Add BuildStream support

The GnomeOS folks are looking into mkosi to build their images instead
of BuildStream. While BuildStream will still take care of providing the
rootfs tree, mkosi would take over the responsibility of packaging that
directory tree into a disk image.

Let's add support for BuildStream to mkosi to make this possible. Unlike
the other supported distributions, BuildStream is not intended to be consumed
by installing individual packages. Instead, BuildStream elements should be
exposed which provide the full rootfs that should go into the image. That's
why we limit the number of packages that can be specified to a single one, which
should always provide all contents that should go into the image.

Author: daandemeyer

mkosi/distribution/__init__.py

@@ -42,6 +42,7 @@ class Distribution(StrEnum):
     rocky = enum.auto()
     alma = enum.auto()
     azure = enum.auto()
+    buildstream = enum.auto()
     custom = enum.auto()
 
     def is_centos_variant(self) -> bool:

mkosi/distribution/buildstream.py

@@ -0,0 +1,61 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+from mkosi.config import Config
+from mkosi.context import Context
+
+from mkosi.log import die
+from mkosi.config import Architecture
+from mkosi.installer.bst import BST
+from mkosi.distribution import (
+    Distribution,
+    DistributionInstaller,
+    PackageType,
+)
+
+class Installer(DistributionInstaller, distribution=Distribution.buildstream):
+    @classmethod
+    def pretty_name(cls) -> str:
+        return "BuildStream"
+
+    @classmethod
+    def filesystem(cls) -> str:
+        return "btrfs"
+
+    @classmethod
+    def package_type(cls) -> PackageType:
+        return PackageType.none
+
+    @classmethod
+    def default_release(cls) -> str:
+        return "snapshot"
+
+    @classmethod
+    def package_manager(cls, config: "Config") -> type[BST]:
+        return BST
+
+    @classmethod
+    def setup(cls, context: Context) -> None:
+        pass
+
+    @classmethod
+    def install(cls, context: Context) -> None:
+        pass
+
+    @classmethod
+    def architecture(cls, arch: Architecture) -> str:
+        a = {
+            Architecture.x86_64: "x86_64",
+        }.get(arch)  # fmt: skip
+
+        if not a:
+            die(f"Architecture {a} is not supported by {cls.pretty_name()}")
+
+        return a
+
+    @classmethod
+    def latest_snapshot(cls, config: Config) -> str:
+        die(f"Latest snapshot not supported by {cls.pretty_name()}")
+
+    @classmethod
+    def is_kernel_package(cls, package: str) -> bool:
+        return False

mkosi/installer/bst.py

@@ -0,0 +1,73 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+from collections.abc import Sequence
+from pathlib import Path
+
+from mkosi.config import Config
+from mkosi.context import Context
+from mkosi.installer import PackageManager
+from mkosi.run import run
+from mkosi.log import die
+
+
+class BST(PackageManager):
+    @classmethod
+    def executable(cls, config: Config) -> str:
+        return "bst"
+
+    @classmethod
+    def subdir(cls, config: Config) -> Path:
+        return Path("bst")
+
+    @classmethod
+    def architecture(cls, context: Context) -> str:
+        return context.config.distribution.installer.architecture(context.config.architecture)
+
+    @classmethod
+    def setup(cls, context: Context) -> None:
+        if len(context.config.packages) > 1:
+            die("Only a single element can be specified in Packages= when using bst")
+
+    @classmethod
+    def install(
+        cls,
+        context: Context,
+        packages: Sequence[str],
+        *,
+        apivfs: bool = True,
+        allow_downgrade: bool = False,
+    ) -> None:
+        options = [
+            "--same-dir",
+            *context.rootoptions(),
+            # bst might need to lookup files/paths across the user's home directory so make sure it is
+            # available.
+            "--bind", Path.home(), Path.home(),
+            "--setenv", "HOME", Path.home(),
+        ]
+
+        # We don't really want to run bst as (fake) root but it uses bubblewrap which stubbornly refuses to
+        # run when invoked unprivileged but with capabilities. We get around this by running as fake root but
+        # still setting $HOME to the user's home to reuse the buildstream cache directory.
+        run(
+            ["bst", "build", *packages],
+            sandbox=cls.sandbox(context, apivfs=apivfs, options=options),
+            env=cls.finalize_environment(context),
+        )
+        run(
+            ["bst", "artifact", "checkout", "--force", "--directory=/buildroot", *packages],
+            sandbox=cls.sandbox(context, apivfs=apivfs, options=options),
+            env=cls.finalize_environment(context),
+        )
+
+    @classmethod
+    def remove(cls, context: Context, packages: Sequence[str]) -> None:
+        die("Removing packages is not supported for bst")
+
+    @classmethod
+    def sync(cls, context: Context, force: bool) -> None:
+        pass
+
+    @classmethod
+    def createrepo(cls, context: Context) -> None:
+        die("Creating package repositories is not supported for bst")

mkosi/resources/man/mkosi.1.md

@@ -488,9 +488,9 @@ boolean argument: either `1`, `yes`, or `true` to enable, or `0`, `no`,
 :   The distribution to install in the image. Takes one of the following
     arguments: `fedora`, `debian`, `kali`, `ubuntu`, `arch`, `opensuse`,
     `mageia`, `centos`, `rhel`, `rhel-ubi`, `openmandriva`, `rocky`, `alma`,
-    `azure` or `custom`. If not specified, defaults to the distribution of
-    the host or `custom` if the distribution of the host is not a supported
-    distribution.
+    `azure`, `buildstream` or `custom`. If not specified, defaults to the
+    distribution of the host or `custom` if the distribution of the host is
+    not a supported distribution.
 
 `Release=`, `--release=`, `-r`
 :   The release of the distribution to install in the image. The precise
@@ -536,6 +536,7 @@ boolean argument: either `1`, `yes`, or `true` to enable, or `0`, `no`,
     | `mageia`       | https://www.mageia.org             |                                |
     | `openmandriva` | http://mirrors.openmandriva.org    |                                |
     | `azure`        | https://packages.microsoft.com/    |                                |
+    | `buildstream`  |                                    |                                |
 
 `Snapshot=`
 :   Download packages from the given snapshot instead of downloading the latest

mkosi/resources/mkosi-initrd/mkosi.conf.d/buildstream.conf

@@ -0,0 +1,9 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+[Match]
+Distribution=buildstream
+
+[Content]
+# BuildStream is a generic distribution so we don't know which package to install, hence override to the
+# empty list.
+Packages=

mkosi/resources/mkosi-initrd/mkosi.conf.d/stub.conf

@@ -3,6 +3,7 @@
 [Match]
 Format=uki
 Distribution=!arch
+Distribution=!buildstream
 
 [Content]
 Packages=systemd-boot

mkosi/resources/mkosi-tools/mkosi.conf.d/buildstream.conf

@@ -0,0 +1,9 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+[Match]
+Distribution=buildstream
+
+[Content]
+# BuildStream is a generic distribution so we don't know which package to install, hence override to the
+# empty list.
+Packages=