repl: display prompt once after error callback

Do not call `.displayPrompt()` twice after the `eval` callback
resulted in an error.

(This does not affect the default eval because it doesn’t use
the callback if an error occurs.)

PR-URL: #38314
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>

Author: addaleax

lib/repl.js

@@ -889,8 +889,11 @@ function REPLServer(prompt,
         self.output.write(self.writer(ret) + '\n');
       }
 
-      // Display prompt again
-      self.displayPrompt();
+      // Display prompt again (unless we already did by emitting the 'error'
+      // event on the domain instance).
+      if (!e) {
+        self.displayPrompt();
+      }
     }
   });
 

test/parallel/test-repl-uncaught-exception-evalcallback.js

@@ -0,0 +1,23 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const repl = require('repl');
+const { PassThrough } = require('stream');
+const input = new PassThrough();
+const output = new PassThrough();
+
+const r = repl.start({
+  input, output,
+  eval: common.mustCall((code, context, filename, cb) => {
+    r.setPrompt('prompt! ');
+    cb(new Error('err'));
+  })
+});
+
+input.end('foo\n');
+
+// The output includes exactly one post-error prompt.
+const out = output.read().toString();
+assert.match(out, /prompt!/);
+assert.doesNotMatch(out, /prompt![\S\s]*prompt!/);
+output.on('data', common.mustNotCall());