(fix) site_cache_dir() must return DVC_SITE_CACHE_DIR if it exists (#10776)

* DVC_SITE_CACHE_DIR must be sent first, deal with platformdirs afterwards

* Test should check default behaviour when no env var is set

* Add test: site_cache_dir() must return the env var if set

dvc/dirs.py

@@ -24,6 +24,9 @@ def site_cache_dir():
     from platformdirs import PlatformDirs
     from platformdirs.unix import Unix
 
+    if dvc_site_cache_dir := os.getenv(env.DVC_SITE_CACHE_DIR):
+        return dvc_site_cache_dir
+
     if issubclass(Unix, PlatformDirs):
         # Return the cache directory shared by users, e.g. `/var/tmp/$appname`
         # NOTE: platformdirs>=5 changed `site_cache_dir` to return /var/cache/$appname.
@@ -34,6 +37,4 @@ def site_cache_dir():
         #        In the future, consider migrating to a more secure directory.
         return f"/var/tmp/{APPNAME}"  # noqa: S108
 
-    return os.getenv(env.DVC_SITE_CACHE_DIR) or platformdirs.site_cache_dir(
-        APPNAME, APPAUTHOR, opinion=True
-    )
+    return platformdirs.site_cache_dir(APPNAME, APPAUTHOR, opinion=True)

tests/unit/test_dirs.py

@@ -3,7 +3,7 @@ import sys
 import pytest
 
 from dvc.dirs import global_config_dir, site_cache_dir
-from dvc.env import DVC_GLOBAL_CONFIG_DIR
+from dvc.env import DVC_GLOBAL_CONFIG_DIR, DVC_SITE_CACHE_DIR
 
 
 def test_global_config_dir_respects_env_var(monkeypatch):
@@ -13,5 +13,11 @@ def test_global_config_dir_respects_env_var(monkeypatch):
 
 
 @pytest.mark.skipif(sys.platform != "linux", reason="Only for Unix platforms")
-def test_site_cache_dir_on_unix():
+def test_site_cache_dir_on_unix(monkeypatch):
+    monkeypatch.delenv(DVC_SITE_CACHE_DIR, raising=False)
     assert site_cache_dir() == "/var/tmp/dvc"
+
+
+def test_site_cache_dir_env_var(monkeypatch):
+    monkeypatch.setenv(DVC_SITE_CACHE_DIR, "foo_bar")
+    assert site_cache_dir() == "foo_bar"