Read only used bytes from MmapedDict files, not all the zeroes too

akx · akx · commit 0f544ebdd52e · 2019-06-06T14:08:31.000+03:00
Signed-off-by: Aarni Koskela &lt;akx@iki.fi&gt;
diff --git a/prometheus_client/mmap_dict.py b/prometheus_client/mmap_dict.py
@@ -81,8 +81,13 @@ def __init__(self, filename, read_mode=False):
     @staticmethod
     def read_all_values_from_file(filename):
         with open(filename, 'rb') as infp:
-            data = infp.read()
-        return _read_all_values(data)
+            # Read the first block of data, including the first 4 bytes which tell us
+            # how much of the file (which is preallocated to _INITIAL_MMAP_SIZE bytes) is occupied.
+            data = infp.read(65535)
+            used = _unpack_integer(data, 0)[0]
+            if used > len(data):  # Then read in the rest, if needed.
+                data += infp.read(used - len(data))
+        return _read_all_values(data, used)
 
     def _init_value(self, key):
         """Initialize a value. Lock must be held by caller."""
