backup: support multiple roots; remove "relative absolute path" nonsense

Support multiple roots in config file, not just one.
The absolute path stuff before would match against exclusions/inclusions
based on paths from the root dir, but that doesn't make sense when we
have multiple roots, and added needless complexity.

backup.py

@@ -23,7 +23,7 @@ def pstr(path: bytes) -> str:
     return path.decode(errors='backslashreplace')
 
 class Config:
-    root: bytes
+    roots: list[bytes]
     max_file_size: typing.Optional[int]
     one_file_system: bool
     exclude_caches: bool
@@ -35,7 +35,6 @@ class Config:
         # Read config
         with open(configfile, 'r') as f:
             config = yaml.safe_load(f)
-            self.root = config['root'].encode()
             self.one_file_system = config.get('one-file-system', False)
             self.exclude_caches = config.get('exclude-caches', False)
 
@@ -45,6 +44,14 @@ class Config:
             else:
                 self.max_file_size = None
 
+            raw = config.get('roots', '').encode().split(b'\n')
+            self.roots = []
+            for x in raw:
+                if not len(x):
+                    continue
+                self.roots.append(x)
+            self.roots.sort(key=len)
+
             def process_match_list(config_name):
                 raw = config.get(config_name, '').encode().split(b'\n')
                 pats = []
@@ -96,6 +103,7 @@ class Backup:
     def __init__(self, config: Config, dry_run: bool):
         self.config = config
         self.dry_run = dry_run
+        self.root_seen: dict[bytes, bool] = {}
 
         # All logged messages, with severity
         self.logs: list[tuple[str, str]] = []
@@ -114,28 +122,29 @@ class Backup:
 
     def run(self, outfile: typing.IO[bytes]):
         self.outfile = outfile
-        # Base should not end with a slash, but full path should
-        if self.config.root.endswith(b'/'):
-            base = self.config.root[:-1]
-            path = self.config.root
-        else:
-            base = self.config.root
-            path = self.config.root + b'/'
-        self.scan(base, path)
+        for root in self.config.roots:
+            if root in self.root_seen:
+                self.log('I', f"ignoring root, already seen: {pstr(root)}")
+                continue
 
-    def scan(self, base: bytes, path: bytes,
-             parent_st: os.stat_result=None):
+            try:
+                st = os.lstat(root)
+                if not stat.S_ISDIR(st.st_mode):
+                    raise NotADirectoryError
+            except FileNotFoundError:
+                self.log('W', f"ignoring root, does not exist: {pstr(root)}")
+                continue
+            except NotADirectoryError:
+                self.log('W', f"ignoring root, not a directory: {pstr(root)}")
+                continue
+
+            self.log('I', f"processing root {pstr(root)}")
+            self.scan(root)
+
+    def scan(self, path: bytes, parent_st: os.stat_result=None):
         """If the given path should be backed up, print it.  If it's
            a directory and its contents should be included, recurse.
         """
-
-        if base.endswith(b'/'):
-            raise Exception("base must not end with /")
-        relpath = path[len(base):]
-        if not relpath.startswith(b'/'):
-            raise Exception(f"relative path (from {repr(base)}, {repr(path)})"
-                            + f" must start with /")
-
         try:
             st = os.lstat(path)
             is_dir = stat.S_ISDIR(st.st_mode)
@@ -187,6 +196,11 @@ class Backup:
             # Process directories
             if is_dir:
 
+                if path in self.config.roots:
+                    self.root_seen[path] = True
+                if decorated_path in self.config.roots:
+                    self.root_seen[decorated_path] = True
+
                 # Skip if it contains CACHEDIR.TAG
                 # (mirroring the --exclude-caches borg option)
                 if self.config.exclude_caches:
@@ -203,8 +217,7 @@ class Backup:
                 # Recurse
                 with os.scandir(path) as it:
                     for entry in it:
-                        self.scan(base=base, path=entry.path,
-                                  parent_st=st)
+                        self.scan(path=entry.path, parent_st=st)
 
         except PermissionError as e:
             self.log('E', f"can't read {pstr(path)}")

config.yaml

@@ -1,4 +1,13 @@
-root: "/"
+# List multiple roots, in case they come from different file systems.
+# Any paths already included by another root will be excluded, so it's
+# OK if these paths actually live on the same filesystem.
+roots: |
+  /
+  /boot
+  /efi
+  /usr
+  /var
+
 one-file-system: true
 exclude-caches: true
 
@@ -9,7 +18,6 @@ exclude-caches: true
 max-file-size: 500MiB
 
 # Files/dirs to exclude from backup.
-# Absolute paths here start at the root directory.
 # Relative paths are treated as if starting with **/
 # Paths ending in / will only match directories.
 exclude: |