ENH: Create gzip header deterministically by default

nibabel/openers.py

@@ -42,7 +42,24 @@
     HAVE_INDEXED_GZIP = False
 
 
-def _gzip_open(filename, mode='rb', compresslevel=9, keep_open=False):
+class DeterministicGzipFile(gzip.GzipFile):
+    """ Deterministic variant of GzipFile
+
+    This writer does not add filename information to the header, and defaults
+    to a modification time (``mtime``) of 0 seconds.
+    """
+    def __init__(self, filename=None, mode=None, compresslevel=9, fileobj=None, mtime=0):
+        # These two guards are copied from
+        # https://github.com/python/cpython/blob/6ab65c6/Lib/gzip.py#L171-L174
+        if mode and 'b' not in mode:
+            mode += 'b'
+        if fileobj is None:
+            fileobj = self.myfileobj = open(filename, mode or 'rb')
+        return super().__init__(filename="", mode=mode, compresslevel=compresslevel,
+                                fileobj=fileobj, mtime=mtime)
+
+
+def _gzip_open(filename, mode='rb', compresslevel=9, mtime=0, keep_open=False):
 
     # use indexed_gzip if possible for faster read access.  If keep_open ==
     # True, we tell IndexedGzipFile to keep the file handle open. Otherwise
@@ -52,7 +69,7 @@ def _gzip_open(filename, mode='rb', compresslevel=9, keep_open=False):
 
     # Fall-back to built-in GzipFile
     else:
-        gzip_file = gzip.GzipFile(filename, mode, compresslevel)
+        gzip_file = DeterministicGzipFile(filename, mode, compresslevel, mtime=mtime)
 
     return gzip_file
 
@@ -83,7 +100,7 @@ class Opener(object):
         passed to opening method when `fileish` is str.  Change of defaults as
         for \*args
     """
-    gz_def = (_gzip_open, ('mode', 'compresslevel', 'keep_open'))
+    gz_def = (_gzip_open, ('mode', 'compresslevel', 'mtime', 'keep_open'))
     bz2_def = (BZ2File, ('mode', 'buffering', 'compresslevel'))
     zstd_def = (_zstd_open, ('mode', 'level_or_option', 'zstd_dict'))
     compress_ext_map = {
@@ -163,10 +180,7 @@ def name(self):
         self._name will be None if object was created with a fileobj, otherwise
         it will be the filename.
         """
-        try:
-            return self.fobj.name
-        except AttributeError:
-            return self._name
+        return self._name
 
     @property
     def mode(self):

nibabel/tests/test_openers.py

@@ -12,12 +12,15 @@
 from gzip import GzipFile
 from io import BytesIO, UnsupportedOperation
 from distutils.version import StrictVersion
+import hashlib
+import time
 
 from numpy.compat.py3k import asstr, asbytes
 from ..openers import (Opener,
                        ImageOpener,
                        HAVE_INDEXED_GZIP,
                        BZ2File,
+                       DeterministicGzipFile,
                        )
 from ..tmpdirs import InTemporaryDirectory
 from ..volumeutils import BinOpener
@@ -367,3 +370,126 @@ def test_iter():
         lobj = Opener(Lunk(''))
         with pytest.raises(TypeError):
             list(lobj)
+
+
+def md5sum(fname):
+    with open(fname, "rb") as fobj:
+        return hashlib.md5(fobj.read()).hexdigest()
+
+
+def test_DeterministicGzipFile():
+    with InTemporaryDirectory():
+        msg = b"Hello, I'd like to have an argument."
+
+        # No filename, no mtime
+        with open("ref.gz", "wb") as fobj:
+            with GzipFile(filename="", mode="wb", fileobj=fobj, mtime=0) as gzobj:
+                gzobj.write(msg)
+        anon_chksum = md5sum("ref.gz")
+
+        with DeterministicGzipFile("default.gz", "wb") as fobj:
+            internal_fobj = fobj.myfileobj
+            fobj.write(msg)
+        # Check that myfileobj is being closed by GzipFile.close()
+        # This is in case GzipFile changes its internal implementation
+        assert internal_fobj.closed
+
+        assert md5sum("default.gz") == anon_chksum
+
+        # No filename, current mtime
+        now = time.time()
+        with open("ref.gz", "wb") as fobj:
+            with GzipFile(filename="", mode="wb", fileobj=fobj, mtime=now) as gzobj:
+                gzobj.write(msg)
+        now_chksum = md5sum("ref.gz")
+
+        with DeterministicGzipFile("now.gz", "wb", mtime=now) as fobj:
+            fobj.write(msg)
+
+        assert md5sum("now.gz") == now_chksum
+
+        # Change in default behavior
+        with mock.patch("time.time") as t:
+            t.return_value = now
+
+            # GzipFile will use time.time()
+            with open("ref.gz", "wb") as fobj:
+                with GzipFile(filename="", mode="wb", fileobj=fobj) as gzobj:
+                    gzobj.write(msg)
+            assert md5sum("ref.gz") == now_chksum
+
+            # DeterministicGzipFile will use 0
+            with DeterministicGzipFile("now.gz", "wb") as fobj:
+                fobj.write(msg)
+            assert md5sum("now.gz") == anon_chksum
+
+        # GzipFile is filename dependent, DeterministicGzipFile is independent
+        with GzipFile("filenameA.gz", mode="wb", mtime=0) as gzobj:
+            gzobj.write(msg)
+        fnameA_chksum = md5sum("filenameA.gz")
+        assert fnameA_chksum != anon_chksum
+
+        with DeterministicGzipFile("filenameA.gz", "wb") as fobj:
+            fobj.write(msg)
+
+        # But the contents are the same with different filenames
+        assert md5sum("filenameA.gz") == anon_chksum
+
+
+def test_DeterministicGzipFile_fileobj():
+    with InTemporaryDirectory():
+        msg = b"Hello, I'd like to have an argument."
+        with open("ref.gz", "wb") as fobj:
+            with GzipFile(filename="", mode="wb", fileobj=fobj, mtime=0) as gzobj:
+                gzobj.write(msg)
+        ref_chksum = md5sum("ref.gz")
+
+        with open("test.gz", "wb") as fobj:
+            with DeterministicGzipFile(filename="", mode="wb", fileobj=fobj) as gzobj:
+                gzobj.write(msg)
+        md5sum("test.gz") == ref_chksum
+
+        with open("test.gz", "wb") as fobj:
+            with DeterministicGzipFile(fileobj=fobj, mode="wb") as gzobj:
+                gzobj.write(msg)
+        md5sum("test.gz") == ref_chksum
+
+        with open("test.gz", "wb") as fobj:
+            with DeterministicGzipFile(filename="test.gz", mode="wb", fileobj=fobj) as gzobj:
+                gzobj.write(msg)
+        md5sum("test.gz") == ref_chksum
+
+
+def test_bitwise_determinism():
+    with InTemporaryDirectory():
+        msg = b"Hello, I'd like to have an argument."
+        # Canonical reference: No filename, no mtime
+        # Use default compresslevel
+        with open("ref.gz", "wb") as fobj:
+            with GzipFile(filename="", mode="wb",
+                          compresslevel=1, fileobj=fobj,
+                          mtime=0) as gzobj:
+                gzobj.write(msg)
+        anon_chksum = md5sum("ref.gz")
+
+        # Different times, different filenames
+        now = time.time()
+        with mock.patch("time.time") as t:
+            t.return_value = now
+            with Opener("a.gz", "wb") as fobj:
+                fobj.write(msg)
+            t.return_value = now + 1
+            with Opener("b.gz", "wb") as fobj:
+                fobj.write(msg)
+
+        assert md5sum("a.gz") == anon_chksum
+        assert md5sum("b.gz") == anon_chksum
+
+        # Users can still set mtime, but filenames will not be embedded
+        with Opener("filenameA.gz", "wb", mtime=0xCAFE10C0) as fobj:
+            fobj.write(msg)
+        with Opener("filenameB.gz", "wb", mtime=0xCAFE10C0) as fobj:
+            fobj.write(msg)
+        fnameA_chksum = md5sum("filenameA.gz")
+        fnameB_chksum = md5sum("filenameB.gz")
+        assert fnameA_chksum == fnameB_chksum != anon_chksum