feat(plugin): Allow plugins to receive empty arg

Empty arguments were being swallowed by the plugin parsing logic; this
commit fixes that.

This was not a big problem as empty arguments are probably quite rare.
However, the shell completion logic does have a use case for an empty
argument.  Plugins using Cobra have a '__complete' command that can be
called to obtain the valid completion choices.  That command takes as
arguments what the user has typed.  When the user has not typed anything
the command needs to be passed an explicit empty argument.  For example:

  helm 2to3 __complete ''

but since the empty argument is not being passed to the plugin we get a
missing argument error because the __complete command requires at least
one argument.

Signed-off-by: Marc Khouzam <marc.khouzam@montreal.ca>

cmd/helm/load_plugins.go

@@ -170,7 +170,9 @@ func manuallyProcessArgs(args []string) ([]string, []string) {
 				return v
 			}
 		}
-		return ""
+		// Return something different than what we received by adding a random prefix.
+		// We cannot simply return an empty string as v could be an empty string itself.
+		return fmt.Sprintf("x%s", v)
 	}
 
 	for i := 0; i < len(args); i++ {

cmd/helm/plugin_test.go

@@ -116,6 +116,8 @@ func TestLoadPlugins(t *testing.T) {
 		{"env", "env stuff", "show the env", "env\n", []string{}, 0},
 		{"exitwith", "exitwith code", "This exits with the specified exit code", "", []string{"2"}, 2},
 		{"fullenv", "show env vars", "show all env vars", envs + "\n", []string{}, 0},
+		// Test that empty arguments are passed to the plugin
+		{"numargs", "echo num args", "echo number of arguments received", "3\n", []string{"-a", "", "-b"}, 0},
 	}
 
 	plugins := cmd.Commands()
@@ -196,6 +198,7 @@ func TestLoadPluginsForCompletion(t *testing.T) {
 				{"more", []string{"one", "two"}, []string{"b", "ball"}, []staticCompletionDetails{}},
 			}},
 		}},
+		{"numargs", []string{}, []string{}, []staticCompletionDetails{}},
 	}
 	checkCommand(t, cmd.Commands(), tests)
 }

cmd/helm/testdata/helmhome/helm/plugins/numargs/numargs.sh

@@ -0,0 +1,2 @@
+#!/bin/bash
+echo $#

cmd/helm/testdata/helmhome/helm/plugins/numargs/plugin.yaml

@@ -0,0 +1,4 @@
+name: numargs
+usage: "echo num args"
+description: "echo number of arguments received"
+command: "$HELM_PLUGIN_DIR/numargs.sh"

cmd/helm/testdata/output/plugin_list_comp.txt

@@ -3,5 +3,6 @@ echo	echo stuff
 env	env stuff
 exitwith	exitwith code
 fullenv	show env vars
+numargs	echo num args
 :4
 Completion ended with directive: ShellCompDirectiveNoFileComp

cmd/helm/testdata/output/plugin_repeat_comp.txt

@@ -2,5 +2,6 @@ echo	echo stuff
 env	env stuff
 exitwith	exitwith code
 fullenv	show env vars
+numargs	echo num args
 :4
 Completion ended with directive: ShellCompDirectiveNoFileComp