Fix wrong container runtime detection on Linux

Use "podman" when on Linux, and "docker" otherwise.
Show what the error was when the command failed.

dangerzone/gui/__init__.py

@@ -5,7 +5,7 @@ import platform
 import signal
 import sys
 import typing
-from typing import Dict, List, Optional
+from typing import List, Optional
 
 import click
 import colorama

dangerzone/gui/main_window.py

@@ -10,14 +10,18 @@ from typing import List, Optional
 # FIXME: See https://github.com/freedomofpress/dangerzone/issues/320 for more details.
 if typing.TYPE_CHECKING:
     from PySide2 import QtCore, QtGui, QtSvg, QtWidgets
-    from PySide2.QtWidgets import QAction
+    from PySide2.QtCore import Qt
+    from PySide2.QtWidgets import QAction, QTextEdit
 else:
     try:
         from PySide6 import QtCore, QtGui, QtSvg, QtWidgets
+        from PySide6.QtCore import Qt
         from PySide6.QtGui import QAction
+        from PySide6.QtWidgets import QTextEdit
     except ImportError:
         from PySide2 import QtCore, QtGui, QtSvg, QtWidgets
-        from PySide2.QtWidgets import QAction
+        from PySide2.QtWidgets import QAction, QTextEdit
+        from PySide2.QtCore import Qt
 
 from .. import errors
 from ..document import SAFE_EXTENSION, Document
@@ -434,10 +438,29 @@ class WaitingWidgetContainer(WaitingWidget):
         self.buttons = QtWidgets.QWidget()
         self.buttons.setLayout(buttons_layout)
 
+        # Error
+        self.error_text = QTextEdit()
+        self.error_text.setReadOnly(True)
+        self.error_text.setStyleSheet(
+            """
+        QTextEdit {
+            font-family: Consolas, Monospace;
+            font-size: 12px;
+            background-color: #fff;
+            color: #000;
+            padding: 10px;
+        }
+        """
+        )
+        self.error_text.setVisible(False)
+        # Enable copying
+        self.error_text.setTextInteractionFlags(Qt.TextSelectableByMouse)
+
         # Layout
         layout = QtWidgets.QVBoxLayout()
         layout.addStretch()
         layout.addWidget(self.label)
+        layout.addWidget(self.error_text)
         layout.addStretch()
         layout.addWidget(self.buttons)
         layout.addStretch()
@@ -448,49 +471,84 @@ class WaitingWidgetContainer(WaitingWidget):
 
     def check_state(self) -> None:
         state: Optional[str] = None
+        error: Optional[str] = None
 
         try:
             if isinstance(  # Sanity check
                 self.dangerzone.isolation_provider, Container
             ):
                 container_runtime = self.dangerzone.isolation_provider.get_runtime()
+                runtime_name = self.dangerzone.isolation_provider.get_runtime_name()
         except NoContainerTechException as e:
             log.error(str(e))
             state = "not_installed"
 
         else:
-            # Can we run `docker image ls` without an error
+            # Can we run `docker/podman image ls` without an error
             with subprocess.Popen(
                 [container_runtime, "image", "ls"],
                 stdout=subprocess.DEVNULL,
-                stderr=subprocess.DEVNULL,
+                stderr=subprocess.PIPE,
                 startupinfo=get_subprocess_startupinfo(),
             ) as p:
-                p.communicate()
+                _, stderr = p.communicate()
                 if p.returncode != 0:
-                    log.error("Docker is not running")
+                    log.error(f"{runtime_name} is not running")
                     state = "not_running"
+                    error = stderr.decode()
                 else:
                     # Always try installing the container
                     state = "install_container"
 
         # Update the state
-        self.state_change(state)
+        self.state_change(state, error)
 
-    def state_change(self, state: str) -> None:
+    def state_change(self, state: str, error: str | None = None) -> None:
         if state == "not_installed":
-            self.label.setText(
+            if platform.system() == "Linux":
-                "<strong>Dangerzone Requires Docker Desktop</strong><br><br><a href='https://www.docker.com/products/docker-desktop'>Download Docker Desktop</a>, install it, and open it."
+                self.label.setText(
-            )
+                    (
+                        "<strong>Dangerzone requires Podman</strong><br><br>"
+                        "Install it and retry."
+                    )
+                )
+            else:
+                self.label.setText(
+                    (
+                        "<strong>Dangerzone requires Docker Desktop</strong><br><br>"
+                        "<a href='https://www.docker.com/products/docker-desktop'>Download Docker Desktop</a>"
+                        ", install it, and open it."
+                    )
+                )
             self.buttons.show()
         elif state == "not_running":
-            self.label.setText(
+            if platform.system() == "Linux":
-                "<strong>Dangerzone Requires Docker Desktop</strong><br><br>Docker is installed but isn't running.<br><br>Open Docker and make sure it's running in the background."
+                # "not_running" here means that the `podman image ls` command failed.
-            )
+                message = (
+                    "<strong>Dangerzone requires Podman</strong><br><br>"
+                    "Podman is installed but cannot run properly. See errors below"
+                )
+                if error:
+                    self.error_text.setPlainText(error)
+                    self.error_text.setVisible(True)
+
+                self.label.setText(message)
+
+            else:
+                self.label.setText(
+                    (
+                        "<strong>Dangerzone requires Docker Desktop</strong><br><br>"
+                        "Docker is installed but isn't running.<br><br>"
+                        "Open Docker and make sure it's running in the background."
+                    )
+                )
             self.buttons.show()
         else:
             self.label.setText(
-                "Installing the Dangerzone container image.<br><br>This might take a few minutes..."
+                (
+                    "Installing the Dangerzone container image.<br><br>"
+                    "This might take a few minutes..."
+                )
             )
             self.buttons.hide()
             self.install_container_t = InstallContainerThread(self.dangerzone)

dangerzone/isolation_provider/container.py

@@ -242,7 +242,7 @@ class Container(IsolationProvider):
         # `int`.
         #
         # See https://stackoverflow.com/a/37888668
-        if not type(val) == _type:
+        if type(val) is not _type:
             raise ValueError("Status field has incorrect type")
 
     def parse_progress_trusted(self, document: Document, line: str) -> None: