feat: add incoming ref data to non-flakes entrypoint

We can now implement a Nix library for Buildbot CI. :)

We dump it into a file, it's better to pass large stuff and easier to
escape things.

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

buildbot_nix/__init__.py

@@ -8,6 +8,7 @@ from collections.abc import Generator
 from dataclasses import dataclass, field
 from pathlib import Path
 from typing import TYPE_CHECKING, Any
+import tempfile
 
 import buildbot
 from buildbot.configurators import ConfiguratorBase
@@ -374,6 +375,11 @@ class NixEvalCommand(buildstep.ShellMixin, steps.BuildStep):
         self.addLogObserver("stdio", self.observer)
         self.supported_systems = supported_systems
 
+    def __del__(self):
+        if os.path.exists('../incoming-ref-json'):
+            # Cleanup the incoming ref parameter file.
+            os.unlink('../incoming-ref-json')
+
     @defer.inlineCallbacks
     def run(self) -> Generator[Any, object, Any]:
         # run nix-eval-jobs --flake .#$FLAKE_TARGET_ATTRIBUTE_FOR_JOBS to generate the dict of stages
@@ -453,13 +459,23 @@ class NixEvalCommand(buildstep.ShellMixin, steps.BuildStep):
 
         return result
 
-def make_job_evaluator(name: str, settings: EvaluatorSettings, flake: bool) -> NixEvalCommand:
+def make_job_evaluator(name: str, settings: EvaluatorSettings, flake: bool, incoming_ref_data: dict[str, Any]) -> NixEvalCommand:
     actual_command = []
 
+    try:
+        eval_parameter_json_repr = json.dumps(incoming_ref_data)
+    except ValueError as e:
+        msg = f"Failed to serialize incoming ref data for {name}"
+        raise BuildbotNixError(msg) from e
+
+    with open('incoming-ref-json', 'w') as f:
+        f.write(eval_parameter_json_repr)
+
     if flake:
         actual_command += ["--flake", f".#{FLAKE_TARGET_ATTRIBUTE_FOR_JOBS}"]
     else:
-        actual_command += ["--expr", "import ./.ci/buildbot.nix"]
+        actual_command += ["--expr",
+                           f"import ./.ci/buildbot.nix {{ incoming_ref_data = builtins.fromJSON (builtins.readFile ../incoming-ref-json); }}"]
 
     return NixEvalCommand(
         env={},
@@ -504,6 +520,14 @@ class NixConfigure(buildstep.CommandMixin, steps.BuildStep):
         except Exception:
             configure_log: Log = yield self.addLog("stdio")
 
+        # This information can be passed at job evaluation time
+        # to skip some jobs, e.g. expensive jobs, etc.
+        incoming_ref_data = {
+            # TODO: please just denormalize it properly.
+            'event': {
+                key: value for (key, (value, _)) in self.getProperties().asDict().items() if key.startswith('event.')
+            }
+        }
         # Takes precedence.
         configure_log.addStdout("checking if there's a .ci/buildbot.nix...\n")
         ci_buildbot_defn_exists = yield self.pathExists('build/.ci/buildbot.nix')
@@ -514,7 +538,8 @@ class NixConfigure(buildstep.CommandMixin, steps.BuildStep):
                     make_job_evaluator(
                         "evaluate `.ci/buildbot.nix` jobs",
                         self.evaluator_settings,
-                        False
+                        False,
+                        incoming_ref_data
                     )
                 ]
             )
@@ -527,7 +552,8 @@ class NixConfigure(buildstep.CommandMixin, steps.BuildStep):
                     make_job_evaluator(
                         "evaluate `flake.nix` jobs",
                         self.evaluator_settings,
-                        True
+                        True,
+                        incoming_ref_data
                     )
                 ]
             )