property handle missing config files in external projects (#12094)

src/harness/unittests/tsserverProjectSystem.ts

@@ -2249,6 +2249,27 @@ namespace ts.projectSystem {
             assert.equal(diags.length, 0);
         });
 
+        it("should property handle missing config files", () => {
+            const f1 = {
+                path: "/a/b/app.ts",
+                content: "let x = 1"
+            };
+            const config = {
+                path: "/a/b/tsconfig.json",
+                content: "{}"
+            };
+            const projectName = "project1";
+            const host = createServerHost([f1]);
+            const projectService = createProjectService(host);
+            projectService.openExternalProject({ rootFiles: toExternalFiles([f1.path, config.path]), options: {}, projectFileName: projectName });
+
+            // should have one external project since config file is missing
+            projectService.checkNumberOfProjects({ externalProjects: 1 });
+
+            host.reloadFS([f1, config]);
+            projectService.openExternalProject({ rootFiles: toExternalFiles([f1.path, config.path]), options: {}, projectFileName: projectName });
+            projectService.checkNumberOfProjects({ configuredProjects: 1 });
+        });
     });
 
     describe("add the missing module file for inferred project", () => {

src/server/editorServices.ts

@@ -1288,7 +1288,9 @@ namespace ts.server {
             for (const file of proj.rootFiles) {
                 const normalized = toNormalizedPath(file.fileName);
                 if (getBaseFileName(normalized) === "tsconfig.json") {
-                    (tsConfigFiles || (tsConfigFiles = [])).push(normalized);
+                    if (this.host.fileExists(normalized)) {
+                        (tsConfigFiles || (tsConfigFiles = [])).push(normalized);
+                    }
                 }
                 else {
                     rootFiles.push(file);