repl: disable blocking completions by default

It’s not okay for the REPL to be blocked for multiple seconds after
entering `require('` because the completion is performing blocking
fs operations on potentially huge directories. Turning the REPL
completion function asynchronous would be the right thing to do here,
but unfortunately the way the code is structured doesn’t play well
with that (in particular, it breaks the preview feature).
Therefore, disable these blocking calls by default.

Refs: #33282 (comment)

PR-URL: #36564
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

Author: addaleax

lib/repl.js

@@ -298,6 +298,7 @@ function REPLServer(prompt,
     configurable: true
   });
 
+  this.allowBlockingCompletions = !!options.allowBlockingCompletions;
   this.useColors = !!options.useColors;
   this._domain = options.domain || domain.create();
   this.useGlobal = !!useGlobal;
@@ -1204,7 +1205,8 @@ function complete(line, callback) {
     if (completeOn.length) {
       filter = completeOn;
     }
-  } else if (RegExpPrototypeTest(requireRE, line)) {
+  } else if (RegExpPrototypeTest(requireRE, line) &&
+             this.allowBlockingCompletions) {
     // require('...<Tab>')
     const extensions = ObjectKeys(this.context.require.extensions);
     const indexes = ArrayPrototypeMap(extensions,
@@ -1265,7 +1267,8 @@ function complete(line, callback) {
     if (!subdir) {
       ArrayPrototypePush(completionGroups, _builtinLibs);
     }
-  } else if (RegExpPrototypeTest(fsAutoCompleteRE, line)) {
+  } else if (RegExpPrototypeTest(fsAutoCompleteRE, line) &&
+             this.allowBlockingCompletions) {
     [completionGroups, completeOn] = completeFSFunctions(line);
   // Handle variable member lookup.
   // We support simple chained expressions like the following (no function

test/parallel/test-repl-tab-complete.js

@@ -51,7 +51,12 @@ function getNoResultsFunction() {
 
 const works = [['inner.one'], 'inner.o'];
 const putIn = new ArrayStream();
-const testMe = repl.start('', putIn);
+const testMe = repl.start({
+  prompt: '',
+  input: putIn,
+  output: process.stdout,
+  allowBlockingCompletions: true
+});
 
 // Some errors are passed to the domain, but do not callback
 testMe._domain.on('error', assert.ifError);