sandbox/_mounter: Remove default mutable arguments stderr/out

stderr and stdout were passed as default arguments and would therefore
retain the first value they had when the module was imported, which
means they wouldn't get overriden by the pytest capture of stderr/out.

This also meant that depending how the mounter was imported, if the
stdout/err was patched at that moment, and the file closed, we would
get an error.

The bug can be reproduced by running:

tox -e pyXX -- tests/integration/cmake.py

On master, and seeing that it is now fixed.

1 files changed, 23 insertions, 6 deletions

diff --git a/src/buildstream/sandbox/_mounter.py b/src/buildstream/sandbox/_mounter.py
index e6054c20d..4e31ef67a 100644
--- a/src/buildstream/sandbox/_mounter.py
+++ b/src/buildstream/sandbox/_mounter.py
@@ -28,9 +28,14 @@ from .. import utils, _signals
 class Mounter():
     @classmethod
     def _mount(cls, dest, src=None, mount_type=None,
-               stdout=sys.stdout, stderr=sys.stderr, options=None,
+               stdout=None, stderr=None, options=None,
                flags=None):
 
+        if stdout is None:
+            stdout = sys.stdout
+        if stderr is None:
+            stderr = sys.stderr
+
         argv = [utils.get_host_tool('mount')]
         if mount_type:
             argv.extend(['-t', mount_type])
@@ -57,7 +62,11 @@ class Mounter():
         return dest
 
     @classmethod
-    def _umount(cls, path, stdout=sys.stdout, stderr=sys.stderr):
+    def _umount(cls, path, stdout=None, stderr=None):
+        if stdout is None:
+            stdout = sys.stdout
+        if stderr is None:
+            stderr = sys.stderr
 
         cmd = [utils.get_host_tool('umount'), '-R', path]
         status, _ = utils._call(
@@ -89,8 +98,12 @@ class Mounter():
     #
     @classmethod
     @contextmanager
-    def mount(cls, dest, src=None, stdout=sys.stdout,
-              stderr=sys.stderr, mount_type=None, **kwargs):
+    def mount(cls, dest, src=None, stdout=None,
+              stderr=None, mount_type=None, **kwargs):
+        if stdout is None:
+            stdout = sys.stdout
+        if stderr is None:
+            stderr = sys.stderr
 
         def kill_proc():
             cls._umount(dest, stdout, stderr)
@@ -126,8 +139,12 @@ class Mounter():
     #
     @classmethod
     @contextmanager
-    def bind_mount(cls, dest, src=None, stdout=sys.stdout,
-                   stderr=sys.stderr, **kwargs):
+    def bind_mount(cls, dest, src=None, stdout=None,
+                   stderr=None, **kwargs):
+        if stdout is None:
+            stdout = sys.stdout
+        if stderr is None:
+            stderr = sys.stderr
 
         def kill_proc():
             cls._umount(dest, stdout, stderr)