Collect complete reverse-reading blocks after short reads

boltons/jsonutils.py

@@ -100,6 +100,15 @@ def reverse_iter_lines(file_obj, blocksize=DEFAULT_BLOCKSIZE, preseek=True, enco
         cur_pos -= read_size
         file_obj.seek(cur_pos, os.SEEK_SET)
         cur = file_obj.read(read_size)
+        if len(cur) < read_size:
+            parts, remaining = [cur], read_size - len(cur)
+            while remaining:
+                part = file_obj.read(remaining)
+                if not part:
+                    break
+                parts.append(part)
+                remaining -= len(part)
+            cur = empty.join(parts)
         buff = cur + buff
         lines = buff.splitlines()
 

tests/test_reverse_short_reads.py

@@ -0,0 +1,33 @@
+import io
+
+import pytest
+
+from boltons.jsonutils import reverse_iter_lines
+
+
+class ShortReader:
+    def __init__(self, stream):
+        self.stream = stream
+
+    def read(self, size=-1):
+        return self.stream.read(min(size, 2) if size >= 0 else 2)
+
+    def seek(self, *args):
+        return self.stream.seek(*args)
+
+    def tell(self):
+        return self.stream.tell()
+
+
+@pytest.mark.parametrize('binary', [False, True])
+@pytest.mark.parametrize('blocksize', [4, 9])
+@pytest.mark.parametrize('trailing', [False, True])
+def test_reverse_lines_collects_short_reads(binary, blocksize, trailing):
+    data = 'first\nsecond\nthird' + ('\n' if trailing else '')
+    expected = ([''] if trailing else []) + ['third', 'second', 'first']
+    if binary:
+        stream = io.BytesIO(data.encode('ascii'))
+        expected = [line.encode('ascii') for line in expected]
+    else:
+        stream = io.StringIO(data)
+    assert list(reverse_iter_lines(ShortReader(stream), blocksize=blocksize)) == expected