From 190aa0535e13be89c06d859a0ea2a326b3d46e6c Mon Sep 17 00:00:00 2001
From: Hector <hector.dearman@sentry.io>
Date: Fri, 27 Feb 2026 09:36:35 +0000
Subject: [PATCH] Add seek() method to ZstdCompressionReader

Even though ZstdCompressionReader doesn't support seek()ing we should
probably follow the interface of:
https://docs.python.org/3/library/io.html#io.IOBase.seek
and define a seek method which raises OSError.
---
 c-ext/compressionreader.c              |  7 +++++++
 rust-ext/src/compression_reader.rs     |  4 ++++
 tests/test_compressor_stream_reader.py | 14 ++++++++++++++
 zstandard/__init__.pyi                 |  1 +
 zstandard/backend_cffi.py              |  3 +++
 5 files changed, 29 insertions(+)

diff --git a/c-ext/compressionreader.c b/c-ext/compressionreader.c
index b14a63b1..67c4fba9 100644
--- a/c-ext/compressionreader.c
+++ b/c-ext/compressionreader.c
@@ -83,6 +83,11 @@ static PyObject *compressionreader_seekable(ZstdCompressionReader *self) {
     Py_RETURN_FALSE;
 }
 
+static PyObject *compressionreader_seek(PyObject *self, PyObject *args) {
+    PyErr_SetString(PyExc_OSError, "stream is not seekable");
+    return NULL;
+}
+
 static PyObject *compressionreader_readline(PyObject *self, PyObject *args) {
     set_io_unsupported_operation();
     return NULL;
@@ -760,6 +765,8 @@ static PyMethodDef compressionreader_methods[] = {
      PyDoc_STR("Not implemented")},
     {"readlines", (PyCFunction)compressionreader_readlines, METH_VARARGS,
      PyDoc_STR("Not implemented")},
+    {"seek", (PyCFunction)compressionreader_seek, METH_VARARGS,
+     PyDoc_STR("Raises OSError")},
     {"seekable", (PyCFunction)compressionreader_seekable, METH_NOARGS,
      PyDoc_STR("Returns False")},
     {"tell", (PyCFunction)compressionreader_tell, METH_NOARGS,
diff --git a/rust-ext/src/compression_reader.rs b/rust-ext/src/compression_reader.rs
index 47626e34..0f6d85c9 100644
--- a/rust-ext/src/compression_reader.rs
+++ b/rust-ext/src/compression_reader.rs
@@ -169,6 +169,10 @@ impl ZstdCompressionReader {
         false
     }
 
+    fn seek(&self, _data: &Bound<'_, PyAny>) -> PyResult<()> {
+        Err(PyOSError::new_err("stream is not seekable"))
+    }
+
     fn readline(&self, py: Python) -> PyResult<()> {
         let io = py.import("io")?;
         let exc = io.getattr("UnsupportedOperation")?;
diff --git a/tests/test_compressor_stream_reader.py b/tests/test_compressor_stream_reader.py
index 6c1fca21..81cb55bb 100644
--- a/tests/test_compressor_stream_reader.py
+++ b/tests/test_compressor_stream_reader.py
@@ -54,6 +54,20 @@ def test_not_implemented(self):
             with self.assertRaises(OSError):
                 reader.write(b"foo")
 
+    def test_seek(self):
+        cctx = zstd.ZstdCompressor()
+
+        with cctx.stream_reader(b"foo" * 60) as reader:
+            with self.assertRaises(OSError):
+                reader.seek(0)
+
+            reader.read(1)
+            with self.assertRaises(OSError):
+                reader.seek(0)
+
+        with self.assertRaises(OSError):
+            reader.seek(0)
+
     def test_constant_methods(self):
         cctx = zstd.ZstdCompressor()
 
diff --git a/zstandard/__init__.pyi b/zstandard/__init__.pyi
index f341405f..3f7eb22f 100644
--- a/zstandard/__init__.pyi
+++ b/zstandard/__init__.pyi
@@ -216,6 +216,7 @@ class ZstdCompressionReader(BinaryIO):
     def readable(self) -> bool: ...
     def writable(self) -> bool: ...
     def seekable(self) -> bool: ...
+    def seek(self, pos: int, whence: int = ...) -> int: ...
     def readline(self, limit: int = ...) -> bytes: ...
     def readlines(self, hint: int = ...) -> List[bytes]: ...
     def write(self, data: ByteString): ...
diff --git a/zstandard/backend_cffi.py b/zstandard/backend_cffi.py
index 9d830cf1..ee21f2cb 100644
--- a/zstandard/backend_cffi.py
+++ b/zstandard/backend_cffi.py
@@ -1427,6 +1427,9 @@ def writable(self):
     def seekable(self):
         return False
 
+    def seek(self, offset, whence=None):
+        raise io.UnsupportedOperation()
+
     def readline(self):
         raise io.UnsupportedOperation()