Make docker_environment's Pants-provided-PBS an absolute path (pantsb…

…uild#20314)

Fixes pantsbuild#20313 by ensuring that the `PEX_PYTHON` used for Pex CLI
invocations inside a docker environment is an absolute path.

I also did some drive-by improvements of the test code.

src/python/pants/core/util_rules/adhoc_binaries.py

@@ -104,10 +104,11 @@ async def download_python_binary(
             cp -r python "{installation_root}"
             touch "{installation_root}/DONE"
         fi
+        echo "$(realpath "{installation_root}")/bin/python3"
     """
     )
 
-    await Get(
+    result = await Get(
         ProcessResult,
         Process(
             [bash_binary.path, "-c", installation_script],
@@ -123,7 +124,7 @@ async def download_python_binary(
         ),
     )
 
-    return _PythonBuildStandaloneBinary(f"{installation_root}/bin/python3")
+    return _PythonBuildStandaloneBinary(result.stdout.decode().splitlines()[-1].strip())
 
 
 @dataclass(frozen=True)

src/python/pants/core/util_rules/adhoc_binaries_test.py

@@ -6,14 +6,17 @@
 import pytest
 
 from pants.build_graph.address import Address
-from pants.core.target_types import FileTarget
 from pants.core.util_rules import adhoc_binaries
 from pants.core.util_rules.adhoc_binaries import (
-    PythonBuildStandaloneBinary,
     _DownloadPythonBuildStandaloneBinaryRequest,
     _PythonBuildStandaloneBinary,
 )
-from pants.core.util_rules.environments import EnvironmentTarget, LocalEnvironmentTarget
+from pants.core.util_rules.environments import (
+    DockerEnvironmentTarget,
+    EnvironmentTarget,
+    LocalEnvironmentTarget,
+)
+from pants.engine.environment import EnvironmentName
 from pants.testutil.rule_runner import MockGet, QueryRule, RuleRunner, run_rule_with_mocks
 
 
@@ -27,7 +30,7 @@ def rule_runner() -> RuleRunner:
                 [_DownloadPythonBuildStandaloneBinaryRequest],
             ),
         ],
-        target_types=[LocalEnvironmentTarget, FileTarget],
+        target_types=[LocalEnvironmentTarget, DockerEnvironmentTarget],
     )
 
 
@@ -47,32 +50,29 @@ def test_local(env_tgt) -> None:
     assert result == adhoc_binaries.PythonBuildStandaloneBinary(sys.executable)
 
 
-def test_docker_uses_helper() -> None:
-    result = run_rule_with_mocks(
-        adhoc_binaries.get_python_for_scripts,
-        rule_args=[EnvironmentTarget("docker", FileTarget({"source": ""}, address=Address("")))],
-        mock_gets=[
-            MockGet(
-                output_type=_PythonBuildStandaloneBinary,
-                input_types=(_DownloadPythonBuildStandaloneBinaryRequest,),
-                mock=lambda _: _PythonBuildStandaloneBinary(""),
-            )
+def test_docker_uses_helper(rule_runner: RuleRunner) -> None:
+    rule_runner = RuleRunner(
+        rules=[
+            *adhoc_binaries.rules(),
+            QueryRule(
+                _PythonBuildStandaloneBinary,
+                [_DownloadPythonBuildStandaloneBinaryRequest],
+            ),
         ],
+        target_types=[DockerEnvironmentTarget],
+        inherent_environment=EnvironmentName("docker"),
     )
-    assert result == PythonBuildStandaloneBinary("")
-
-
-def test_docker_helper(rule_runner: RuleRunner):
     rule_runner.write_files(
         {
-            "BUILD": "local_environment(name='local')",
+            "BUILD": "docker_environment(name='docker', image='ubuntu:latest')",
         }
     )
     rule_runner.set_options(
-        ["--environments-preview-names={'local': '//:local'}"], env_inherit={"PATH"}
+        ["--environments-preview-names={'docker': '//:docker'}"], env_inherit={"PATH"}
     )
     pbs = rule_runner.request(
         _PythonBuildStandaloneBinary,
         [_DownloadPythonBuildStandaloneBinaryRequest()],
     )
-    assert not pbs.path.startswith("/")
+    assert pbs.path.startswith("/pants-named-caches")
+    assert pbs.path.endswith("/bin/python3")