feat: support non-flake entrypoint

The way to use Buildbot jobs without Flakes is via `.ci/buildbot.nix`.

Signed-off-by: Raito Bezarius <raito@lix.systems>

buildbot_nix/__init__.py

@@ -23,6 +23,7 @@ from buildbot.reporters.generators.build import BuildStatusGenerator
 from buildbot.reporters.message import MessageFormatterFunction
 from buildbot.process.buildstep import EXCEPTION
 from buildbot.process.buildstep import SUCCESS
+from buildbot.process.buildstep import BuildStepFailed
 from buildbot.process.results import worst_status
 
 if TYPE_CHECKING:
@@ -37,6 +38,14 @@ log = Logger()
 
 FLAKE_TARGET_ATTRIBUTE_FOR_JOBS = "buildbotJobs"
 
+@dataclass
+class EvaluatorSettings:
+    supported_systems: list[str]
+    worker_count: int
+    max_memory_size: int
+    gc_roots_dir: str
+    lock: util.MasterLock
+
 @dataclass
 class NixBuilder:
     protocol: str
@@ -397,7 +406,7 @@ class NixEvalCommand(buildstep.ShellMixin, steps.BuildStep):
                 [
                     BuildTrigger(
                         builds_scheduler_group=f"{project_name}-nix-build",
-                        name="build flake",
+                        name="build derivations",
                         jobs=filtered_jobs,
                         all_deps=all_deps,
                     ),
@@ -406,6 +415,78 @@ class NixEvalCommand(buildstep.ShellMixin, steps.BuildStep):
 
         return result
 
+def make_job_evaluator(name: str, settings: EvaluatorSettings, flake: bool) -> NixEvalCommand:
+    actual_command = []
+
+    if flake:
+        actual_command += ["--flake", f".#{FLAKE_TARGET_ATTRIBUTE_FOR_JOBS}"]
+    else:
+        actual_command += ["--expr", "'import ./.ci/buildbot.nix'"]
+
+    return NixEvalCommand(
+        env={},
+        name=name,
+        supported_systems=settings.supported_systems,
+        command=[
+            "nix-eval-jobs",
+            "--workers",
+            str(settings.worker_count),
+            "--max-memory-size",
+            str(settings.max_memory_size),
+            "--gc-roots-dir",
+            settings.gc_roots_dir,
+            "--force-recurse",
+            "--check-cache-status",
+        ] + actual_command,
+        haltOnFailure=True,
+        locks=[settings.lock.access("exclusive")]
+    )
+
+class NixConfigure(buildstep.CommandMixin, steps.BuildStep):
+    """
+    Determine what `NixEvalCommand` step should be added after
+    based on the existence of:
+
+    - flake.nix
+    - .ci/buildbot.nix
+    """
+
+    def __init__(self, eval_settings: EvaluatorSettings, **kwargs: Any) -> None:
+        self.evaluator_settings = eval_settings
+        super().__init__(**kwargs)
+        self.observer = logobserver.BufferLogObserver()
+        self.addLogObserver("stdio", self.observer)
+
+    @defer.inlineCallbacks
+    def run(self) -> Generator[Any, object, Any]:
+        configure_log: Log = yield self.getLog("stdio")
+
+        # Takes precedence.
+        configure_log.addStdout("checking if there's a .ci/buildbot.nix...")
+        ci_buildbot_defn_exists = yield self.pathExists('.ci/buildbot.nix')
+        if ci_buildbot_defn_exists:
+            configure_log.addStdout(".ci/buildbot.nix found, configured for non-flake CI")
+            self.build.addStepAfterCurrentStep(
+                make_job_evaluator(
+                    "evaluate `.ci/buildbot.nix` jobs",
+                    self.evaluator_settings,
+                    False
+                )
+            )
+
+        flake_exists = yield self.pathExists('flake.nix')
+        if flake_exists:
+            configure_log.addStdout(f"flake.nix found")
+            self.build.addStepAfterCurrentStep(
+                make_job_evaluator(
+                    "evaluate `flake.nix` jobs",
+                    self.evaluator_settings,
+                    True
+                )
+            )
+            return
+
+        configure_log.addStdout("neither flake.nix found neither .ci/buildbot.nix, no CI to run!")
 
 class NixBuildCommand(buildstep.ShellMixin, steps.BuildStep):
     """Builds a nix derivation."""
@@ -445,7 +526,8 @@ def nix_eval_config(
     worker_count: int,
     max_memory_size: int,
 ) -> util.BuilderConfig:
-    """Uses nix-eval-jobs to evaluate $FLAKE_TARGET_ATTRIBUTE_FOR_JOBS (`.#hydraJobs` by default) from flake.nix in parallel.
+    """
+    Uses nix-eval-jobs to evaluate the entrypoint of this project.
     For each evaluated attribute a new build pipeline is started.
     """
     factory = util.BuildFactory()
@@ -467,27 +549,22 @@ def nix_eval_config(
         "/nix/var/nix/gcroots/per-user/buildbot-worker/%(prop:project)s/drvs/%(prop:workername)s/",
     )
 
+    eval_settings = EvaluatorSettings(
+        supported_systems=supported_systems,
+        worker_count=worker_count,
+        max_memory_size=max_memory_size,
+        gc_roots_dir=drv_gcroots_dir,
+        lock=eval_lock
+    )
+
+    # NixConfigure will choose
+    # how to add a NixEvalCommand job
+    # based on whether there's a flake.nix or
+    # a .ci/buildbot.nix.
     factory.addStep(
-        NixEvalCommand(
+        NixConfigure(
-            env={},
+            eval_settings
-            name="evaluate flake",
+        )
-            supported_systems=supported_systems,
-            command=[
-                "nix-eval-jobs",
-                "--workers",
-                str(worker_count),
-                "--max-memory-size",
-                str(max_memory_size),
-                "--gc-roots-dir",
-                drv_gcroots_dir,
-                "--force-recurse",
-                "--check-cache-status",
-                "--flake",
-                f".#{FLAKE_TARGET_ATTRIBUTE_FOR_JOBS}"
-            ],
-            haltOnFailure=True,
-            locks=[eval_lock.access("exclusive")],
-        ),
     )
 
     factory.addStep(