Fix Podman crashes due to old conmon version

Switching from mounting files to writing to stdout has introduced some
Podman crashes in specific environments (Ubuntu Jammy / Debian Bullseye)
due to a conmon bug that affects version 2.0.25.

Fixing it for various permutations of the environments we support
requires the following:

1. CI tests: Install conmon from the oldstable-proposed-updates in
   our Debian Bullseye / Ubuntu Jammy dev/end-user environments.
2. Developers: Add a line in BUILD.md that suggests users to install
   conmon from the oldstable-proposed-updates repo, or some other repo
   they prefer.
3. End-user installations: We will build conmon for Ubuntu Jammy, and
   wait until the proposed updates repo gets merged in Debian Bullseye.

Fixes #685

BUILD.md

@@ -4,6 +4,27 @@
 
 Install dependencies:
 
+<details>
+  <summary><i>:memo: Expand this section if you are on Ubuntu 22.04 (Jammy).</i></summary>
+  </br>
+
+  The `conmon` version that Podman uses and Ubuntu Jammy ships, has a bug
+  that gets triggered by Dangerzone
+  (more details in https://github.com/freedomofpress/dangerzone/issues/685).
+  If you want to run Dangerzone from source, you are advised to install a
+  patched `conmon` version. A simple way to do so is to enable the Debian
+  Bullseye Proposed Updates repo, just for the `conmon` package:
+
+  ```bash
+  sudo cp ./dev_scripts/oldstable-pu-ubuntu.sources /etc/apt/sources.list.d/
+  sudo cp ./dev_scripts/oldstable-pu.pref /etc/apt/preferences.d/
+  ```
+
+  Alternatively, you can install a `conmon` version higher than `v2.0.25` from
+  any repo you prefer.
+
+</details>
+
 ```sh
 sudo apt install -y podman dh-python build-essential fakeroot make libqt6gui6 \
     pipx python3 python3-dev python3-stdeb python3-all

dev_scripts/env.py

@@ -115,6 +115,14 @@
 RUN touch /var/mail/ubuntu && chown ubuntu /var/mail/ubuntu && userdel -r ubuntu
 """
 
+# On Ubuntu Jammy / Debian Bullseye, use a different conmon version, as acquired from
+# Debian's oldstable proposed updates. For more details, read:
+# https://github.com/freedomofpress/dangerzone/issues/685
+DOCKERFILE_CONMON_UPDATE = r"""
+COPY oldstable-pu.sources /etc/apt/sources.list.d/
+COPY oldstable-pu.pref /etc/apt/preferences.d/
+"""
+
 # FIXME: Do we really need the python3-venv packages?
 DOCKERFILE_BUILD_DEV_DEBIAN_DEPS = r"""
 ARG DEBIAN_FRONTEND=noninteractive
@@ -557,6 +565,11 @@ def build_dev(self, show_dockerfile=DEFAULT_SHOW_DOCKERFILE):
                 # Ubuntu Jammy misses a dependency to `libxkbcommon-x11-0`, which we can
                 # install indirectly via `qt6-qpa-plugins`.
                 qt_deps += " qt6-qpa-plugins"
+                # Ubuntu Jammy and Debian Bullseye require a more up-to-date conmon
+                # package (see https://github.com/freedomofpress/dangerzone/issues/685)
+                install_deps = (
+                    DOCKERFILE_CONMON_UPDATE + DOCKERFILE_BUILD_DEV_DEBIAN_DEPS
+                )
             elif self.distro == "ubuntu" and self.version in (
                 "23.04",
                 "23.10",
@@ -569,6 +582,11 @@ def build_dev(self, show_dockerfile=DEFAULT_SHOW_DOCKERFILE):
             elif self.distro == "debian" and self.version in ("bullseye-backports",):
                 # Debian Bullseye misses a dependency to libgl1.
                 qt_deps += " libgl1"
+                # Ubuntu Jammy and Debian Bullseye require a more up-to-date conmon
+                # package (see https://github.com/freedomofpress/dangerzone/issues/685)
+                install_deps = (
+                    DOCKERFILE_CONMON_UPDATE + DOCKERFILE_BUILD_DEV_DEBIAN_DEPS
+                )
 
             install_deps = install_deps.format(qt_deps=qt_deps)
 
@@ -586,6 +604,12 @@ def build_dev(self, show_dockerfile=DEFAULT_SHOW_DOCKERFILE):
         shutil.copy(git_root() / "pyproject.toml", build_dir)
         shutil.copy(git_root() / "poetry.lock", build_dir)
         shutil.copy(git_root() / "dev_scripts" / "storage.conf", build_dir)
+        if self.distro in ("debian", "ubuntu"):
+            shutil.copy(git_root() / "dev_scripts" / "oldstable-pu.pref", build_dir)
+            shutil.copy(
+                git_root() / "dev_scripts" / f"oldstable-pu-{self.distro}.sources",
+                build_dir / "oldstable-pu.sources",
+            )
         with open(build_dir / "Dockerfile", mode="w") as f:
             f.write(dockerfile)
 
@@ -636,6 +660,12 @@ def build(
                 install_deps = (
                     DOCKERFILE_UBUNTU_2004_DEPS + DOCKERFILE_BUILD_DEBIAN_DEPS
                 )
+            elif (self.distro == "ubuntu" and self.version in ("22.04", "jammy")) or (
+                self.distro == "debian" and self.version in ("bullseye-backports",)
+            ):
+                # Ubuntu Jammy and Debian Bullseye require a more up-to-date conmon
+                # package (see https://github.com/freedomofpress/dangerzone/issues/685)
+                install_deps = DOCKERFILE_CONMON_UPDATE + DOCKERFILE_BUILD_DEBIAN_DEPS
             elif self.distro == "ubuntu" and self.version in (
                 "23.04",
                 "23.10",
@@ -664,6 +694,12 @@ def build(
         # Populate the build context.
         shutil.copy(package_src, package_dst)
         shutil.copy(git_root() / "dev_scripts" / "storage.conf", build_dir)
+        if self.distro in ("debian", "ubuntu"):
+            shutil.copy(git_root() / "dev_scripts" / "oldstable-pu.pref", build_dir)
+            shutil.copy(
+                git_root() / "dev_scripts" / f"oldstable-pu-{self.distro}.sources",
+                build_dir / "oldstable-pu.sources",
+            )
         with open(build_dir / "Dockerfile", mode="w") as f:
             f.write(dockerfile)
 

dev_scripts/oldstable-pu-debian.sources

@@ -0,0 +1,4 @@
+Types: deb
+URIs: http://deb.debian.org/debian/
+Suites: oldstable-proposed-updates
+Components: main

dev_scripts/oldstable-pu-ubuntu.sources

@@ -0,0 +1,193 @@
+Types: deb
+URIs: http://deb.debian.org/debian/
+Suites: oldstable-proposed-updates
+Components: main
+# Taken from Debian 11's FTP master key:
+# https://ftp-master.debian.org/keys.html
+Signed-By:
+  -----BEGIN PGP PUBLIC KEY BLOCK-----
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dev_scripts/oldstable-pu.pref

@@ -0,0 +1,7 @@
+Package: *
+Pin: release a=oldstable-proposed-updates
+Pin-Priority: 100
+
+Package: conmon
+Pin: release a=oldstable-proposed-updates
+Pin-Priority: 500

dev_scripts/qa.py

@@ -198,6 +198,27 @@
 
 Install dependencies:
 
+<details>
+  <summary><i>:memo: Expand this section if you are on Ubuntu 22.04 (Jammy).</i></summary>
+  </br>
+
+  The `conmon` version that Podman uses and Ubuntu Jammy ships, has a bug
+  that gets triggered by Dangerzone
+  (more details in https://github.com/freedomofpress/dangerzone/issues/685).
+  If you want to run Dangerzone from source, you are advised to install a
+  patched `conmon` version. A simple way to do so is to enable the Debian
+  Bullseye Proposed Updates repo, just for the `conmon` package:
+
+  ```bash
+  sudo cp ./dev_scripts/oldstable-pu-ubuntu.sources /etc/apt/sources.list.d/
+  sudo cp ./dev_scripts/oldstable-pu.pref /etc/apt/preferences.d/
+  ```
+
+  Alternatively, you can install a `conmon` version higher than `v2.0.25` from
+  any repo you prefer.
+
+</details>
+
 ```sh
 sudo apt install -y podman dh-python build-essential fakeroot make libqt6gui6 \
     pipx python3 python3-dev python3-stdeb python3-all