Fix Reader.peek() IndexError when index exceeds buffer length

lib/yaml/reader.py

@@ -89,7 +89,9 @@ def peek(self, index=0):
             return self.buffer[self.pointer+index]
         except IndexError:
             self.update(index+1)
-            return self.buffer[self.pointer+index]
+            if self.pointer+index < len(self.buffer):
+                return self.buffer[self.pointer+index]
+            return '\0'
 
     def prefix(self, length=1):
         if self.pointer+length >= len(self.buffer):

tests/test_reader_peek.py

@@ -0,0 +1,56 @@
+import io
+
+import yaml
+import yaml.reader
+
+
+class TestReaderPeekBeyondBuffer:
+    """Tests for Reader.peek() when index exceeds available data.
+
+    Regression test for https://github.com/yaml/pyyaml/issues/904
+    """
+
+    def test_peek_beyond_end_returns_null(self):
+        """peek() past end of a string stream should return '\\0', not raise IndexError."""
+        reader = yaml.reader.Reader('abc')
+        assert reader.peek(4) == '\0'
+
+    def test_peek_at_null_terminator(self):
+        """peek() at the exact position of the null terminator should return '\\0'."""
+        reader = yaml.reader.Reader('abc')
+        # buffer is 'abc\0', so index 3 is the '\0'
+        assert reader.peek(3) == '\0'
+
+    def test_peek_far_beyond_end(self):
+        """peek() with a very large index should return '\\0'."""
+        reader = yaml.reader.Reader('abc')
+        assert reader.peek(100) == '\0'
+
+    def test_peek_within_range(self):
+        """peek() within range should still return the correct character."""
+        reader = yaml.reader.Reader('abc')
+        assert reader.peek(0) == 'a'
+        assert reader.peek(1) == 'b'
+        assert reader.peek(2) == 'c'
+
+    def test_peek_empty_string(self):
+        """peek() on an empty string should return '\\0'."""
+        reader = yaml.reader.Reader('')
+        assert reader.peek(0) == '\0'
+        assert reader.peek(1) == '\0'
+
+    def test_peek_beyond_end_bytes(self):
+        """peek() past end of a bytes stream should return '\\0'."""
+        reader = yaml.reader.Reader(b'abc')
+        assert reader.peek(4) == '\0'
+
+    def test_peek_beyond_end_file_stream(self):
+        """peek() past end of a file-like stream should return '\\0'."""
+        reader = yaml.reader.Reader(io.StringIO('abc'))
+        assert reader.peek(4) == '\0'
+
+    def test_loader_peek_beyond_end(self):
+        """Original reproducer from issue #904."""
+        obj = yaml.loader.Loader('abc')
+        ret = obj.peek(4)
+        assert ret == '\0'