C#: Only create a single empty location element.

Author: michaelnebel

csharp/extractor/Semmle.Extraction.CSharp/Entities/Locations/EmptyLocation.cs

@@ -0,0 +1,43 @@
+using System.IO;
+
+namespace Semmle.Extraction.CSharp.Entities
+{
+    /// <summary>
+    /// Used to create a single canonical empty location entity.
+    /// </summary>
+    public class EmptyLocation : GeneratedLocation
+    {
+        private EmptyLocation(Context cx) : base(cx) { }
+
+        public override void Populate(TextWriter trapFile)
+        {
+            trapFile.locations_default(this, GenFile, -1, -1, -1, -1);
+        }
+
+        public override void WriteId(EscapingTextWriter trapFile)
+        {
+            if (Context.ExtractionContext.IsStandalone)
+            {
+                WriteStarId(trapFile);
+                return;
+            }
+
+            trapFile.Write("loc,");
+            trapFile.WriteSubId(GenFile);
+            trapFile.Write(",-1,-1,-1,-1");
+        }
+
+        public static new EmptyLocation Create(Context cx) => EmptyLocationFactory.Instance.CreateEntity(cx, typeof(EmptyLocation), null);
+
+        private class EmptyLocationFactory : CachedEntityFactory<string?, EmptyLocation>
+        {
+            public static EmptyLocationFactory Instance { get; } = new EmptyLocationFactory();
+
+            /// <summary>
+            /// The QL library assumes the presence of a single empty location element.
+            /// </summary>
+            public override EmptyLocation Create(Context cx, string? init) => new EmptyLocation(cx);
+        }
+    }
+
+}

csharp/extractor/Semmle.Extraction.CSharp/Entities/Locations/GeneratedLocation.cs

@@ -4,17 +4,17 @@ namespace Semmle.Extraction.CSharp.Entities
 {
     public class GeneratedLocation : SourceLocation
     {
-        private readonly File generatedFile;
+        protected File GenFile { get; init; }
 
-        private GeneratedLocation(Context cx)
+        protected GeneratedLocation(Context cx)
             : base(cx, null)
         {
-            generatedFile = GeneratedFile.Create(cx);
+            GenFile = GeneratedFile.Create(cx);
         }
 
         public override void Populate(TextWriter trapFile)
         {
-            trapFile.locations_default(this, generatedFile, 0, 0, 0, 0);
+            trapFile.locations_default(this, GenFile, 0, 0, 0, 0);
         }
 
         public override void WriteId(EscapingTextWriter trapFile)
@@ -26,7 +26,7 @@ public override void WriteId(EscapingTextWriter trapFile)
             }
 
             trapFile.Write("loc,");
-            trapFile.WriteSubId(generatedFile);
+            trapFile.WriteSubId(GenFile);
             trapFile.Write(",0,0,0,0");
         }
 

csharp/extractor/Semmle.Extraction.CSharp/Extractor/Analyser.cs

@@ -238,6 +238,9 @@ private void DoAnalyseCompilation()
 
                 compilationEntity = Entities.Compilation.Create(cx);
 
+                // Add a single empty location
+                Entities.EmptyLocation.Create(cx);
+
                 ExtractionContext.CompilationInfos.ForEach(ci => trapWriter.Writer.compilation_info(compilationEntity, ci.key, ci.value));
 
                 ReportProgressTaskDone(currentTaskId, assemblyPath, trapWriter.TrapFile, stopwatch.Elapsed, AnalysisAction.Extracted);