Avoid concurrent generation of the html document within a razor code document. (#11986)

I see about 40% of edits within an html scope generate the html document
multiple times, primarily due to concurrent
RazorCodeCode.GetHtmlDocument requests from the callstacks below
(measured on release bits without a debugger attached). As these
requests are on the same code document, they can be collapsed into a
single request.

*** callstack 1 ***
RazorCodeDocument.GetHtmlDocument
RazorCodeDocumentExtensions.GetHtmlSourceText
GeneratedDocumentSynchronizer.DocumentProcessed
OpenDocumentGenerator.ProcessBatchAsync

*** callstack 2 ***
RazorCodeDocument.GetHtmlDocument
IDocumentMappingServiceExtensions.GetPositionInfo
DelegatedCompletionListProvider.GetCompletionListAsync
CompletionListProvider.GetCompletionListCoreAsync

Author: ToddGrun

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/RazorCodeDocument.cs

@@ -24,6 +24,7 @@ public sealed partial class RazorCodeDocument
     public RazorFileKind FileKind => ParserOptions.FileKind;
 
     private readonly PropertyTable _properties = new();
+    private readonly object _htmlDocumentLock = new();
 
     private RazorCodeDocument(
         RazorSourceDocument source,
@@ -182,8 +183,15 @@ internal RazorHtmlDocument GetHtmlDocument()
             return result;
         }
 
-        result = RazorHtmlWriter.GetHtmlDocument(this);
-        _properties.HtmlDocument.SetValue(result);
+        // Perf: Avoid concurrent requests generating the same html document
+        lock (_htmlDocumentLock)
+        {
+            if (!_properties.HtmlDocument.TryGetValue(out result))
+            {
+                result = RazorHtmlWriter.GetHtmlDocument(this);
+                _properties.HtmlDocument.SetValue(result);
+            }
+        }
 
         return result;
     }