WIP: Add nix backend

src/python/pants/backend/nix/goals/run.py

@@ -0,0 +1,64 @@
+import os
+
+from pants.backend.nix.target_types import NixBinaryExprField, NixBinaryRelativePathField
+from pants.core.goals.run import RunFieldSet, RunInSandboxBehavior, RunRequest
+from pants.core.util_rules.system_binaries import SystemBinariesSubsystem
+from pants.engine.fs import EMPTY_DIGEST
+from pants.engine.process import Process, ProcessResult
+from pants.engine.rules import Get, collect_rules, rule
+from pants.engine.target import WrappedTarget, WrappedTargetRequest
+
+
+class NixBinaryFieldSet(RunFieldSet):
+    required_fields = (
+        NixBinaryExprField,
+        NixBinaryRelativePathField,
+    )
+    run_in_sandbox_behavior = RunInSandboxBehavior.RUN_REQUEST_HERMETIC
+
+
+@rule
+async def run_nix_binary(
+    field_set: NixBinaryFieldSet,
+    system_binaries: SystemBinariesSubsystem.EnvironmentAware,
+) -> RunRequest:
+    wrapped_tgt = await Get(
+        WrappedTarget,
+        WrappedTargetRequest(field_set.address, description_of_origin="<infallible>"),
+    )
+    expr = wrapped_tgt.target[NixBinaryExprField].value
+    assert expr
+    rel_path = wrapped_tgt.target[NixBinaryRelativePathField].value
+    assert rel_path
+
+    # TODO search path
+    nix_instantiate = "/nix/var/nix/profiles/default/bin/nix-instantiate"
+    instantiate_result = await Get(
+        ProcessResult,
+        Process(
+            argv=(nix_instantiate, "--expr", expr),
+            description="Create nix .drv file",
+        ),
+    )
+    drv_path = instantiate_result.stdout.decode("utf-8").strip()
+
+    # TODO search path
+    nix_store = "/nix/var/nix/profiles/default/bin/nix-store"
+    realise_result = await Get(
+        ProcessResult,
+        Process(
+            argv=(nix_store, "--realise", drv_path),
+            description="Build nix derivation",
+        ),
+    )
+    derivation_dir = realise_result.stdout.decode("utf-8").strip()
+
+    binary_path = os.path.join(derivation_dir, rel_path)
+    return RunRequest(args=[binary_path], digest=EMPTY_DIGEST)
+
+
+def rules():
+    return [
+        *collect_rules(),
+        *NixBinaryFieldSet.rules(),
+    ]

src/python/pants/backend/nix/register.py

@@ -0,0 +1,10 @@
+from pants.backend.nix.goals import run
+from pants.backend.nix.target_types import NixBinaryTarget, NixSourceTarget
+
+
+def target_types():
+    return [NixBinaryTarget, NixSourceTarget]
+
+
+def rules():
+    return [*run.rules()]

src/python/pants/backend/nix/target_types.py

@@ -0,0 +1,50 @@
+from pants.engine.target import (
+    COMMON_TARGET_FIELDS,
+    Dependencies,
+    SingleSourceField,
+    StringField,
+    Target,
+)
+
+
+class NixSourceField(SingleSourceField):
+    pass
+
+
+class NixDependenciesField(Dependencies):
+    pass
+
+
+class NixSourceTarget(Target):
+    alias = "nix_source"
+    core_fields = (
+        *COMMON_TARGET_FIELDS,
+        NixSourceField,
+        NixDependenciesField,
+    )
+
+
+class NixBinaryExprField(StringField):
+    alias = "expr"
+    required = True  # TODO make expr optional
+    help = "Passed to nix-instantiate --expr."
+
+
+class NixBinaryRelativePathField(StringField):
+    alias = "rel_path"
+    required = True
+    help = "Binary path relative to derivation root."
+
+
+class NixBinaryDependencies(Dependencies):
+    pass
+
+
+class NixBinaryTarget(Target):
+    alias = "nix_binary"
+    core_fields = (
+        *COMMON_TARGET_FIELDS,
+        NixBinaryExprField,
+        NixBinaryDependencies,
+        NixBinaryRelativePathField,
+    )