Refactor cache sync options and introduce new cache preference

Add new borg create option '--prefer-adhoc-cache' to prefer the
AdHocCache over the NewCache implementation.

Adjust a test to match the previous default behaviour (== use the
AdHocCache) with --no-cache-sync.

src/borg/archiver/create_cmd.py

@@ -224,8 +224,9 @@ class CreateMixIn:
                 manifest,
                 progress=args.progress,
                 lock_wait=self.lock_wait,
-                permit_adhoc_cache=args.no_cache_sync,
+                no_cache_sync_permitted=args.no_cache_sync,
-                force_adhoc_cache=args.no_cache_sync_forced,
+                no_cache_sync_forced=args.no_cache_sync_forced,
+                prefer_adhoc_cache=args.prefer_adhoc_cache,
                 cache_mode=args.files_cache_mode,
                 iec=args.iec,
             ) as cache:
@@ -802,13 +803,19 @@ class CreateMixIn:
             "--no-cache-sync",
             dest="no_cache_sync",
             action="store_true",
-            help="experimental: do not synchronize the cache. Implies not using the files cache.",
+            help="experimental: do not synchronize the chunks cache.",
         )
         subparser.add_argument(
             "--no-cache-sync-forced",
             dest="no_cache_sync_forced",
             action="store_true",
-            help="experimental: do not synchronize the cache (forced). Implies not using the files cache.",
+            help="experimental: do not synchronize the chunks cache (forced).",
+        )
+        subparser.add_argument(
+            "--prefer-adhoc-cache",
+            dest="prefer_adhoc_cache",
+            action="store_true",
+            help="experimental: prefer AdHocCache (w/o files cache) over NewCache (with files cache).",
         )
         subparser.add_argument(
             "--stdin-name",

src/borg/cache.py

@@ -350,8 +350,9 @@ class Cache:
         warn_if_unencrypted=True,
         progress=False,
         lock_wait=None,
-        permit_adhoc_cache=False,
+        no_cache_sync_permitted=False,
-        force_adhoc_cache=False,
+        no_cache_sync_forced=False,
+        prefer_adhoc_cache=False,
         cache_mode=FILES_CACHE_MODE_DISABLED,
         iec=False,
     ):
@@ -381,14 +382,14 @@ class Cache:
         def adhoc():
             return AdHocCache(manifest=manifest, lock_wait=lock_wait, iec=iec)
 
-        if force_adhoc_cache:
+        if no_cache_sync_forced:
-            return adhoc()
+            return adhoc() if prefer_adhoc_cache else newcache()
 
-        if not permit_adhoc_cache:
+        if not no_cache_sync_permitted:
             return local()
 
-        # ad-hoc cache may be permitted, but if the local cache is in sync it'd be stupid to invalidate
+        # no cache sync may be permitted, but if the local cache is in sync it'd be stupid to invalidate
-        # it by needlessly using the ad-hoc cache.
+        # it by needlessly using the AdHocCache or the NewCache.
         # Check if the local cache exists and is in sync.
 
         cache_config = CacheConfig(repository, path, lock_wait)
@@ -400,8 +401,12 @@ class Cache:
                 # Local cache is in sync, use it
                 logger.debug("Cache: choosing local cache (in sync)")
                 return local()
-        logger.debug("Cache: choosing ad-hoc cache (local cache does not exist or is not in sync)")
+        if prefer_adhoc_cache:
-        return adhoc()
+            logger.debug("Cache: choosing AdHocCache (local cache does not exist or is not in sync)")
+            return adhoc()
+        else:
+            logger.debug("Cache: choosing NewCache (local cache does not exist or is not in sync)")
+            return newcache()
 
 
 class CacheStatsMixin:

src/borg/testsuite/archiver/create_cmd.py

@@ -540,20 +540,20 @@ def test_create_pattern_intermediate_folders_first(archivers, request):
     assert out_list.index("d x/b") < out_list.index("- x/b/foo_b")
 
 
-def test_create_no_cache_sync(archivers, request):
+def test_create_no_cache_sync_adhoc(archivers, request):  # TODO: add test for NewCache
     archiver = request.getfixturevalue(archivers)
     create_test_files(archiver.input_path)
     cmd(archiver, "rcreate", RK_ENCRYPTION)
     cmd(archiver, "rdelete", "--cache-only")
     create_json = json.loads(
-        cmd(archiver, "create", "--no-cache-sync", "--json", "--error", "test", "input")
+        cmd(archiver, "create", "--no-cache-sync", "--prefer-adhoc-cache", "--json", "--error", "test", "input")
     )  # ignore experimental warning
     info_json = json.loads(cmd(archiver, "info", "-a", "test", "--json"))
     create_stats = create_json["cache"]["stats"]
     info_stats = info_json["cache"]["stats"]
     assert create_stats == info_stats
     cmd(archiver, "rdelete", "--cache-only")
-    cmd(archiver, "create", "--no-cache-sync", "test2", "input")
+    cmd(archiver, "create", "--no-cache-sync", "--prefer-adhoc-cache", "test2", "input")
     cmd(archiver, "rinfo")
     cmd(archiver, "check")