fix(41736): tsserver should not crash if external file does not exist

src/server/project.ts

@@ -1188,8 +1188,8 @@ namespace ts.server {
                 // by the host for files in the program when the program is retrieved above but
                 // the program doesn't contain external files so this must be done explicitly.
                 inserted => {
-                    const scriptInfo = this.projectService.getOrCreateScriptInfoNotOpenedByClient(inserted, this.currentDirectory, this.directoryStructureHost)!;
-                    scriptInfo.attachToProject(this);
+                    const scriptInfo = this.projectService.getOrCreateScriptInfoNotOpenedByClient(inserted, this.currentDirectory, this.directoryStructureHost);
+                    scriptInfo?.attachToProject(this);
                 },
                 removed => this.detachScriptInfoFromProject(removed)
             );

src/testRunner/unittests/tsserver/externalProjects.ts

@@ -898,5 +898,44 @@ namespace ts.projectSystem {
             const jsConfigProject = service.configuredProjects.get(jsConfig.path.toLowerCase())!;
             checkProjectActualFiles(jsConfigProject, [jsConfig.path, jsFilePath, libFile.path]);
         });
+
+        it("does not crash if external file does not exist", () => {
+            const f1 = {
+                path: "/a/file1.ts",
+                content: "let x = [1, 2];",
+            };
+            const p1 = {
+                projectFileName: "/a/proj1.csproj",
+                rootFiles: [toExternalFile(f1.path)],
+                options: {},
+            };
+
+            const host = createServerHost([f1]);
+            host.require = (_initialPath, moduleName) => {
+                assert.equal(moduleName, "myplugin");
+                return {
+                    module: () => ({
+                        create(info: server.PluginCreateInfo) {
+                            return Harness.LanguageService.makeDefaultProxy(info);
+                        },
+                        getExternalFiles() {
+                            return ["/does/not/exist"];
+                        },
+                    }),
+                    error: undefined,
+                };
+            };
+            const session = createSession(host, {
+                globalPlugins: ["myplugin"],
+            });
+            const projectService = session.getProjectService();
+            // When the external project is opened, the graph will be updated,
+            // and in the process getExternalFiles() above will be called.
+            // Since the external file does not exist, there will not be a script
+            // info for it. If tsserver does not handle this case, the following
+            // method call will crash.
+            projectService.openExternalProject(p1);
+            checkNumberOfProjects(projectService, { externalProjects: 1 });
+        });
     });
 }