buffer: truncate buffer after string decode

When our estimates for a storage size are higher than the actual length
of decoded data, the destination buffer should be truncated. Otherwise
`Buffer::Length` will give misleading information to C++ layer.

fix #7365

Signed-off-by: Fedor Indutny <fedor@indutny.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/lib/buffer.js b/lib/buffer.js
index ab16cd8ad7..91c13521ac 100644
--- a/lib/buffer.js
+++ b/lib/buffer.js
@@ -23,6 +23,7 @@ var buffer = process.binding('buffer');
 var smalloc = process.binding('smalloc');
 var util = require('util');
 var alloc = smalloc.alloc;
+var truncate = smalloc.truncate;
 var sliceOnto = smalloc.sliceOnto;
 var kMaxLength = smalloc.kMaxLength;
 var internal = {};
@@ -79,7 +80,13 @@ function Buffer(subject, encoding) {
       // In the case of base64 it's possible that the size of the buffer
       // allocated was slightly too large. In this case we need to rewrite
       // the length to the actual length written.
-      this.length = this.write(subject, encoding);
+      var len = this.write(subject, encoding);
+
+      // Buffer was truncated after decode, realloc internal ExternalArray
+      if (len !== this.length) {
+        this.length = len;
+        truncate(this, this.length);
+      }
     } else {
       if (util.isBuffer(subject))
         subject.copy(this, 0, 0, this.length);