Merge pull request github#15325 from RasmusWL/c#-filter-order

C#: Respect order of `LGTM_INDEX_FILTERS` in buildless extraction

Author: RasmusWL

csharp/extractor/Semmle.Extraction.CSharp.DependencyFetching/FilePathFilter.cs

@@ -74,16 +74,18 @@ public IEnumerable<FileInfo> Filter(IEnumerable<FileInfo> files)
                         includeByDefault)
                 });
 
-            // Move included pathfilters to the front of the list:
-            pathFilters.Sort((pf1, pf2) => -1 * pf1.Include.CompareTo(pf2.Include));
             return unfilteredResult.Where(f =>
             {
                 var include = f.FileInclusion.Include;
-                foreach (var pathFilter in pathFilters)
+                // LGTM_INDEX_FILTERS is a prioritized list, where later filters take
+                // priority over earlier ones.
+                for (int i = pathFilters.Count - 1; i >= 0; i--)
                 {
+                    var pathFilter = pathFilters[i];
                     if (pathFilter.Regex.IsMatch(f.FileInclusion.Path))
                     {
                         include = pathFilter.Include;
+                        break;
                     }
                 }
 

csharp/extractor/Semmle.Extraction.Tests/FilePathFilter.cs

@@ -165,6 +165,7 @@ public void TestFiltersWithIncludeExcludeExcludeFirst()
         {
             (var testSubject, var progressMonitor, var files) = TestSetup();
 
+            // NOTE: the ordering DOES matter, later filters takes priority, so the exclude will end up not mattering at all.
             Environment.SetEnvironmentVariable("LGTM_INDEX_FILTERS", """
                 exclude:c/x/z
                 include:c/x
@@ -174,7 +175,8 @@ public void TestFiltersWithIncludeExcludeExcludeFirst()
 
             var expected = GetExpected(
                 [
-                    "/a/b/c/x/y/i.cs"
+                    "/a/b/c/x/y/i.cs",
+                    "/a/b/c/x/z/i.cs"
                 ]);
 
             AssertFileInfoEquivalence(expected, filtered);