Added environment interpolation rule.

Allows rules/targets to easily support values that can be pulled from
the environment.

Alternative solution to pantsbuild#12797.

src/python/pants/engine/environment.py

@@ -16,6 +16,7 @@
 
 name_value_re = re.compile(r"([A-Za-z_]\w*)=(.*)")
 shorthand_re = re.compile(r"([A-Za-z_]\w*)")
+interpolation_re = re.compile(r"^\$\{([A-Za-z_]{1,}[A-Za-z0-9]{0,})\}$")
 
 
 class CompleteEnvironment(FrozenDict):
@@ -84,6 +85,21 @@ def __init__(self, requested: Sequence[str], allowed: Optional[Sequence[str]] =
         self.allowed = None if allowed is None else FrozenOrderedSet(allowed)
 
 
+class InterpolatedEnvironmentRequest(FrozenDict[str, str]):
+    """Request a map of key:values w/ simple environment variable interpolation.
+
+    e.g.
+
+        {
+            "FOO_BAR": "${FOO_BAR}",
+            "WIZ_BOING": "WIZ_BOING"
+        } == {
+            "FOO_BAR": os.getenv("FOO_BAR", None),
+            "WIZ_BOING": "WIZ_BOING",
+        }
+    """
+
+
 class Environment(FrozenDict[str, str]):
     """A subset of the variables set in the environment."""
 
@@ -105,5 +121,26 @@ def environment_subset(session_values: SessionValues, request: EnvironmentReques
     )
 
 
+@rule
+def interpolated_subset(
+    session_values: SessionValues, request: InterpolatedEnvironmentRequest
+) -> Environment:
+    env = session_values[CompleteEnvironment]
+    values = {}
+
+    for key, value in request.items():
+        match = interpolation_re.match(value)
+        if match:
+            env_key = match.groups()[0]
+            try:
+                values[key] = env[env_key]
+            except KeyError:
+                pass
+        else:
+            values[key] = value
+
+    return Environment(values)
+
+
 def rules():
     return collect_rules()

src/python/pants/engine/environment_test.py

@@ -5,7 +5,11 @@
 
 import pytest
 
-from pants.engine.environment import CompleteEnvironment
+from pants.engine.environment import (
+    CompleteEnvironment,
+    InterpolatedEnvironmentRequest,
+    interpolated_subset,
+)
 
 
 @pytest.mark.parametrize(
@@ -37,3 +41,22 @@ def test_invalid_variable() -> None:
         "An invalid variable was requested via the --test-extra-env-var mechanism: 3INVALID"
         in str(exc)
     )
+
+
+@pytest.mark.parametrize(
+    "input_dict, environment, expected",
+    [
+        ({"foo": "FOO"}, {}, {"foo": "FOO"}),
+        ({"foo": "$FOO"}, {}, {"foo": "$FOO"}),
+        ({"foo": "${FOO}"}, {}, {}),
+        ({"foo": "${FOO}"}, {"FOO": "bar"}, {"foo": "bar"}),
+        ({"foo": "${FOO}", "bar": "BAR"}, {"FOO": "bar"}, {"foo": "bar", "bar": "BAR"}),
+    ],
+)
+def test_interpolated_subset(input_dict, environment, expected):
+    session_values = {CompleteEnvironment: environment}
+    request = InterpolatedEnvironmentRequest(input_dict)
+
+    subset = interpolated_subset(session_values, request)
+
+    assert dict(subset) == expected