Separate config dirs from temp dirs

Do not store temporary directories in the Dangerzone's config directory.
There are two reasons for that:

1. They are ephemeral, and they need a temporary place to be stored,
   preferably RAM-backed.
2. We need to set them while running our CI tests.

dangerzone/isolation_provider/container.py

@@ -9,10 +9,8 @@ import subprocess
 import tempfile
 from typing import Callable, List, Optional, Tuple
 
-import appdirs
-
 from ..document import Document
-from ..util import get_resource_path, get_subprocess_startupinfo
+from ..util import get_resource_path, get_subprocess_startupinfo, get_tmp_dir
 from .base import IsolationProvider
 
 # Define startupinfo for subprocesses
@@ -225,9 +223,7 @@ class Container(IsolationProvider):
         else:
             ocr = "0"
 
-        dz_tmp = os.path.join(appdirs.user_config_dir("dangerzone"), "tmp")
-        os.makedirs(dz_tmp, exist_ok=True)
-
+        dz_tmp = get_tmp_dir()
         tmpdir = tempfile.TemporaryDirectory(dir=dz_tmp)
         pixel_dir = os.path.join(tmpdir.name, "pixels")
         safe_dir = os.path.join(tmpdir.name, "safe")

dangerzone/logic.py

@@ -9,10 +9,9 @@ import subprocess
 import sys
 from typing import Callable, List, Optional
 
-import appdirs
 import colorama
 
-from . import errors
+from . import errors, util
 from .document import Document
 from .isolation_provider.base import IsolationProvider
 from .settings import Settings
@@ -31,7 +30,7 @@ class DangerzoneCore(object):
         colorama.init(autoreset=True)
 
         # App data folder
-        self.appdata_path = appdirs.user_config_dir("dangerzone")
+        self.appdata_path = util.get_config_dir()
 
         # Languages supported by tesseract
         with open(get_resource_path("ocr-languages.json"), "r") as f:

dangerzone/util.py

@@ -2,6 +2,24 @@ import pathlib
 import platform
 import subprocess
 import sys
+from typing import Optional
+
+import appdirs
+
+
+def get_config_dir() -> str:
+    return appdirs.user_config_dir("dangerzone")
+
+
+def get_tmp_dir() -> Optional[str]:
+    """Get the parent dir for the Dangerzone temporary dirs.
+
+    This function returns the parent directory where Dangerzone will store its temporary
+    directories. The default behavior is to let Python choose for us (e.g., in `/tmp`
+    for Linux), which is why we return None. However, we still need to define this
+    function in order to be able to set this dir via mocking in our tests.
+    """
+    return None
 
 
 def get_resource_path(filename: str) -> str: