Improve performance of lsif generation.

src/Features/Lsif/Generator/Writing/BatchingLsifJsonWriter.cs

@@ -57,6 +57,14 @@ public void Write(Element element)
             }
         }
 
+        public void Write(List<Element> elements)
+        {
+            lock (_elementsGate)
+            {
+                _elements.AddRange(elements);
+            }
+        }
+
         public void FlushToUnderlyingAndEmpty()
         {
             lock (_writingGate)
@@ -69,10 +77,7 @@ public void FlushToUnderlyingAndEmpty()
                     _elements = new List<Element>();
                 }
 
-                foreach (var element in localElements)
-                {
-                    _underlyingWriter.Write(element);
-                }
+                _underlyingWriter.Write(localElements);
             }
         }
     }

src/Features/Lsif/Generator/Writing/ILsifJsonWriter.cs

@@ -2,12 +2,14 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Collections.Generic;
 using Microsoft.CodeAnalysis.LanguageServerIndexFormat.Generator.Graph;
 
 namespace Microsoft.CodeAnalysis.LanguageServerIndexFormat.Generator.Writing
 {
     internal interface ILsifJsonWriter
     {
         void Write(Element element);
+        void Write(List<Element> elements);
     }
 }

src/Features/Lsif/Generator/Writing/JsonModeLsifJsonWriter.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
+using System.Collections.Generic;
 using System.IO;
 using Microsoft.CodeAnalysis.LanguageServerIndexFormat.Generator.Graph;
 using Newtonsoft.Json;
@@ -44,6 +45,15 @@ public void Write(Element element)
             }
         }
 
+        public void Write(List<Element> elements)
+        {
+            lock (_writeGate)
+            {
+                foreach (var element in elements)
+                    _jsonSerializer.Serialize(_jsonTextWriter, element);
+            }
+        }
+
         public void Dispose()
         {
             _jsonTextWriter.WriteWhitespace(Environment.NewLine);

src/Features/Lsif/Generator/Writing/LineModeLsifJsonWriter.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Collections.Generic;
 using System.IO;
 using Microsoft.CodeAnalysis.LanguageServerIndexFormat.Generator.Graph;
 using Newtonsoft.Json;
@@ -40,5 +41,18 @@ public void Write(Element element)
                 _outputWriter.WriteLine(line);
             }
         }
+
+        public void Write(List<Element> elements)
+        {
+            var lines = new List<string>();
+            foreach (var element in elements)
+                lines.Add(JsonConvert.SerializeObject(element, _settings));
+
+            lock (_writeGate)
+            {
+                foreach (var line in lines)
+                    _outputWriter.WriteLine(line);
+            }
+        }
     }
 }

src/Features/Lsif/GeneratorTest/Utilities/TestLsifJsonWriter.vb

@@ -18,6 +18,12 @@ Namespace Microsoft.CodeAnalysis.LanguageServerIndexFormat.Generator.UnitTests.U
         Private ReadOnly _elementsById As Dictionary(Of Id(Of Element), Element) = New Dictionary(Of Id(Of Element), Element)
         Private ReadOnly _edgesByOutVertex As Dictionary(Of Vertex, List(Of Edge)) = New Dictionary(Of Vertex, List(Of Edge))
 
+        Private Sub ILsifJsonWriter_Write(elements As List(Of Element)) Implements ILsifJsonWriter.Write
+            For Each element In elements
+                ILsifJsonWriter_Write(element)
+            Next
+        End Sub
+
         Private Sub ILsifJsonWriter_Write(element As Element) Implements ILsifJsonWriter.Write
             SyncLock _gate
                 ' We intentionally use Add so it'll throw if we have a duplicate ID.