fix(provider): ensure /root/.cache exists (#2134)

Fixes #2125
CRAFT-4037

Test failures are unrelated. I will be putting up a separate PR to fix
those. Reporter has confirmed that [the fix works for
them](#2125 (comment))

charmcraft/services/provider.py

@@ -126,10 +126,12 @@ def instance(
             try:
                 # Use /root/.cache even if we're in the snap.
                 instance.execute_run(
-                    ["rm", "-rf", "/root/snap/charmcraft/common/cache"]
+                    ["rm", "-rf", "/root/snap/charmcraft/common/cache"], check=True
                 )
+                instance.execute_run(["mkdir", "-p", "/root/.cache"], check=True)
                 instance.execute_run(
-                    ["ln", "-s", "/root/.cache", "/root/snap/charmcraft/common/cache"]
+                    ["ln", "-s", "/root/.cache", "/root/snap/charmcraft/common/cache"],
+                    check=True,
                 )
                 yield instance
             finally:

tests/integration/services/test_provider.py

@@ -114,3 +114,30 @@ def test_locked_cache_no_cache(
         )
 
         assert not (tmp_path / "cache_cached").exists()
+
+
+@pytest.mark.skipif(sys.platform == "win32", reason="no cache on windows")
+@pytest.mark.skipif(
+    sys.platform == "darwin", reason="multipass sometimes fails weirdly for this test"
+)
+def test_cache_symlink(
+    service_factory: services.CharmcraftServiceFactory,
+    tmp_path: pathlib.Path,
+    default_build_info: BuildInfo,
+    emitter: RecordingEmitter,
+):
+    cache_path = tmp_path / "cache"
+    cache_path.mkdir()
+    provider = service_factory.provider
+    provider_kwargs = {
+        "build_info": default_build_info,
+        "work_dir": tmp_path,
+        "cache_path": cache_path,
+    }
+    with provider.instance(**provider_kwargs) as instance:
+        instance.execute_run(["test", "-d", "/root/.cache"], check=True)
+        instance.execute_run(
+            ["test", "-d", "/root/snap/charmcraft/common/cache"], check=True
+        )
+        with pytest.raises(subprocess.CalledProcessError):
+            instance.execute_run(["test", "-d", "/blorp"], check=True)