packer: Parallelize synopsis lookup to speed up help output

Using `time` to calculate the average of 100 iterations on my machine,
`packer` went from 130ms on average to 70ms.

Previously, the load time would scale linearly about 30ms (on my
machine) on average per new command added. Now that is much much
smaller.

CHANGELOG.md

@@ -8,6 +8,7 @@ FEATURES:
 
 IMPROVEMENTS:
 
+* core: packer help output now loads much faster.
 * builder/virtualbox: Do not check for VirtualBox as part of template
   validation; only check at execution.
 * builder/vmware: Do not check for VMware as part of template validation;

packer/environment.go

@@ -8,6 +8,7 @@ import (
 	"os"
 	"sort"
 	"strings"
+	"sync"
 )
 
 // The function type used to lookup Builder implementations.
@@ -68,6 +69,12 @@ type EnvironmentConfig struct {
 	Ui         Ui
 }
 
+type helpCommandEntry struct {
+	i        int
+	key      string
+	synopsis string
+}
+
 // DefaultEnvironmentConfig returns a default EnvironmentConfig that can
 // be used to create a new enviroment with NewEnvironment with sane defaults.
 func DefaultEnvironmentConfig() *EnvironmentConfig {
@@ -276,29 +283,69 @@ func (e *coreEnvironment) printHelp() {
 	// Sort the keys
 	sort.Strings(e.commands)
 
+	// Create the communication/sync mechanisms to get the synopsis' of
+	// the various commands. We do this in parallel since the overhead
+	// of the subprocess underneath is very expensive and this speeds things
+	// up an incredible amount.
+	var wg sync.WaitGroup
+	ch := make(chan *helpCommandEntry)
+
+	for i, key := range e.commands {
+		wg.Add(1)
+
+		// Get the synopsis in a goroutine since it may take awhile
+		// to subprocess out.
+		go func(i int, key string) {
+			defer wg.Done()
+			var synopsis string
+			command, err := e.components.Command(key)
+			if err != nil {
+				synopsis = fmt.Sprintf("Error loading command: %s", err.Error())
+			} else if command == nil {
+				return
+			} else {
+				synopsis = command.Synopsis()
+			}
+
+			// Pad the key with spaces so that they're all the same width
+			key = fmt.Sprintf("%s%s", key, strings.Repeat(" ", maxKeyLen-len(key)))
+
+			// Output the command and the synopsis
+			ch <- &helpCommandEntry{
+				i:        i,
+				key:      key,
+				synopsis: synopsis,
+			}
+		}(i, key)
+	}
+
 	e.ui.Say("usage: packer [--version] [--help] <command> [<args>]\n")
 	e.ui.Say("Available commands are:")
-	for _, key := range e.commands {
-		var synopsis string
 
-		command, err := e.components.Command(key)
-		if err != nil {
-			synopsis = fmt.Sprintf("Error loading command: %s", err.Error())
-		} else if command == nil {
-			continue
-		} else {
-			synopsis = command.Synopsis()
+	// Make a goroutine that just waits for all the synopsis gathering
+	// to complete, and then output it.
+	synopsisDone := make(chan struct{})
+	go func() {
+		defer close(synopsisDone)
+		entries := make([]string, len(e.commands))
+
+		for entry := range ch {
+			e.ui.Machine("command", entry.key, entry.synopsis)
+			message := fmt.Sprintf("    %s    %s", entry.key, entry.synopsis)
+			entries[entry.i] = message
 		}
 
-		// Machine-readable output of the available command
-		e.ui.Machine("command", key, synopsis)
+		for _, message := range entries {
+			if message != "" {
+				e.ui.Say(message)
+			}
+		}
+	}()
 
-		// Pad the key with spaces so that they're all the same width
-		key = fmt.Sprintf("%s%s", key, strings.Repeat(" ", maxKeyLen-len(key)))
-
-		// Output the command and the synopsis
-		e.ui.Say(fmt.Sprintf("    %s     %s", key, synopsis))
-	}
+	// Wait to complete getting the synopsis' then close the channel
+	wg.Wait()
+	close(ch)
+	<-synopsisDone
 
 	e.ui.Say("\nGlobally recognized options:")
 	e.ui.Say("    -machine-readable    Machine-readable output format.")