mercurial/store.py

--- a/mercurial/store.py	Fri Mar 08 10:20:33 2019 -0800
+++ b/mercurial/store.py	Thu Nov 22 15:14:24 2018 +0300
@@ -8,6 +8,7 @@
 from __future__ import absolute_import
 
 import errno
+import functools
 import hashlib
 import os
 import stat
@@ -23,6 +24,9 @@
 )
 
 parsers = policy.importmod(r'parsers')
+# how much bytes should be read from fncache in one read
+# It is done to prevent loading large fncache files into memory
+fncache_chunksize = 10 ** 6
 
 def _matchtrackedpath(path, matcher):
     """parses a fncache entry and returns whether the entry is tracking a path
@@ -463,7 +467,20 @@
             # skip nonexistent file
             self.entries = set()
             return
-        self.entries = set(decodedir(fp.read()).splitlines())
+
+        self.entries = set()
+        chunk = b''
+        for c in iter(functools.partial(fp.read, fncache_chunksize), b''):
+            chunk += c
+            try:
+                p = chunk.rindex(b'\n')
+                self.entries.update(decodedir(chunk[:p + 1]).splitlines())
+                chunk = chunk[p + 1:]
+            except ValueError:
+                # substring '\n' not found, maybe the entry is bigger than the
+                # chunksize, so let's keep iterating
+                pass
+
         self._checkentries(fp)
         fp.close()