIntPtr((ulong)optionalPayload.Length) : UIntPtr.Zero;
-                int maxBytes = MarshalUtils.GetMaxBytesUTF8(status.Detail);
+
                 const int MaxStackAllocBytes = 256;
+                int maxBytes = MarshalUtils.GetMaxByteCountUTF8(status.Detail);
+                if (maxBytes > MaxStackAllocBytes)
+                {
+                    // pay the extra to get the *actual* size; this could mean that
+                    // it ends up fitting on the stack after all, but even if not
+                    // it will mean that we ask for a *much* smaller buffer
+                    maxBytes = MarshalUtils.GetByteCountUTF8(status.Detail);
+                }
 
                 if (maxBytes <= MaxStackAllocBytes)
                 {   // for small status, we can encode on the stack without touching arrays

src/csharp/Grpc.Core/Internal/MarshalUtils.cs

@@ -66,9 +66,17 @@ namespace Grpc.Core.Internal
         /// <summary>
         /// Returns the maximum number of bytes required to encode a given string.
         /// </summary>
-        public static int GetMaxBytesUTF8(string str)
+        public static int GetMaxByteCountUTF8(string str)
         {
             return EncodingUTF8.GetMaxByteCount(str.Length);
         }
+
+        /// <summary>
+        /// Returns the actual number of bytes required to encode a given string.
+        /// </summary>
+        public static int GetByteCountUTF8(string str)
+        {
+            return EncodingUTF8.GetByteCount(str);
+        }
     }
 }