avoid previous_location mismatch, fixes #1741

due to the changed canonicalization for relative pathes in PR #1711 (implement /./ relpath hack),
there would be a changed repo location warning and the user would be asked if this is ok.
this would break automation and require manual intervention, which is unwanted.

thus, we automatically fix the previous_location config entry, if it only changed in the
expected way, but still means the same location.

borg/cache.py

@@ -10,7 +10,7 @@ from .key import PlaintextKey
 from .logger import create_logger
 logger = create_logger()
 from .helpers import Error, get_cache_dir, decode_dict, int_to_bigint, \
-    bigint_to_int, format_file_size, yes, bin_to_hex
+    bigint_to_int, format_file_size, yes, bin_to_hex, Location
 from .locking import Lock
 from .hashindex import ChunkIndex
 
@@ -140,10 +140,7 @@ Chunk index:    {0.total_unique_chunks:20d} {0.total_chunks:20d}"""
         with open(os.path.join(self.path, 'files'), 'wb') as fd:
             pass  # empty file
 
-    def _do_open(self):
-        self.config = configparser.ConfigParser(interpolation=None)
-        config_path = os.path.join(self.path, 'config')
-        self.config.read(config_path)
+    def _check_upgrade(self, config_path):
         try:
             cache_version = self.config.getint('cache', 'version')
             wanted_version = 1
@@ -154,6 +151,25 @@ Chunk index:    {0.total_unique_chunks:20d} {0.total_chunks:20d}"""
         except configparser.NoSectionError:
             self.close()
             raise Exception('%s does not look like a Borg cache.' % config_path) from None
+        # borg < 1.0.8rc1 had different canonicalization for the repo location (see #1655 and #1741).
+        cache_loc = self.config.get('cache', 'previous_location', fallback=None)
+        if cache_loc:
+            repo_loc = self.repository._location.canonical_path()
+            rl = Location(repo_loc)
+            cl = Location(cache_loc)
+            if cl.proto == rl.proto and cl.user == rl.user and cl.host == rl.host and cl.port == rl.port \
+                    and \
+                    cl.path and rl.path and \
+                    cl.path.startswith('/~/') and rl.path.startswith('/./') and cl.path[3:] == rl.path[3:]:
+                # everything is same except the expected change in relative path canonicalization,
+                # update previous_location to avoid warning / user query about changed location:
+                self.config.set('cache', 'previous_location', repo_loc)
+
+    def _do_open(self):
+        self.config = configparser.ConfigParser(interpolation=None)
+        config_path = os.path.join(self.path, 'config')
+        self.config.read(config_path)
+        self._check_upgrade(config_path)
         self.id = self.config.get('cache', 'repository')
         self.manifest_id = unhexlify(self.config.get('cache', 'manifest'))
         self.timestamp = self.config.get('cache', 'timestamp', fallback=None)