Add investigation: can dbus-broker replace dbus-daemon for ruby-dbus?

Document findings on dbus-broker compatibility. The library works
transparently with dbus-broker since it only connects via socket
addresses. The test suite explicitly spawns dbus-daemon and would
need changes (e.g. migrating to dbus-run-session) to be fully
daemon-agnostic.

https://claude.ai/code/session_017TKMdbvByVBNUpfkLHMJea

Author: claude

doc/dbus-broker-investigation.md

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+# Can dbus-broker replace dbus-daemon for ruby-dbus?
+
+## Summary
+
+**Yes, mostly.** For normal library usage (connecting to the system or session
+bus), dbus-broker is a transparent drop-in — ruby-dbus simply connects to a
+socket address and doesn't care which daemon is behind it. For **testing**,
+the situation is more nuanced because the test suite explicitly spawns
+`dbus-daemon`.
+
+## What is dbus-broker?
+
+dbus-broker is a Linux-native reimplementation of the D-Bus message bus. It is
+wire-compatible with dbus-daemon and is already the **default** on Fedora and
+Arch Linux. Key improvements over dbus-daemon:
+
+- **Reliability**: no silently dropped messages
+- **Performance**: O(log n) lookups vs O(n) in dbus-daemon
+- **Accounting**: per-user resource quotas resistant to chaining-exhaustion
+- **Security**: resistant to deadlocks and DoS attacks
+
+The main caveat: `dbus-broker-launch` (the compatibility launcher) **requires
+systemd** for service activation. The broker binary itself has no hard
+dependencies beyond libc.
+
+## How ruby-dbus uses dbus-daemon
+
+### 1. Runtime library (no direct dependency)
+
+The library code in `lib/dbus/` never spawns or references `dbus-daemon`
+directly. It connects to a bus via:
+
+- **Session bus** (`lib/dbus/bus.rb`): reads `DBUS_SESSION_BUS_ADDRESS` env
+  var, or falls back to `~/.dbus/session-bus/*` files, or macOS launchd.
+- **System bus** (`lib/dbus/bus.rb`): reads `DBUS_SYSTEM_BUS_ADDRESS` env var,
+  or defaults to `unix:path=/var/run/dbus/system_bus_socket`.
+- **Connection** (`lib/dbus/message_queue.rb`): connects to unix sockets, TCP,
+  or launchd — pure socket-level, daemon-agnostic.
+
+**Conclusion**: If the system runs dbus-broker instead of dbus-daemon, the
+library works without any changes. The bus address and wire protocol are
+identical.
+
+### 2. Test infrastructure (explicit dbus-daemon dependency)
+
+The test suite spawns a **private session bus** to run integration tests in
+isolation:
+
+- **`spec/spec_helper.rb`** — `with_private_bus` spawns:
+  ```
+  dbus-daemon --nofork --config-file=spec/tools/dbus-limited-session.conf \
+    --print-address=3 --print-pid=4
+  ```
+- **`spec/tools/dbus-launch-simple`** — shell script that does the same, used
+  by `spec/tools/test_env`.
+- **`spec/tools/dbus-limited-session.conf`** — XML config file for the test
+  bus (session type, custom limits, multiple listen addresses including TCP).
+
+Both launchers wait for the daemon to write its address and PID to temp files,
+then set `DBUS_SESSION_BUS_ADDRESS` for the tests.
+
+### 3. CI
+
+`.travis.yml` installs the `dbus` apt package. GitHub Actions
+(`.github/workflows/ruby.yml`) relies on the system-installed dbus.
+
+## Can dbus-broker replace dbus-daemon here?
+
+### For users of the library: YES, transparently
+
+No changes needed. If the system session/system bus is provided by dbus-broker,
+ruby-dbus connects to it exactly the same way.
+
+### For the test suite: NOT a drop-in replacement
+
+The test suite invokes `dbus-daemon` directly with `--nofork`,
+`--config-file`, `--print-address`, and `--print-pid` flags. dbus-broker's
+`dbus-broker-launch` is **not** a CLI-compatible replacement for
+`dbus-daemon` — it has a different command-line interface and requires systemd
+for service activation.
+
+To support running tests against dbus-broker, we would need to either:
+
+1. **Keep using dbus-daemon for tests** (simplest, it remains available even on
+   dbus-broker systems since dbus-broker doesn't remove dbus-daemon).
+2. **Use `dbus-run-session`** instead of invoking dbus-daemon directly.
+   `dbus-run-session` is a small wrapper (from the dbus package) that starts a
+   private session bus and runs a command within it. It works with whichever
+   daemon is installed and is simpler than our custom launcher. This would be
+   the cleanest approach.
+3. **Abstract the daemon launch** so tests can use either dbus-daemon or
+   dbus-broker-launch, selected by an env var. This is more complex and
+   probably not worth it.
+
+### Recommendation
+
+- The **library itself** needs no changes for dbus-broker compatibility.
+- For **testing**, consider migrating from the hand-rolled `dbus-launch-simple`
+  / `with_private_bus` to `dbus-run-session` which is daemon-agnostic. Even
+  without this change, tests will continue to work since dbus-daemon remains
+  installable alongside dbus-broker on all major distributions.
+
+## References
+
+- [dbus-broker wiki](https://github.com/bus1/dbus-broker/wiki)
+- [Fedora: Make dbus-broker the default](https://fedoraproject.org/wiki/Changes/DbusBrokerAsTheDefaultDbusImplementation)
+- [Arch Linux RFC 0025](https://rfc.archlinux.page/0025-dbus-broker-default/)
+- [Arch Linux D-Bus wiki](https://wiki.archlinux.org/title/D-Bus)
+- [LWN: The D-Bus Broker project](https://lwn.net/Articles/731755/)