hashindex: add NSIndex1 write support, fix iteritems

to get the unit tests working, we better have write support for the
borg 1.x format.

iteritems grew support for the marker kwarg.

src/borg/hashindex.pyx

@@ -116,11 +116,16 @@ NSIndex1Entry = namedtuple('NSIndex1bEntry', 'segment offset')
 class NSIndex1:  # legacy borg 1.x
 
     MAX_VALUE = 2**32 - 1  # borghash has the full uint32_t range
+    MAGIC = b"BORG_IDX"  # borg 1.x
+    HEADER_FMT = "<8sIIBB"  # magic, entries, buckets, ksize, vsize
+    VALUE_FMT = "<II"  # borg 1.x on-disk: little-endian segment, offset
+    KEY_SIZE = 32
+    VALUE_SIZE = 8
 
     def __init__(self, capacity=1000, path=None, permit_compact=False, usable=None):
         if usable is not None:
             capacity = usable * 2  # load factor 0.5
-        self.ht = _borghash.HashTableNT(key_size=32, value_format="<II", namedtuple_type=NSIndex1Entry, capacity=capacity)
+        self.ht = _borghash.HashTableNT(key_size=self.KEY_SIZE, value_format=self.VALUE_FMT, namedtuple_type=NSIndex1Entry, capacity=capacity)
         if path:
             self._read(path)
 
@@ -155,8 +160,13 @@ class NSIndex1:  # legacy borg 1.x
                 return default
             raise
 
-    def iteritems(self):
-        yield from self.ht.iteritems()
+    def iteritems(self, marker=None):
+        do_yield = marker is None
+        for key, value in self.ht.iteritems():
+            if do_yield:
+                yield key, value
+            else:
+                do_yield = key == marker
 
     def compact(self):
         return 0
@@ -168,12 +178,16 @@ class NSIndex1:  # legacy borg 1.x
     def read(cls, path, permit_compact=False):
         return cls(path=path)
 
-    def write(self, path):
-        self.ht.write(path)  # only for unit tests
-
     def size(self):
         return self.ht.size()  # not quite correct as this is not the on-disk read-only format.
 
+    def write(self, path):
+        if isinstance(path, str):
+            with open(path, 'wb') as fd:
+                self._write_fd(fd)
+        else:
+            self._write_fd(path)
+
     def _read(self, path):
         if isinstance(path, str):
             with open(path, 'rb') as fd:
@@ -181,27 +195,28 @@ class NSIndex1:  # legacy borg 1.x
         else:
             self._read_fd(path)
 
+    def _write_fd(self, fd):
+        used = len(self.ht)
+        header_bytes = struct.pack(self.HEADER_FMT, self.MAGIC, used, used, self.KEY_SIZE, self.VALUE_SIZE)
+        fd.write(header_bytes)
+        count = 0
+        for key, _ in self.ht.iteritems():
+            value = self.ht._get_raw(key)
+            fd.write(key)
+            fd.write(value)
+            count += 1
+        assert count == used
+
     def _read_fd(self, fd):
-        magic = fd.read(8)
-        fd.seek(0)
-        if magic == b"BORG_IDX":  # used for borg transfer borg 1.x -> borg 2
-            self._read_fd_borg1(fd)
-        if magic == b"BORGHASH":  # only for unit tests
-            self.ht = _borghash.HashTableNT.read(fd)
-
-    def _read_fd_borg1(self, fd):
-        MAGIC = b"BORG_IDX"  # borg 1.x
-        HEADER_FMT = "<8sIIBB"  # magic, entries, buckets, ksize, vsize
-        header_size = struct.calcsize(HEADER_FMT)
+        header_size = struct.calcsize(self.HEADER_FMT)
         header_bytes = fd.read(header_size)
         if len(header_bytes) < header_size:
             raise ValueError(f"Invalid file, file is too short (header).")
-        magic, entries, buckets, ksize, vsize = struct.unpack(HEADER_FMT, header_bytes)
-        if magic != MAGIC:
-            raise ValueError(f"Invalid file, magic {MAGIC.decode()} not found.")
-        VALUE_FMT = "<II"  # borg 1.x on-disk: little-endian segment, offset
-        assert ksize == 32, "invalid key size"
-        assert vsize == struct.calcsize(VALUE_FMT), "invalid value size"
+        magic, entries, buckets, ksize, vsize = struct.unpack(self.HEADER_FMT, header_bytes)
+        if magic != self.MAGIC:
+            raise ValueError(f"Invalid file, magic {self.MAGIC.decode()} not found.")
+        assert ksize == self.KEY_SIZE, "invalid key size"
+        assert vsize == self.VALUE_SIZE, "invalid value size"
         buckets_size = buckets * (ksize + vsize)
         current_pos = fd.tell()
         end_of_file = fd.seek(0, os.SEEK_END)