unify scanning and listing of segment dirs / segment files and apply good practices

+ os.scandir instead of os.listdir
  Improved speed and added flexibility with attributes (name, path, is_dir(), is_file())
+ use is_dir / is_file to make sure  we're reading only dirs / files respectively
+ Filtering to particular start, end index range built in
+ Move value bounds of segment (index) into constants module and use them instead

Resolves #7597

(forward patch from commits c9f35a16e9bf9e7073c486553177cef79ff1cb06^..edb5e749f512b7737b6933e13b7e61fefcd17bcb)

src/borg/constants.py

@@ -75,6 +75,12 @@
 
 FD_MAX_AGE = 4 * 60  # 4 minutes
 
+# Some bounds on segment / segment_dir indexes
+MIN_SEGMENT_INDEX = 0
+MAX_SEGMENT_INDEX = 2**32 - 1
+MIN_SEGMENT_DIR_INDEX = 0
+MAX_SEGMENT_DIR_INDEX = 2**32 - 1
+
 # chunker algorithms
 CH_BUZHASH = "buzhash"
 CH_FIXED = "fixed"

src/borg/repository.py

@@ -1376,39 +1376,51 @@ def _close_fd(self, ts_fd):
         safe_fadvise(fd.fileno(), 0, 0, "DONTNEED")
         fd.close()
 
+    def get_segment_dirs(self, data_dir, start_index=MIN_SEGMENT_DIR_INDEX, end_index=MAX_SEGMENT_DIR_INDEX):
+        """Returns generator yielding required segment dirs in data_dir as `os.DirEntry` objects.
+        Start and end are inclusive.
+        """
+        segment_dirs = (
+            f
+            for f in os.scandir(data_dir)
+            if f.is_dir() and f.name.isdigit() and start_index <= int(f.name) <= end_index
+        )
+        return segment_dirs
+
+    def get_segment_files(self, segment_dir, start_index=MIN_SEGMENT_INDEX, end_index=MAX_SEGMENT_INDEX):
+        """Returns generator yielding required segment files in segment_dir as `os.DirEntry` objects.
+        Start and end are inclusive.
+        """
+        segment_files = (
+            f
+            for f in os.scandir(segment_dir)
+            if f.is_file() and f.name.isdigit() and start_index <= int(f.name) <= end_index
+        )
+        return segment_files
+
     def segment_iterator(self, start_segment=None, end_segment=None, reverse=False):
         if start_segment is None:
-            start_segment = 0 if not reverse else 2**32 - 1
+            start_segment = MIN_SEGMENT_INDEX if not reverse else MAX_SEGMENT_INDEX
         if end_segment is None:
-            end_segment = 2**32 - 1 if not reverse else 0
+            end_segment = MAX_SEGMENT_INDEX if not reverse else MIN_SEGMENT_INDEX
         data_path = os.path.join(self.path, "data")
         start_segment_dir = start_segment // self.segments_per_dir
         end_segment_dir = end_segment // self.segments_per_dir
-        dirs = os.listdir(data_path)
         if not reverse:
-            dirs = [dir for dir in dirs if dir.isdigit() and start_segment_dir <= int(dir) <= end_segment_dir]
+            dirs = self.get_segment_dirs(data_path, start_index=start_segment_dir, end_index=end_segment_dir)
         else:
-            dirs = [dir for dir in dirs if dir.isdigit() and start_segment_dir >= int(dir) >= end_segment_dir]
-        dirs = sorted(dirs, key=int, reverse=reverse)
+            dirs = self.get_segment_dirs(data_path, start_index=end_segment_dir, end_index=start_segment_dir)
+        dirs = sorted(dirs, key=lambda dir: int(dir.name), reverse=reverse)
         for dir in dirs:
-            filenames = os.listdir(os.path.join(data_path, dir))
             if not reverse:
-                filenames = [
-                    filename
-                    for filename in filenames
-                    if filename.isdigit() and start_segment <= int(filename) <= end_segment
-                ]
+                files = self.get_segment_files(dir, start_index=start_segment, end_index=end_segment)
             else:
-                filenames = [
-                    filename
-                    for filename in filenames
-                    if filename.isdigit() and start_segment >= int(filename) >= end_segment
-                ]
-            filenames = sorted(filenames, key=int, reverse=reverse)
-            for filename in filenames:
+                files = self.get_segment_files(dir, start_index=end_segment, end_index=start_segment)
+            files = sorted(files, key=lambda file: int(file.name), reverse=reverse)
+            for file in files:
                 # Note: Do not filter out logically deleted segments  (see "File system interaction" above),
                 # since this is used by cleanup and txn state detection as well.
-                yield int(filename), os.path.join(data_path, dir, filename)
+                yield int(file.name), file.path
 
     def get_latest_segment(self):
         for segment, filename in self.segment_iterator(reverse=True):