Replace getEncodedFile with shared getFileBySourceArchiveName predicate

While also making it work with paths for databases created on Windows.

python/ql/src/analysis/DefinitionTracking.qll

@@ -4,6 +4,7 @@
 
 import python
 import semmle.python.pointsto.PointsTo
+import IDEContextual
 
 private newtype TDefinition =
   TLocalDefinition(AstNode a) { a instanceof Expr or a instanceof Stmt or a instanceof Module }
@@ -513,11 +514,3 @@ Definition definitionOf(NiceLocationExpr use, string kind) {
     not result.getLocation().hasLocationInfo(f, l, _, _, _)
   )
 }
-
-/**
- * Returns an appropriately encoded version of a filename `name`
- * passed by the VS Code extension in order to coincide with the
- * output of `.getFile()` on locatable entities.
- */
-cached
-File getEncodedFile(string name) { result.getAbsolutePath().replaceAll(":", "_") = name }

python/ql/src/analysis/IDEContextual.qll

@@ -0,0 +1,22 @@
+/*
+ * Provides shared predicates related to contextual queries in the code viewer.
+ */
+
+import semmle.files.FileSystem
+
+/**
+ * Returns the `File` matching the given source file name as encoded by the VS
+ * Code extension.
+ */
+cached
+File getFileBySourceArchiveName(string name) {
+  // The name provided for a file in the source archive by the VS Code extension
+  // has some differences from the absolute path in the database:
+  // 1. colons are replaced by underscores
+  // 2. there's a leading slash, even for Windows paths: "C:/foo/bar" ->
+  //    "/C_/foo/bar"
+  // 3. double slashes in UNC prefixes are replaced with a single slash
+  // We can handle 2 and 3 together by unconditionally adding a leading slash
+  // before replacing double slashes.
+  name = ("/" + result.getAbsolutePath().replaceAll(":", "_")).replaceAll("//", "/")
+}

python/ql/src/analysis/LocalDefinitions.ql

@@ -16,5 +16,5 @@ from NiceLocationExpr use, Definition defn, string kind, string f
 where
   defn = definitionOf(use, kind) and
   use.hasLocationInfo(f, _, _, _, _) and
-  getEncodedFile(selectedSourceFile()).getAbsolutePath() = f
+  getFileBySourceArchiveName(selectedSourceFile()).getAbsolutePath() = f
 select use, defn, kind

python/ql/src/analysis/LocalReferences.ql

@@ -15,5 +15,5 @@ external string selectedSourceFile();
 from NiceLocationExpr use, Definition defn, string kind
 where
   defn = definitionOf(use, kind) and
-  defn.getLocation().getFile() = getEncodedFile(selectedSourceFile())
+  defn.getLocation().getFile() = getFileBySourceArchiveName(selectedSourceFile())
 select use, defn, kind