fsmonitor: don't write out state if identity has changed (issue5581)

Inspired by the dirstate fix in dc7efa2826e4, this should fix any race
conditions with the fsmonitor state changing from underneath.

Since we now grab the wlock for any non-invalidate writes, the only situation
this appears to happen in is with a concurrent invalidation. Test that.

hgext/fsmonitor/state.py

@@ -13,7 +13,10 b' import socket'
 import struct
 
 from mercurial.i18n import _
-from mercurial import pathutil
+from mercurial import (
+    pathutil,
+    util,
+)
 
 _version = 4
 _versionformat = ">I"
@@ -24,6 +27,7 b' class state(object):'
         self._ui = repo.ui
         self._rootdir = pathutil.normasprefix(repo.root)
         self._lastclock = None
+        self._identity = util.filestat(None)
 
         self.mode = self._ui.config('fsmonitor', 'mode', default='on')
         self.walk_on_invalidate = self._ui.configbool(
@@ -35,10 +39,13 b' class state(object):'
         try:
             file = self._vfs('fsmonitor.state', 'rb')
         except IOError as inst:
+            self._identity = util.filestat(None)
             if inst.errno != errno.ENOENT:
                 raise
             return None, None, None
 
+        self._identity = util.filestat.fromfp(file)
+
         versionbytes = file.read(4)
         if len(versionbytes) < 4:
             self._ui.log(
@@ -90,8 +97,16 b' class state(object):'
             self.invalidate()
             return
 
+        # Read the identity from the file on disk rather than from the open file
+        # pointer below, because the latter is actually a brand new file.
+        identity = util.filestat.frompath(self._vfs.join('fsmonitor.state'))
+        if identity != self._identity:
+            self._ui.debug('skip updating fsmonitor.state: identity mismatch\n')
+            return
+
         try:
-            file = self._vfs('fsmonitor.state', 'wb', atomictemp=True)
+            file = self._vfs('fsmonitor.state', 'wb', atomictemp=True,
+                checkambig=True)
         except (IOError, OSError):
             self._ui.warn(_("warning: unable to write out fsmonitor state\n"))
             return
@@ -111,6 +126,7 b' class state(object):'
         except OSError as inst:
             if inst.errno != errno.ENOENT:
                 raise
+        self._identity = util.filestat(None)
 
     def setlastclock(self, clock):
         self._lastclock = clock

mercurial/util.py

@@ -1519,6 +1519,11 b' class filestat(object):'
             stat = None
         return cls(stat)
 
+    @classmethod
+    def fromfp(cls, fp):
+        stat = os.fstat(fp.fileno())
+        return cls(stat)
+
     __hash__ = object.__hash__
 
     def __eq__(self, old):

tests/test-dirstate-race.t

@@ -160,6 +160,34 b' treated differently in _checklookup() ac'
 
   $ rm b
 
+#if fsmonitor
+
+Create fsmonitor state.
+
+  $ hg status
+  $ f --type .hg/fsmonitor.state
+  .hg/fsmonitor.state: file
+
+Test that invalidating fsmonitor state in the middle (which doesn't require the
+wlock) causes the fsmonitor update to be skipped.
+hg debugrebuilddirstate ensures that the dirstaterace hook will be called, but
+it also invalidates the fsmonitor state. So back it up and restore it.
+
+  $ mv .hg/fsmonitor.state .hg/fsmonitor.state.tmp
+  $ hg debugrebuilddirstate
+  $ mv .hg/fsmonitor.state.tmp .hg/fsmonitor.state
+
+  $ cat > $TESTTMP/dirstaterace.sh <<EOF
+  > rm .hg/fsmonitor.state
+  > EOF
+
+  $ hg status --config extensions.dirstaterace=$TESTTMP/dirstaterace.py --debug
+  skip updating fsmonitor.state: identity mismatch
+  $ f .hg/fsmonitor.state
+  .hg/fsmonitor.state: file not found
+
+#endif
+
 Set up a rebase situation for issue5581.
 
   $ echo c2 > a