string_decoder: reset decoder on end

This resets the StringDecoder's state after calling `#end`. Further
writes to the decoder will act as if it were a brand new instance,
allowing simple reuse.

PR-URL: https://github.com/nodejs/node/pull/18494
Fixes: https://github.com/nodejs/node/issues/16564
Refs: https://github.com/nodejs/node/pull/16594
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

1 files changed, 12 insertions, 3 deletions

diff --git a/lib/string_decoder.js b/lib/string_decoder.js
index 891ab5bdba..1e569ba6b2 100644
--- a/lib/string_decoder.js
+++ b/lib/string_decoder.js
@@ -210,8 +210,11 @@ function utf8Text(buf, i) {
 // character.
 function utf8End(buf) {
   const r = (buf && buf.length ? this.write(buf) : '');
-  if (this.lastNeed)
+  if (this.lastNeed) {
+    this.lastNeed = 0;
+    this.lastTotal = 0;
     return r + '\ufffd';
+  }
   return r;
 }
 
@@ -246,6 +249,8 @@ function utf16End(buf) {
   const r = (buf && buf.length ? this.write(buf) : '');
   if (this.lastNeed) {
     const end = this.lastTotal - this.lastNeed;
+    this.lastNeed = 0;
+    this.lastTotal = 0;
     return r + this.lastChar.toString('utf16le', 0, end);
   }
   return r;
@@ -269,8 +274,12 @@ function base64Text(buf, i) {
 
 function base64End(buf) {
   const r = (buf && buf.length ? this.write(buf) : '');
-  if (this.lastNeed)
-    return r + this.lastChar.toString('base64', 0, 3 - this.lastNeed);
+  if (this.lastNeed) {
+    const end = 3 - this.lastNeed;
+    this.lastNeed = 0;
+    this.lastTotal = 0;
+    return r + this.lastChar.toString('base64', 0, end);
+  }
   return r;
 }