C#: Only extract a single empty location.

Author: michaelnebel

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

@@ -0,0 +1,58 @@
+using System.IO;
+using System.Threading;
+
+namespace Semmle.Extraction.CSharp.Entities
+{
+    public class EmptyLocation : SourceLocation
+    {
+        private readonly File generatedFile;
+
+        // The Ql library assumes the presence of a single "empty" location.
+        private static EmptyLocation? instance;
+        private static readonly Lock instanceLock = new();
+
+        private EmptyLocation(Context cx)
+            : base(cx, null)
+        {
+            generatedFile = GeneratedFile.Create(cx);
+        }
+
+        public override void Populate(TextWriter trapFile)
+        {
+            trapFile.locations_default(this, generatedFile, 0, 0, 0, 0);
+        }
+
+        public override void WriteId(EscapingTextWriter trapFile)
+        {
+            if (Context.ExtractionContext.IsStandalone)
+            {
+                WriteStarId(trapFile);
+                return;
+            }
+
+            trapFile.Write("loc,");
+            trapFile.WriteSubId(generatedFile);
+            trapFile.Write(",0,0,0,0");
+        }
+
+        public override int GetHashCode() => 98732567;
+
+        public override bool Equals(object? obj) => obj is not null && obj.GetType() == typeof(EmptyLocation);
+
+        public static EmptyLocation Create(Context cx)
+        {
+            lock (instanceLock)
+            {
+                instance ??= EmptyLocationFactory.Instance.CreateEntity(cx, typeof(EmptyLocation), null);
+                return instance;
+            }
+        }
+
+        private class EmptyLocationFactory : CachedEntityFactory<string?, EmptyLocation>
+        {
+            public static EmptyLocationFactory Instance { get; } = new EmptyLocationFactory();
+
+            public override EmptyLocation Create(Context cx, string? init) => new EmptyLocation(cx);
+        }
+    }
+}

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

@@ -247,7 +247,7 @@ public override void Populate(TextWriter trapFile)
             var typeKey = VarargsType.Create(Context);
             // !! Maybe originaldefinition is wrong
             trapFile.@params(this, "", typeKey, Ordinal, Kind.None, Parent!, this);
-            trapFile.param_location(this, GeneratedLocation.Create(Context));
+            trapFile.param_location(this, EmptyLocation.Create(Context));
         }
 
         protected override int Ordinal => ((Method)Parent!).OriginalDefinition.Symbol.Parameters.Length;

csharp/extractor/Semmle.Extraction.CSharp/Entities/Parameter.cs

@@ -238,6 +238,9 @@ private void DoAnalyseCompilation()
 
                 compilationEntity = Entities.Compilation.Create(cx);
 
+                // Ensure that the empty location is always created.
+                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);

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

@@ -582,14 +582,14 @@ public void WithDuplicationGuard(Key key, Action a)
         public Entities.Location CreateLocation()
         {
             return SourceTree is null
-                ? Entities.GeneratedLocation.Create(this)
+                ? Entities.EmptyLocation.Create(this)
                 : CreateLocation(Microsoft.CodeAnalysis.Location.Create(SourceTree, Microsoft.CodeAnalysis.Text.TextSpan.FromBounds(0, 0)));
         }
 
         public Entities.Location CreateLocation(Microsoft.CodeAnalysis.Location? location)
         {
             return (location is null || location.Kind == LocationKind.None)
-                ? Entities.GeneratedLocation.Create(this)
+                ? Entities.EmptyLocation.Create(this)
                 : location.IsInSource
                     ? Entities.NonGeneratedSourceLocation.Create(this, location)
                     : Entities.Assembly.Create(this, location);