Fix local exec pushes in server mode

The local-exec push strategy was assuming it was running from a CLI and
so it wouldn't be a big deal for it to straight up `exec` and replace
its running with the user command. That command will just do its thing
and we want the exit code for the CLI command to match anyways, right?

Sure that works for a shell, but in a GRPC server setting it's decidedly
Not Cool to suddenly swap out the running process!

As you can imagine - the effect of doing this was all sorts of broken
pipes and unexpected EOFs and a very confused @phinze.

Luckily we had a subprocess strategy sitting right there for Windows
compat, so it was just a matter of switching to that in the server
context as well. Long and winding debugging process; simple fix;
just another classic!

plugins/pushes/local-exec/push.rb

@@ -7,6 +7,8 @@ require_relative "errors"
 module VagrantPlugins
   module LocalExecPush
     class Push < Vagrant.plugin("2", :push)
+      @@logger = Log4r::Logger.new("vagrant::push::local_exec")
+
       def push
         if config.inline
           execute_inline!(config.inline, config.args)
@@ -47,7 +49,7 @@ module VagrantPlugins
 
       # Execute the script, raising an exception if it fails.
       def execute!(*cmd)
-        if Vagrant::Util::Platform.windows?
+        if Vagrant::Util::Platform.windows? || Vagrant.server_mode?
           execute_subprocess!(*cmd)
         else
           execute_exec!(*cmd)
@@ -63,11 +65,13 @@ module VagrantPlugins
 
       # Run the command as exec (unix).
       def execute_exec!(*cmd)
+        @@logger.debug("executing command via exec: #{cmd.inspect}")
         Vagrant::Util::SafeExec.exec(cmd[0], *cmd[1..-1])
       end
 
       # Run the command as a subprocess (windows).
       def execute_subprocess!(*cmd)
+        @@logger.debug("executing command via subprocess: #{cmd.inspect}")
         cmd = cmd.dup << { notify: [:stdout, :stderr] }
         result = Vagrant::Util::Subprocess.execute(*cmd) do |type, data|
           if type == :stdout
@@ -77,7 +81,7 @@ module VagrantPlugins
           end
         end
 
-        Kernel.exit(result.exit_code)
+        Kernel.exit(result.exit_code) unless Vagrant.server_mode?
       end
     end
   end

test/unit/plugins/pushes/local-exec/push_test.rb

@@ -122,5 +122,12 @@ describe VagrantPlugins::LocalExecPush::Push do
         expect(e).to be_a(SystemExit)
       }
     end
+
+    it "uses subprocess when running in server mode, and does not exit" do
+      allow(Vagrant).to receive(:server_mode?).and_return(true)
+      result = double("result", exit_code: 0)
+      expect(Vagrant::Util::Subprocess).to receive(:execute).and_return(result)
+      expect { subject.execute! }.to_not raise_error
+    end
   end
 end