avoid race condition in profile creation

ignores EEXIST, which can happen when two processes
are starting up at the same time on the same new profile.

Centralizes some of the mkdir code in a new method,
reducing some duplication.

IPython/core/profiledir.py

@@ -88,21 +88,54 b' class ProfileDir(LoggingConfigurable):'
     def _log_dir_changed(self, name, old, new):
         self.check_log_dir()
 
+    def _mkdir(self, path, mode=None):
+        """ensure a directory exists at a given path
+        
+        This is a version of os.mkdir, with the following differences:
+        
+        - returns True if it created the directory, False otherwise
+        - ignores EEXIST, protecting against race conditions where
+          the dir may have been created in between the check and
+          the creation
+        - sets permissions if requested and the dir already exists
+        """
+        if os.path.exists(path):
+            if mode and os.stat(path).st_mode != mode:
+                try:
+                    os.chmod(path, mode)
+                except OSError:
+                    self.log.warn(
+                        "Could not set permissions on %s",
+                        path
+                    )
+            return False
+        try:
+            if mode:
+                os.mkdir(path, mode)
+            else:
+                os.mkdir(path)
+        except OSError as e:
+            if e.errno == errno.EEXIST:
+                return False
+            else:
+                raise
+        
+        return True
+
     def check_log_dir(self):
-        if not os.path.isdir(self.log_dir):
-            os.mkdir(self.log_dir)
+        self._mkdir(self.log_dir)
 
     def _startup_dir_changed(self, name, old, new):
         self.check_startup_dir()
 
     def check_startup_dir(self):
-        if not os.path.isdir(self.startup_dir):
-            os.mkdir(self.startup_dir)
+        self._mkdir(self.startup_dir)
+
         readme = os.path.join(self.startup_dir, 'README')
         src = os.path.join(get_ipython_package_dir(), u'config', u'profile', u'README_STARTUP')
 
         if not os.path.exists(src):
-            self.log.warn("Could not copy README_STARTUP to startup dir. Source file %s does not exist." % src)
+            self.log.warn("Could not copy README_STARTUP to startup dir. Source file %s does not exist.", src)
 
         if os.path.exists(src) and not os.path.exists(readme):
             shutil.copy(src, readme)
@@ -111,25 +144,13 b' class ProfileDir(LoggingConfigurable):'
         self.check_security_dir()
 
     def check_security_dir(self):
-        if not os.path.isdir(self.security_dir):
-            os.mkdir(self.security_dir, 0o700)
-        else:
-            try:
-                os.chmod(self.security_dir, 0o700)
-            except OSError:
-                self.log.warn("Could not set security dir permissions to private.")
+        self._mkdir(self.security_dir, 0o40700)
 
     def _pid_dir_changed(self, name, old, new):
         self.check_pid_dir()
 
     def check_pid_dir(self):
-        if not os.path.isdir(self.pid_dir):
-            os.mkdir(self.pid_dir, 0o700)
-        else:
-            try:
-                os.chmod(self.pid_dir, 0o700)
-            except OSError:
-                self.log.warn("Could not set pid dir permissions to private.")
+        self._mkdir(self.pid_dir, 0o40700)
 
     def check_dirs(self):
         self.check_security_dir()