fix: initialise provider cache in same process as running TF

This ensures that they're executed on the same machine.
As separate processes, a remote execution might execute these on different machines.
As well, it's possible that locally the mkdir is cached even  if it needs to be run again

src/python/pants/backend/terraform/goals/deploy_test.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import json
+import shlex
 
 import pytest
 
@@ -16,6 +17,14 @@
 standard_deployment = standard_deployment
 
 
+def _extract_terraform_argv_from_script(script_argv) -> tuple[str, ...]:
+    """Extract the argv of the invocation of Terraform from its launcher script."""
+    script = script_argv[2]
+    tf_invocation = script.split("&&")
+    argv = shlex.split(tf_invocation[-1])
+    return tuple(argv)
+
+
 def test_run_terraform_deploy(rule_runner: RuleRunner, standard_deployment, tmpdir) -> None:
     """Test end-to-end running of a deployment."""
     rule_runner.write_files(standard_deployment.files)
@@ -44,7 +53,7 @@ def test_deploy_terraform_forwards_args(rule_runner: RuleRunner, standard_deploy
     deploy_process = rule_runner.request(DeployProcess, [field_set])
     assert deploy_process.process
 
-    argv = deploy_process.process.process.argv
+    argv = _extract_terraform_argv_from_script(deploy_process.process.process.argv)
 
     assert "-chdir=src/tf" in argv, "Did not find expected -chdir"
     assert "-var-file=stg.tfvars" in argv, "Did not find expected -var-file"
@@ -70,7 +79,7 @@ def test_deploy_terraform_adheres_to_dry_run_flag(
     deploy_process = rule_runner.request(DeployProcess, [field_set])
     assert deploy_process.process
 
-    argv = deploy_process.process.process.argv
+    argv = _extract_terraform_argv_from_script(deploy_process.process.process.argv)
 
     assert action in argv, f"Expected {action} in argv"
     assert not_action not in argv, f"Did not expect {not_action} in argv"

src/python/pants/backend/terraform/tool.py

@@ -20,7 +20,7 @@
 import shlex
 from dataclasses import dataclass
 from pathlib import Path
-from typing import Tuple
+from typing import Iterable, Tuple
 
 from pants.core.goals.resolves import ExportableTool
 from pants.core.util_rules import external_tool
@@ -29,12 +29,12 @@
     ExternalToolRequest,
     TemplatedExternalTool,
 )
-from pants.core.util_rules.system_binaries import MkdirBinary
+from pants.core.util_rules.system_binaries import BashBinary, MkdirBinary
 from pants.engine.env_vars import EXTRA_ENV_VARS_USAGE_HELP, EnvironmentVars, EnvironmentVarsRequest
 from pants.engine.fs import EMPTY_DIGEST, Digest
 from pants.engine.internals.selectors import Get
 from pants.engine.platform import Platform
-from pants.engine.process import FallibleProcessResult, Process
+from pants.engine.process import Process
 from pants.engine.rules import collect_rules, rule
 from pants.engine.unions import UnionRule
 from pants.option.option_types import ArgsListOption, BoolOption, StrListOption
@@ -418,10 +418,17 @@ class TerraformProcess:
     chdir: str = "."  # directory for terraform's `-chdir` argument
 
 
+def _make_launcher_script(bash: BashBinary, commands: Iterable[Iterable[str]]) -> tuple[str, ...]:
+    """Assemble several command invocations into an inline launcher script, suitable for passing as
+    `Process(argv=(bash.path, "-c", script), ...)`"""
+    return (bash.path, "-c", " && ".join([shlex.join(command) for command in commands]))
+
+
 @rule
 async def setup_terraform_process(
     request: TerraformProcess,
     terraform: TerraformTool,
+    bash: BashBinary,
     mkdir: MkdirBinary,
     platform: Platform,
 ) -> Process:
@@ -453,18 +460,20 @@ def prepend_paths(paths: Tuple[str, ...]) -> Tuple[str, ...]:
         return tuple((Path(request.chdir) / path).as_posix() for path in paths)
 
     # Initialise the Terraform provider cache, since Terraform expects the directory to already exist.
-    await Get(
-        FallibleProcessResult,
-        Process(
-            argv=[mkdir.path, "-p", tf_plugin_cache_dir],
-            append_only_caches=terraform.append_only_caches,
-            description="initialise Terraform plugin cache dir",
-            level=LogLevel.DEBUG,
-        ),
-    )
+    initialize_provider_cache_cmd = (mkdir.path, "-p", tf_plugin_cache_dir)
+    run_terraform_cmd = (
+        "__terraform/terraform",
+        f"-chdir={shlex.quote(request.chdir)}",
+    ) + request.args
 
     return Process(
-        argv=("__terraform/terraform", f"-chdir={shlex.quote(request.chdir)}") + request.args,
+        argv=_make_launcher_script(
+            bash,
+            (
+                initialize_provider_cache_cmd,
+                run_terraform_cmd,
+            ),
+        ),
         input_digest=request.input_digest,
         immutable_input_digests=immutable_input_digests,
         output_files=prepend_paths(request.output_files),