Silence CRC check during seeking

Fixes: #73

rarfile.py

@@ -2240,6 +2240,7 @@ class RarExtFile(io.RawIOBase):
     _remain = 0
     _returncode = 0
     _md_context = None
+    _seeking = False
 
     def _open_extfile(self, parser, inf):
         self.name = inf.filename
@@ -2248,7 +2249,10 @@ def _open_extfile(self, parser, inf):
 
         if self._fd:
             self._fd.close()
-        md_class = self._inf._md_class or NoHashContext
+        if self._seeking:
+            md_class = NoHashContext
+        else:
+            md_class = self._inf._md_class or NoHashContext
         self._md_context = md_class()
         self._fd = None
         self._remain = self._inf.file_size
@@ -2339,7 +2343,9 @@ def seek(self, offset, whence=0):
         """
 
         # disable crc check when seeking
-        self._md_context = NoHashContext()
+        if not self._seeking:
+            self._md_context = NoHashContext()
+            self._seeking = True
 
         fsize = self._inf.file_size
         cur_ofs = self.tell()

test/test_seek.py

@@ -104,3 +104,31 @@ def test_seek_ioopen():
     with io.open(ARC, "rb") as f:
         run_arc(f, "io.open")
 
+
+def run_seek_middle(fn, entry):
+    rar = rarfile.RarFile(fn)
+    file = rar.open(entry)
+    assert file.read(1) == b"0"
+    assert file.read(1) == b"0"
+    assert file.read(1) == b"0"
+    assert file.read(1) == b"\n"
+    file.read()
+
+    file.seek(0)
+    assert file.read(1) == b"0"
+    assert file.read(1) == b"0"
+    assert file.read(1) == b"0"
+    assert file.read(1) == b"\n"
+    file.read()
+
+    file.seek(2)
+    assert file.read(1) == b"0"
+    assert file.read(1) == b"\n"
+    file.read()
+
+def test_seek_middle1():
+    run_seek_middle("test/files/seektest.rar", "stest1.txt")
+
+def test_seek_middle2():
+    run_seek_middle("test/files/seektest.rar", "stest2.txt")
+