repl: keep entries added while history file is loading

lib/internal/repl/history.js

@@ -4,6 +4,7 @@ const {
   ArrayPrototypeIndexOf,
   ArrayPrototypeJoin,
   ArrayPrototypePop,
+  ArrayPrototypePushApply,
   ArrayPrototypeShift,
   ArrayPrototypeSplice,
   ArrayPrototypeUnshift,
@@ -302,10 +303,21 @@ class ReplHistory {
         return this[kHandleHistoryInitError](err, onReadyCallback);
       }
 
-      if (data) {
-        this[kHistory] = RegExpPrototypeSymbolSplit(/\r?\n+/, data, this[kSize]);
+      const loadedHistory = data ?
+        RegExpPrototypeSymbolSplit(/\r?\n+/, data, this[kSize]) :
+        [];
+
+      // Lines can be evaluated while the history file is still being read,
+      // e.g. when the input stream does not support pausing. Such entries
+      // are already in the in-memory history (newest first), so append the
+      // persisted entries to them instead of discarding them.
+      if (this[kHistory].length > 0) {
+        ArrayPrototypePushApply(this[kHistory], loadedHistory);
+        if (this[kHistory].length > this[kSize]) {
+          ArrayPrototypeSplice(this[kHistory], this[kSize]);
+        }
       } else {
-        this[kHistory] = [];
+        this[kHistory] = loadedHistory;
       }
 
       validateArray(this[kHistory], 'history');

test/parallel/test-repl-history-load-preserves-pending-entries.js

@@ -0,0 +1,62 @@
+'use strict';
+
+// Lines can be evaluated while the history file is still being loaded
+// asynchronously by `setupHistory()`, e.g. when the input stream does not
+// support pausing. Entries added to the in-memory history in the meantime
+// must not be discarded once the file load completes.
+// Refs: https://github.com/nodejs/node/issues/64508
+
+const common = require('../common');
+const assert = require('assert');
+const fs = require('fs');
+const stream = require('stream');
+const repl = require('repl');
+
+if (process.env.TERM === 'dumb') {
+  common.skip('skipping - dumb terminal');
+}
+
+const tmpdir = require('../common/tmpdir');
+tmpdir.refresh();
+
+const historyPath = tmpdir.resolve('.repl_history');
+fs.writeFileSync(historyPath, 'persisted entry');
+
+// An input stream that, unlike a TTY, does not buffer data while paused.
+class FakeInput extends stream.Stream {
+  resume() {}
+  pause() {}
+}
+FakeInput.prototype.readable = true;
+
+const input = new FakeInput();
+const output = new stream.Writable({
+  write(chunk, encoding, callback) {
+    callback();
+  },
+});
+
+const r = repl.start({
+  input,
+  output,
+  prompt: '',
+  terminal: true,
+  useColors: false,
+});
+
+r.setupHistory(historyPath, common.mustSucceed(() => {
+  // The lines evaluated while the history file was being read must be kept,
+  // newest first, followed by the persisted entries.
+  assert.deepStrictEqual(
+    r.history,
+    ['const b = 2', 'const a = 1', 'persisted entry'],
+  );
+  assert.strictEqual(
+    fs.readFileSync(historyPath, 'utf8'),
+    'const b = 2\nconst a = 1\npersisted entry',
+  );
+  r.close();
+}));
+
+// Evaluated synchronously, before the history file has been read.
+input.emit('data', 'const a = 1\nconst b = 2\n');