run-cache: http: stage cache uploads in http remotes

Currently, there is no way to find run-cache in HTTP remotes.
In the future, we could use directory-listings from supported
webservers to find files.
Temporarily, we are restricting the run-cache uploads. This is a bit
strict, we could have only restricted the downloads, but it might be
more confusing to the users, so I went to categorically restrict
stage cache uploads and downloads for http filesystems.

dvc/stage/cache.py

@@ -22,6 +22,10 @@ class RunCacheNotFoundError(DvcException):
         super().__init__(f"No run-cache for {stage.addressing}")
 
 
+class RunCacheNotSupported(DvcException):
+    pass
+
+
 def _get_cache_hash(cache, key=False):
     from dvc_objects.meta import Meta
 
@@ -213,7 +217,7 @@ class StageCache:
         cached_stage.checkout()
 
     def transfer(self, from_odb, to_odb):
-        from dvc.fs import LocalFileSystem
+        from dvc.fs import HTTPFileSystem, LocalFileSystem
         from dvc.fs.callbacks import Callback
 
         from_fs = from_odb.fs
@@ -221,6 +225,19 @@ class StageCache:
         func = _log_exceptions(fs.generic.copy)
         runs = from_fs.path.join(from_odb.fs_path, "runs")
 
+        http_odb = next(
+            (
+                odb
+                for odb in (from_odb, to_odb)
+                if isinstance(odb.fs, HTTPFileSystem)
+            ),
+            None,
+        )
+        if http_odb:
+            path = http_odb.fs_path
+            message = f"run-cache is not supported for http filesystem: {path}"
+            raise RunCacheNotSupported(message)
+
         ret = []
         if not from_fs.exists(runs):
             return ret

dvc/testing/test_remote.py

@@ -1,6 +1,9 @@
 import os
 import shutil
 
+import pytest
+
+from dvc.stage.cache import RunCacheNotSupported
 from dvc.utils.fs import remove
 
 
@@ -92,6 +95,7 @@ class TestRemote:
         status_dir = dvc.cloud.status(dir_hashes)
         _check_status(status_dir, ok=dir_hashes)
 
+    @pytest.mark.xfail(raises=RunCacheNotSupported, strict=False)
     def test_stage_cache_push_pull(self, tmp_dir, dvc, remote):
         tmp_dir.gen("foo", "foo")
         stage = dvc.stage.add(