Allow root to be null (microsoft#994)

Fixes microsoft#977.

This is a weird case, but did work before my search path cleanups. Revert to the old behavior when the root is set to `null`.

To test, open VS Code and close any folders, make a new file and switch its language mode to Python.

Author: jakebailey

src/LanguageServer/Impl/Implementation/Server.cs

@@ -104,8 +104,10 @@ public async Task<InitializeResult> InitializeAsync(InitializeParams @params, Ca
             _rdt = _services.GetService<IRunningDocumentTable>();
 
             _rootDir = @params.rootUri != null ? @params.rootUri.ToAbsolutePath() : @params.rootPath;
-            _rootDir = PathUtils.NormalizePath(_rootDir);
-            _rootDir = PathUtils.TrimEndSeparator(_rootDir);
+            if (_rootDir != null) {
+                _rootDir = PathUtils.NormalizePath(_rootDir);
+                _rootDir = PathUtils.TrimEndSeparator(_rootDir);
+            }
 
             Version.TryParse(@params.initializationOptions.interpreter.properties?.Version, out var version);
 
@@ -116,14 +118,14 @@ public async Task<InitializeResult> InitializeAsync(InitializeParams @params, Ca
             ) {
                 // 1) Split on ';' to support older VS Code extension versions which send paths as a single entry separated by ';'. TODO: Eventually remove.
                 // 2) Normalize paths.
-                // 3) If a path isn't rooted, then root it relative to the workspace root.
+                // 3) If a path isn't rooted, then root it relative to the workspace root. If _rootDir is null, then accept the path as-is.
                 // 4) Trim off any ending separator for a consistent style.
                 // 5) Filter out any entries which are the same as the workspace root; they are redundant. Also ignore "/" to work around the extension (for now).
                 // 6) Remove duplicates.
                 SearchPaths = @params.initializationOptions.searchPaths
                     .Select(p => p.Split(';', StringSplitOptions.RemoveEmptyEntries)).SelectMany()
                     .Select(PathUtils.NormalizePath)
-                    .Select(p => Path.IsPathRooted(p) ? p : Path.GetFullPath(p, _rootDir))
+                    .Select(p => _rootDir == null || Path.IsPathRooted(p) ? p : Path.GetFullPath(p, _rootDir))
                     .Select(PathUtils.TrimEndSeparator)
                     .Where(p => !string.IsNullOrWhiteSpace(p) && p != "/" && !p.PathEquals(_rootDir))
                     .Distinct(PathEqualityComparer.Instance)

src/LanguageServer/Impl/Indexing/IndexManager.cs

@@ -42,7 +42,6 @@ internal class IndexManager : IIndexManager {
         public IndexManager(IFileSystem fileSystem, PythonLanguageVersion version, string rootPath, string[] includeFiles,
             string[] excludeFiles, IIdleTimeService idleTimeService) {
             Check.ArgumentNotNull(nameof(fileSystem), fileSystem);
-            Check.ArgumentNotNull(nameof(rootPath), rootPath);
             Check.ArgumentNotNull(nameof(includeFiles), includeFiles);
             Check.ArgumentNotNull(nameof(excludeFiles), excludeFiles);
             Check.ArgumentNotNull(nameof(idleTimeService), idleTimeService);