Do not dump the session data on the disk on every request.

Session object is updated, everytime a request is being served, and
that was forcing the session object dumped on the dist on every request.

On windows, it was causing issues on slower system on startup. Because -
windows file system locks the file, when it is opened by any
application. And, frequent requests on the pgAdmin main UI rendering
was causing issues, because of that.

In order to resolve the issue, we will not write the session data on
disk for every request, but - only after certain delay (in seconds),
from it was last written. It can be configured using the attribute
'PGADMIN_SESSION_DISK_WRITE_DELAY' in the configuration file,
default vaule for the delay is 10.
(i.e. 10 seconds)

web/pgadmin/utils/session.py

@@ -22,6 +22,7 @@ import hashlib
 import os
 import random
 import string
+import time
 from uuid import uuid4
 
 try:
@@ -56,6 +57,7 @@ class ManagedSession(CallbackDict, SessionMixin):
         self.new = new
         self.modified = False
         self.randval = randval
+        self.last_write = None
         self.hmac_digest = hmac_digest
 
     def sign(self, secret):
@@ -153,9 +155,10 @@ class CachingSessionManager(SessionManager):
 
 class FileBackedSessionManager(SessionManager):
 
-    def __init__(self, path, secret):
+    def __init__(self, path, secret, disk_write_delay):
         self.path = path
         self.secret = secret
+        self.disk_write_delay = disk_write_delay
         if not os.path.exists(self.path):
             os.makedirs(self.path)
 
@@ -212,10 +215,16 @@ class FileBackedSessionManager(SessionManager):
         )
 
     def put(self, session):
-        'Store a managed session'
+        """Store a managed session"""
+        current_time = time.time()
         if not session.hmac_digest:
             session.sign(self.secret)
+        else:
+            if session.last_write is not None \
+                    and (current_time - float(session.last_write)) < self.disk_write_delay:
+                return
 
+        session.last_write = current_time
         fname = os.path.join(self.path, session.sid)
         with open(fname, 'wb') as f:
             dump(
@@ -281,7 +290,8 @@ def create_session_interface(app, skip_paths=[]):
         CachingSessionManager(
             FileBackedSessionManager(
                 app.config['SESSION_DB_PATH'],
-                app.config['SECRET_KEY']
+                app.config['SECRET_KEY'],
+                app.config.get('PGADMIN_SESSION_DISK_WRITE_DELAY', 10)
             ),
             1000
         ), skip_paths,