Restore detection of missing terminado package

* Restore detection of missing terminado package

* Properly handle terminals_enabled config

* Disambiguate terminado availability from web settings

* Further clarity on terminal availability

* Rename terminals_in_use back to terminals_available

Author: kevin-bates

jupyter_server/serverapp.py

@@ -83,7 +83,6 @@
 from .auth.login import LoginHandler
 from .auth.logout import LogoutHandler
 from .base.handlers import FileFindHandler
-from .terminal import TerminalManager
 
 from traitlets.config import Config
 from traitlets.config.application import catch_config_error, boolean_flag
@@ -117,6 +116,13 @@
 from jupyter_server.extension.config import ExtensionConfigManager
 from jupyter_server.traittypes import TypeFromClasses
 
+# Tolerate missing terminado package.
+try:
+    from .terminal import TerminalManager
+    terminado_available = True
+except ImportError:
+    terminado_available = False
+
 #-----------------------------------------------------------------------------
 # Module globals
 #-----------------------------------------------------------------------------
@@ -285,7 +291,7 @@ def init_settings(self, jupyter_app, kernel_manager, contents_manager,
             allow_password_change=jupyter_app.allow_password_change,
             server_root_dir=root_dir,
             jinja2_env=env,
-            terminals_available=False,  # Set later if terminals are available
+            terminals_available=terminado_available and jupyter_app.terminals_enabled,
             serverapp=jupyter_app
         )
 
@@ -588,8 +594,10 @@ class ServerApp(JupyterApp):
     classes = [
             KernelManager, Session, MappingKernelManager, KernelSpecManager, AsyncMappingKernelManager,
             ContentsManager, FileContentsManager, AsyncContentsManager, AsyncFileContentsManager, NotebookNotary,
-            TerminalManager, GatewayKernelSpecManager, GatewaySessionManager, GatewayClient
+            GatewayKernelManager, GatewayKernelSpecManager, GatewaySessionManager, GatewayClient
         ]
+    if terminado_available:  # Only necessary when terminado is available
+        classes.append(TerminalManager)
 
     subcommands = dict(
         list=(JupyterServerListApp, JupyterServerListApp.description.splitlines()[0]),
@@ -1330,6 +1338,15 @@ def _update_server_extensions(self, change):
          is not available.
          """))
 
+    # Since use of terminals is also a function of whether the terminado package is
+    # available, this variable holds the "final indication" of whether terminal functionality
+    # should be considered (particularly during shutdown/cleanup).  It is enabled only
+    # once both the terminals "service" can be initialized and terminals_enabled is True.
+    # Note: this variable is slightly different from 'terminals_available' in the web settings
+    # in that this variable *could* remain false if terminado is available, yet the terminal
+    # service's initialization still fails.  As a result, this variable holds the truth.
+    terminals_available = False
+
     authenticate_prometheus = Bool(
         True,
         help=""""
@@ -1548,7 +1565,7 @@ def init_terminals(self):
         try:
             from .terminal import initialize
             initialize(server_app=self)
-            self.web_app.settings['terminals_available'] = True
+            self.terminals_available = True
         except ImportError as e:
             self.log.warning(_i18n("Terminals not available (error was %s)"), e)
 
@@ -1694,11 +1711,8 @@ def shutdown_no_activity(self):
         if len(km) != 0:
             return   # Kernels still running
 
-        try:
+        if self.terminals_available:
             term_mgr = self.web_app.settings['terminal_manager']
-        except KeyError:
-            pass  # Terminals not enabled
-        else:
             if term_mgr.terminals:
                 return   # Terminals still running
 
@@ -1853,11 +1867,10 @@ def cleanup_terminals(self):
         The terminals will shutdown themselves when this process no longer exists,
         but explicit shutdown allows the TerminalManager to cleanup.
         """
-        try:
-            terminal_manager = self.web_app.settings['terminal_manager']
-        except KeyError:
-            return  # Terminals not enabled
+        if not self.terminals_available:
+            return
 
+        terminal_manager = self.web_app.settings['terminal_manager']
         n_terminals = len(terminal_manager.list())
         terminal_msg = trans.ngettext('Shutting down %d terminal', 'Shutting down %d terminals', n_terminals)
         self.log.info(terminal_msg % n_terminals)