Use cheroot as default production server for pgAdmin4. Fixes #5017

If DEBUG is true then we used the default flask server.

docs/en_US/release_notes_4_17.rst

@@ -14,6 +14,7 @@ New features
 Housekeeping
 ************
 
+| `Issue #5017 <https://redmine.postgresql.org/issues/5017>`_ -  Use cheroot as default production server for pgAdmin4.
 
 Bug fixes
 *********

requirements.txt

@@ -15,6 +15,7 @@
 #       ignored when building a PIP Wheel.
 ##############################################################################
 blinker==1.4
+cheroot==8.2.1
 Flask==1.0.2
 Werkzeug>=0.15.0
 Flask-Gravatar==0.5.0

web/pgAdmin4.py

@@ -13,6 +13,7 @@ to start a web server."""
 
 import os
 import sys
+from cheroot.wsgi import Server as CherootServer
 
 if sys.version_info[0] >= 3:
     import builtins
@@ -190,15 +191,29 @@ if __name__ == '__main__':
     # Reference:
     # https://github.com/pallets/werkzeug/issues/220#issuecomment-11176538
     try:
-        app.run(
-            host=config.DEFAULT_SERVER,
-            port=server_port,
-            use_reloader=(
-                (not PGADMIN_RUNTIME) and app.debug and
-                os.environ.get("WERKZEUG_RUN_MAIN") is not None
-            ),
-            threaded=config.THREADED_MODE
-        )
-
+        if config.DEBUG:
+            app.run(
+                host=config.DEFAULT_SERVER,
+                port=server_port,
+                use_reloader=(
+                    (not PGADMIN_RUNTIME) and app.debug and
+                    os.environ.get("WERKZEUG_RUN_MAIN") is not None
+                ),
+                threaded=config.THREADED_MODE
+            )
+        else:
+            # Can use cheroot instead of flask dev server when not in debug
+            # 10 is default thread count in CherootServer
+            num_threads = 10 if config.THREADED_MODE else 1
+            prod_server = CherootServer(
+                (config.DEFAULT_SERVER, server_port),
+                wsgi_app=app,
+                numthreads=num_threads,
+                server_name=config.APP_NAME)
+            try:
+                print("Using production server...")
+                prod_server.start()
+            except KeyboardInterrupt:
+                prod_server.stop()
     except IOError:
         app.logger.error("Error starting the app server: %s", sys.exc_info())