Fix nil pointer dereference in snapshotFS.GetFile for non-existent files

internal/project/snapshot_test.go

@@ -101,4 +101,23 @@ func TestSnapshot(t *testing.T) {
 		assert.Check(t, snapshotAfter.fs.diskFiles["/home/projects/ts/p1/a.ts"] == nil)
 		assert.Check(t, snapshotAfter.fs.diskFiles["/home/projects/ts/p2/b.ts"] != nil)
 	})
+
+	t.Run("GetFile returns nil for non-existent files", func(t *testing.T) {
+		t.Parallel()
+		files := map[string]any{
+			"/home/projects/TS/p1/tsconfig.json": "{}",
+			"/home/projects/TS/p1/index.ts":      "console.log('Hello, world!');",
+		}
+		session := setup(files)
+		session.DidOpenFile(context.Background(), "file:///home/projects/TS/p1/index.ts", 1, files["/home/projects/TS/p1/index.ts"].(string), lsproto.LanguageKindTypeScript)
+		snapshot, release := session.Snapshot()
+		defer release()
+
+		handle := snapshot.GetFile("/home/projects/TS/p1/nonexistent.ts")
+		assert.Check(t, handle == nil, "GetFile should return nil for non-existent file")
+
+		// Test that ReadFile returns false for non-existent file
+		_, ok := snapshot.ReadFile("/home/projects/TS/p1/nonexistent.ts")
+		assert.Check(t, !ok, "ReadFile should return false for non-existent file")
+	})
 }

internal/project/snapshotfs.go

@@ -30,7 +30,7 @@ type snapshotFS struct {
 	readFiles collections.SyncMap[tspath.Path, memoizedDiskFile]
 }
 
-type memoizedDiskFile func() *diskFile
+type memoizedDiskFile func() FileHandle
 
 func (s *snapshotFS) FS() vfs.FS {
 	return s.fs
@@ -43,16 +43,14 @@ func (s *snapshotFS) GetFile(fileName string) FileHandle {
 	if file, ok := s.diskFiles[s.toPath(fileName)]; ok {
 		return file
 	}
-	newEntry := memoizedDiskFile(sync.OnceValue(func() *diskFile {
+	newEntry := memoizedDiskFile(sync.OnceValue(func() FileHandle {
 		if contents, ok := s.fs.ReadFile(fileName); ok {
 			return newDiskFile(fileName, contents)
 		}
 		return nil
 	}))
-	if entry, ok := s.readFiles.LoadOrStore(s.toPath(fileName), newEntry); ok {
-		return entry()
-	}
-	return nil
+	entry, _ := s.readFiles.LoadOrStore(s.toPath(fileName), newEntry)
+	return entry()
 }
 
 type snapshotFSBuilder struct {