C#: Write overlay metadata at end of extraction.

Author: michaelnebel

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

@@ -2,6 +2,7 @@
 using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.IO;
 using System.Linq;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
@@ -53,6 +54,20 @@ private static void AnalyseStandalone(
                         }
 
                         progressMonitor.MissingSummary(analyser.ExtractionContext!.MissingTypes.Count(), analyser.ExtractionContext!.MissingNamespaces.Count());
+
+                        // If extracting a base database, we need to create an empty metadata file.
+                        if (EnvironmentVariables.GetBaseMetaDataOutPath() is string baseMetaDataOutPath)
+                        {
+                            try
+                            {
+                                analyser.Logger.LogInfo($"Creating base metadata file at {baseMetaDataOutPath}");
+                                File.WriteAllText(baseMetaDataOutPath, string.Empty);
+                            }
+                            catch (Exception ex)
+                            {
+                                analyser.Logger.LogError($"Failed to create base metadata file: {ex.Message}");
+                            }
+                        }
                     });
             }
             finally

csharp/extractor/Semmle.Util/EnvironmentVariables.cs

@@ -67,5 +67,14 @@ public static IEnumerable<string> GetURLs(string name)
         {
             return Environment.GetEnvironmentVariable("CODEQL_EXTRACTOR_CSHARP_OVERLAY_CHANGES");
         }
+
+        /// <summary>
+        /// If the environment variable is set, the extractor is being called to extract a base database.
+        /// Its value will be a path, and the extractor must create either a file or directory at that location.
+        /// </summary>
+        public static string? GetBaseMetaDataOutPath()
+        {
+            return Environment.GetEnvironmentVariable("CODEQL_EXTRACTOR_CSHARP_OVERLAY_BASE_METADATA_OUT");
+        }
     }
 }