store: append to fncache if there are only new files to write

Before this patch, if we have to add a new entry to fncache, we write the whole
fncache again which slows things down on large fncache which have millions of
entries. Addition of a new entry is common operation while pulling new files or
commiting a new file.

This patch adds a new fncache.addls set which keeps track of the additions
happening and store them. When we write the fncache, we will just read the addls
set and append those entries at the end of fncache.
We make sure that the entries are new entries by loading the fncache and making
sure entry does not exists there. In future if we can check if an entry is new
without loading the fncache, that will speed up things more.

Performance numbers for commiting a new file:

mercurial repo
before: 0.08784651756286621
after: 0.08474504947662354

mozilla-central
before: 1.83314049243927
after: 1.7054164409637451

netbeans
before: 0.7953150272369385
after: 0.7202838659286499

pypy
before: 0.17805707454681396
after: 0.13431048393249512

In our internal repo, the performance improvement is in seconds.

I have used octobus's ASV perf benchmark thing to get the above numbers. I also
see some minute perf improvements related to creating a new commit without a new
file, but I believe that's just some noise.

Differential Revision: https://phab.mercurial-scm.org/D5301

mercurial/store.py

@@ -451,6 +451,8 b' class fncache(object):'
         self.vfs = vfs
         self.entries = None
         self._dirty = False
+        # set of new additions to fncache
+        self.addls = set()
 
     def _load(self):
         '''fill the entries from the fncache file'''
@@ -479,17 +481,28 b' class fncache(object):'
                 fp.write(encodedir('\n'.join(self.entries) + '\n'))
             fp.close()
             self._dirty = False
+        if self.addls:
+            # if we have just new entries, let's append them to the fncache
+            tr.addbackup('fncache')
+            fp = self.vfs('fncache', mode='ab', atomictemp=True)
+            if self.addls:
+                fp.write(encodedir('\n'.join(self.addls) + '\n'))
+            fp.close()
+            self.entries = None
+            self.addls = set()
 
     def add(self, fn):
         if self.entries is None:
             self._load()
         if fn not in self.entries:
-            self._dirty = True
-            self.entries.add(fn)
+            self.addls.add(fn)
 
     def remove(self, fn):
         if self.entries is None:
             self._load()
+        if fn in self.addls:
+            self.addls.remove(fn)
+            return
         try:
             self.entries.remove(fn)
             self._dirty = True
@@ -497,6 +510,8 b' class fncache(object):'
             pass
 
     def __contains__(self, fn):
+        if fn in self.addls:
+            return True
         if self.entries is None:
             self._load()
         return fn in self.entries
@@ -504,7 +519,7 b' class fncache(object):'
     def __iter__(self):
         if self.entries is None:
             self._load()
-        return iter(self.entries)
+        return iter(self.entries | self.addls)
 
 class _fncachevfs(vfsmod.abstractvfs, vfsmod.proxyvfs):
     def __init__(self, vfs, fnc, encode):
@@ -580,6 +595,7 b' class fncachestore(basicstore):'
 
     def invalidatecaches(self):
         self.fncache.entries = None
+        self.fncache.addls = set()
 
     def markremoved(self, fn):
         self.fncache.remove(fn)