create module-specific provider subdir before running

Terraform expects the directory to already exist and will not create it.

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

@@ -29,11 +29,12 @@
     ExternalToolRequest,
     TemplatedExternalTool,
 )
+from pants.core.util_rules.system_binaries import 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 Process
+from pants.engine.process import Process, FallibleProcessResult
 from pants.engine.rules import collect_rules, rule
 from pants.engine.unions import UnionRule
 from pants.option.option_types import ArgsListOption, BoolOption, StrListOption
@@ -451,6 +452,14 @@ async def setup_terraform_process(
     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,
+    ))
+
     return Process(
         argv=("__terraform/terraform", f"-chdir={shlex.quote(request.chdir)}") + request.args,
         input_digest=request.input_digest,