Stop invalidating module resolution cache when saving an open file

src/compiler/resolutionCache.ts

@@ -54,6 +54,7 @@ namespace ts {
         writeLog(s: string): void;
         maxNumberOfFilesToIterateForInvalidation?: number;
         getCurrentProgram(): Program | undefined;
+        fileIsOpen(filePath: Path): boolean;
     }
 
     interface DirectoryWatchesOfFailedLookup {
@@ -713,6 +714,10 @@ namespace ts {
                     if (!isPathWithDefaultFailedLookupExtension(fileOrDirectoryPath) && !customFailedLookupPaths.has(fileOrDirectoryPath)) {
                         return false;
                     }
+                    // prevent saving an open file from over-eagerly triggering invalidation
+                    if (resolutionHost.fileIsOpen(fileOrDirectoryPath)) {
+                        return false;
+                    }
                     // Ignore emits from the program
                     if (isEmittedFileOfProgram(resolutionHost.getCurrentProgram(), fileOrDirectoryPath)) {
                         return false;

src/compiler/watch.ts

@@ -691,6 +691,7 @@ namespace ts {
             hasChangedAutomaticTypeDirectiveNames = true;
             scheduleProgramUpdate();
         };
+        compilerHost.fileIsOpen = returnFalse;
         compilerHost.maxNumberOfFilesToIterateForInvalidation = host.maxNumberOfFilesToIterateForInvalidation;
         compilerHost.getCurrentProgram = getCurrentProgram;
         compilerHost.writeLog = writeLog;

src/server/project.ts

@@ -457,6 +457,11 @@ namespace ts.server {
             return this.getTypeAcquisition().enable ? this.projectService.typingsInstaller.globalTypingsCacheLocation : undefined;
         }
 
+        /*@internal*/
+        fileIsOpen(filePath: Path) {
+            return this.projectService.openFiles.has(filePath);
+        }
+
         /*@internal*/
         writeLog(s: string) {
             this.projectService.logger.info(s);

src/testRunner/unittests/tsserver/resolutionCache.ts

@@ -975,5 +975,33 @@ export const x = 10;`
                 host.checkTimeoutQueueLength(0);
             });
         });
+
+        describe("avoid unnecessary invalidation", () => {
+            it("failed lookup invalidation", () => {
+                const expectedNonRelativeDirectories = [`${projectLocation}/node_modules`, `${projectLocation}/src`];
+                const module1Name = "module1";
+                const module2Name = "module2";
+                const fileContent = `import { module1 } from "${module1Name}";import { module2 } from "${module2Name}";`;
+                const file1: File = {
+                    path: `${projectLocation}/src/file1.ts`,
+                    content: fileContent
+                };
+                const { module1, module2 } = getModules(`${projectLocation}/src/node_modules/module1/index.ts`, `${projectLocation}/node_modules/module2/index.ts`);
+                const files = [module1, module2, file1, configFile, libFile];
+                const host = createServerHost(files);
+                const resolutionTrace = createHostModuleResolutionTrace(host);
+                const service = createProjectService(host);
+                service.openClientFile(file1.path);
+                const project = service.configuredProjects.get(configFile.path)!;
+                (project as ResolutionCacheHost).maxNumberOfFilesToIterateForInvalidation = 1;
+                const expectedTrace = getExpectedNonRelativeModuleResolutionTrace(host, file1, module1, module1Name);
+                getExpectedNonRelativeModuleResolutionTrace(host, file1, module2, module2Name, expectedTrace);
+                verifyTrace(resolutionTrace, expectedTrace);
+                verifyWatchesWithConfigFile(host, files, file1, expectedNonRelativeDirectories);
+
+                host.invokeWatchedDirectoriesRecursiveCallback(projectLocation + "/src", "file1.ts");
+                host.checkTimeoutQueueLengthAndRun(0);
+            });
+        });
     });
 }