communicators/ssh: timeout on connection check

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 BUG FIXES:
 
+  - core: Fix cases where sometimes SSH connection would hang.
   - providers/docker: Create args works. [GH-4526]
   - providers/virtualbox: Show a human-friendly error if VirtualBox didn't
       clean up an existing VM. [GH-4681]

plugins/communicators/ssh/communicator.rb

@@ -265,8 +265,13 @@ module VagrantPlugins
           # There is a chance that the socket is closed despite us checking
           # 'closed?' above. To test this we need to send data through the
           # socket.
+          #
+          # We wrap the check itself in a 5 second timeout because there
+          # are some cases where this will just hang.
           begin
-            @connection.exec!("")
+            Timeout.timeout(5) do
+              @connection.exec!("")
+            end
           rescue Exception => e
             @logger.info("Connection errored, not re-using. Will reconnect.")
             @logger.debug(e.inspect)