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+Contributing to pressure-vessel
+===============================
+
+See [README.md](README.md) for general information about
+`pressure-vessel`, and for information about building and testing it.
+
+More extensive notes should go here eventually, but aren't
+available yet. Sorry!
+
+Adding dependencies
+-------------------
+
+Some of the `pressure-vessel` tools are designed to run on the host
+system, which can in principle be anything, so think carefully before
+adding new dependencies.
+
+It's OK to depend on these from the host system:
+
+  * Linux kernel
+      - Linux 3.8 (first version to have mount and user namespaces)
+        is a hard dependency.
+      - Linux 3.9 (first version to let unprivileged users create a
+        `tmpfs` in a user namespace) is required if your host system
+        does not provide a setuid `bwrap`.
+        This version is available in (at least) Debian 8+,
+        Ubuntu 14.04+, RHEL 7+, openSUSE 42.1+, SteamOS 1+.
+      - Linux 3.15 (first version to have `F_OFD_SETLCKW`) is
+        strongly recommended at runtime. We should try to have fallback
+        paths for older kernels, but if that becomes impossible, it's
+        OK to bump this up to a hard dependency.
+        This version is available in (at least) Debian 8+, Ubuntu 16.04+,
+        RHEL 8+, openSUSE 42.1+, SteamOS 2+.
+  * GNU libc
+      - glibc 2.15 (SteamRT 1 'scout', Ubuntu 12.04 'precise') is
+        a hard dependency, and we currently compile against this version
+        This version is available in (at least) Debian 8+, Ubuntu 12.04+,
+        RHEL 7+, openSUSE 42.1+, SteamOS 1+.
+      - glibc 2.19 (SteamOS 2 'brewmaster', Ubuntu 14.04 'trusty',
+        Debian 8 'jessie') is strongly recommended at runtime. We should
+        try to have fallback paths for older glibc, but if that becomes
+        impossible, it's OK to bump this up to a hard dependency.
+        This version is available in (at least) Debian 8+, Ubuntu 14.04+,
+        RHEL 8+, openSUSE 42.1+, SteamOS 2+.
+  * setuid `bwrap`, if the kernel requires it
+    (always required on RHEL, runtime-configurable and required by default
+    on Debian, runtime-configurable but not required by default on Arch)
+      - It can be the version bundled in Flatpak, or standalone
+
+We bundle libraries from the Steam Runtime 1 'scout' for other
+dependencies. See `build-relocatable-install.py` for those. This means
+we can only rely on those libraries being at least as new as the ones
+in scout, which are pretty old.
+
+The scout build environment has backports of Python 3.5 and Meson, so we
+can rely on those at build-time.
+
+If we gain a dependency on glibc 2.19 or other newer libraries, we'll have
+to increase the build environment to SteamRT 1½ 'heavy', and backport
+Python 3.5 and Meson to 'heavy'.