communicators/ssh: detect if config.ssh.shell is non-functional [GH-3040]

CHANGELOG.md

@@ -71,6 +71,7 @@ BUG FIXES:
       source, use that. [GH-3610]
   - commands/rsync-auto: Interrupt exits properly. [GH-3552]
   - commands/rsync-auto: Run properly on Windows. [GH-3547]
+  - communicators/ssh: Detect if `config.ssh.shell` is invalid. [GH-3040]
   - providers/virtualbox: VirtalBox detection works properly again on
       Windows when the `VBOX_INSTALL_PATH` has multiple elements. [GH-3549]
   - providers/virtualbox: Forcing MAC address on private network works

lib/vagrant/errors.rb

@@ -592,6 +592,10 @@ module Vagrant
       error_key(:ssh_host_down)
     end
 
+    class SSHInvalidShell< VagrantError
+      error_key(:ssh_invalid_shell)
+    end
+
     class SSHIsPuttyLink < VagrantError
       error_key(:ssh_is_putty_link)
     end

plugins/communicators/ssh/communicator.rb

@@ -78,6 +78,8 @@ module VagrantPlugins
               message = "Host appears down."
             rescue Vagrant::Errors::SSHNoRoute
               message = "Host unreachable."
+            rescue Vagrant::Errors::SSHInvalidShell
+              raise
             rescue Vagrant::Errors::SSHKeyTypeNotSupported
               raise
             rescue Vagrant::Errors::VagrantError => e
@@ -120,6 +122,11 @@ module VagrantPlugins
           return false
         end
 
+        # Verify the shell is valid
+        if execute("", error_check: false) != 0
+          raise Vagrant::Errors::SSHInvalidShell
+        end
+
         # If we're already attempting to switch out the SSH key, then
         # just return that we're ready (for Machine#guest).
         @lock.synchronize do

templates/locales/en.yml

@@ -995,6 +995,12 @@ en:
         While attempting to connect with SSH, a "host is down" (EHOSTDOWN)
         error was received. Please verify your SSH settings are correct
         and try again.
+      ssh_invalid_shell: |-
+        The configured shell (config.ssh.shell) is invalid and unable
+        to properly execute commands. The most common cause for this is
+        using a shell that is unavailable on the system. Please verify
+        you're using the full path to the shell and that the shell is
+        executable by the SSH user.
       ssh_is_putty_link: |-
         The `ssh` executable found in the PATH is a PuTTY Link SSH client.
         Vagrant is only compatible with OpenSSH SSH clients. Please install