perf(compiler-cli): minimize filesystem calls when generating shims (#47682)

clydin · thePunderWoman · commit a792bf1703fa · 2022-10-07T09:10:34.000-07:00
Previously when a file was being analyzed to determine if a shim should be generated, up to two calls to the host `fileExists` function per file per generator were made. In the default host, each `fileExists` call made two underlying file system calls. Following these calls, the file was then read via `getSourceFile`. However, `getSourceFile` will return `undefined` if the requested file does not exist. As a result, `getSourceFile` can be used directly to request both potential file names and leverage the return value to determine if the file does not exist. This avoids the need to call `fileExists` at all. PR Close #47682
diff --git a/packages/compiler-cli/src/ngtsc/shims/src/adapter.ts b/packages/compiler-cli/src/ngtsc/shims/src/adapter.ts
@@ -168,31 +168,26 @@ export class ShimAdapter {
       // This _might_ be a shim, if an underlying base file exists. The base file might be .ts or
       // .tsx.
       let baseFileName = absoluteFrom(prefix + '.ts');
-      if (!this.delegate.fileExists(baseFileName)) {
+      // Retrieve the original file for which the shim will be generated.
+      let inputFile = this.delegate.getSourceFile(baseFileName, ts.ScriptTarget.Latest);
+      if (inputFile === undefined) {
         // No .ts file by that name - try .tsx.
         baseFileName = absoluteFrom(prefix + '.tsx');
-        if (!this.delegate.fileExists(baseFileName)) {
-          // This isn't a shim after all since there is no original file which would have triggered
-          // its generation, even though the path is right. There are a few reasons why this could
-          // occur:
-          //
-          // * when resolving an import to an .ngfactory.d.ts file, the module resolution algorithm
-          //   will first look for an .ngfactory.ts file in its place, which will be requested here.
-          // * when the user writes a bad import.
-          // * when a file is present in one compilation and removed in the next incremental step.
-          //
-          // Note that this does not add the filename to `notShims`, so this path is not cached.
-          // That's okay as these cases above are edge cases and do not occur regularly in normal
-          // operations.
-          return undefined;
-        }
+        inputFile = this.delegate.getSourceFile(baseFileName, ts.ScriptTarget.Latest);
       }
-
-      // Retrieve the original file for which the shim will be generated.
-      const inputFile = this.delegate.getSourceFile(baseFileName, ts.ScriptTarget.Latest);
       if (inputFile === undefined || isShim(inputFile)) {
-        // Something strange happened here. This case is also not cached in `notShims`, but this
-        // path is not expected to occur in reality so this shouldn't be a problem.
+        // This isn't a shim after all since there is no original file which would have triggered
+        // its generation, even though the path is right. There are a few reasons why this could
+        // occur:
+        //
+        // * when resolving an import to an .ngfactory.d.ts file, the module resolution algorithm
+        //   will first look for an .ngfactory.ts file in its place, which will be requested here.
+        // * when the user writes a bad import.
+        // * when a file is present in one compilation and removed in the next incremental step.
+        //
+        // Note that this does not add the filename to `notShims`, so this path is not cached.
+        // That's okay as these cases above are edge cases and do not occur regularly in normal
+        // operations.
         return undefined;
       }
 
diff --git a/packages/compiler-cli/test/perform_watch_spec.ts b/packages/compiler-cli/test/perform_watch_spec.ts
@@ -121,7 +121,6 @@ describe('perform watch', () => {
     expectNoDiagnostics(config.options, host.diagnostics);
 
     expect(fileExistsSpy!).not.toHaveBeenCalledWith(mainTsPath);
-    expect(fileExistsSpy!).toHaveBeenCalledWith(utilTsPath);
     expect(getSourceFileSpy!).not.toHaveBeenCalledWith(mainTsPath, jasmine.anything());
     expect(getSourceFileSpy!).toHaveBeenCalledWith(utilTsPath, jasmine.anything());
 
