Fix tell() and forward seek() calls on unseekable IGzipFile inputs

CHANGELOG.rst

@@ -7,6 +7,13 @@ Changelog
 .. This document is user facing. Please word the changes in such a way
 .. that users understand how the changes affect the new version.
 
+version 1.10.0-dev
+------------------
++ Fix ``tell()`` and forward ``seek()`` on ``igzip.IGzipFile`` objects that
+  read from an unseekable file object, such as a pipe or a socket. Versions
+  1.4.0 to 1.9.1 raised ``io.UnsupportedOperation`` for these calls.
+
+
 version 1.9.1
 -----------------
 + Fix BrokenPipeError when piping ``python -m isal.igzip`` into ``head``,

src/isal/isal_zlibmodule.c

@@ -2269,7 +2269,9 @@ GzipReader_writable(GzipReader *self, PyObject *Py_UNUSED(ignore))
 
 static PyObject *
 GzipReader_seekable(GzipReader *self, PyObject *Py_UNUSED(ignore)) {
-    return PyObject_CallMethod(self->fp, "seekable", NULL);
+    /* Same as gzip._PaddedFile.seekable(): seek() moves forward by reading,
+       so tell() and forward seeks also work on unseekable streams. */
+    Py_RETURN_TRUE;
 }
 
 static PyObject *

tests/test_igzip.py

@@ -688,6 +688,32 @@ def test_seek(tmp_path):
         assert gzip_file.read(1) == b"C"
 
 
+def test_tell_and_seek_forward_on_unseekable_file():
+    class UnseekableFile(io.RawIOBase):
+        # Like a pipe or a socket: seekable() is False and seek() fails.
+        def __init__(self, data):
+            self.data = io.BytesIO(data)
+
+        def readable(self):
+            return True
+
+        def readinto(self, buffer):
+            return self.data.readinto(buffer)
+
+    data = bytes(range(256)) * 1000
+    fileobj = UnseekableFile(gzip.compress(data))
+    with igzip.IGzipFile(fileobj=fileobj) as gzip_file:
+        assert gzip_file.read(10) == data[:10]
+        assert gzip_file.tell() == 10
+        assert gzip_file.seek(200_000) == 200_000
+        assert gzip_file.read(10) == data[200_000:200_010]
+        assert gzip_file.seek(5, io.SEEK_CUR) == 200_015
+        assert gzip_file.read(10) == data[200_015:200_025]
+        # Rewinding needs fileobj.seek(), as in gzip.
+        with pytest.raises(io.UnsupportedOperation):
+            gzip_file.seek(0)
+
+
 def test_bgzip():
     bgzip_file = Path(__file__).parent / "data" / "test.fastq.bgzip.gz"
     gzip_file = Path(__file__).parent / "data" / "test.fastq.gz"