Do not run parent node on missing child err with usageOnMissing option

Refactors usageOnMissing option to not run parent commands after
printing usage when a child node is missing.

Signed-off-by: hasheddan <georgedanielmangum@gmail.com>

context.go

@@ -187,9 +187,6 @@ func (c *Context) Validate() error { // nolint: gocyclo
 	}
 
 	if err := checkMissingChildren(node); err != nil {
-		if c.Kong.usageOnMissing {
-			return c.PrintUsage(false)
-		}
 		return err
 	}
 	if err := checkMissingPositionals(positionals, node.Positional); err != nil {
@@ -628,10 +625,13 @@ func (c *Context) Run(binds ...interface{}) (err error) {
 	node := c.Selected()
 	if node == nil {
 		if c.Kong.usageOnMissing {
-			return nil
+			return c.PrintUsage(false)
 		}
 		return fmt.Errorf("no command selected")
 	}
+	if c.Kong.usageOnMissing && isMissingChildError(c.Error) {
+		return c.PrintUsage(false)
+	}
 	return c.RunNode(node, binds...)
 }
 
@@ -700,13 +700,7 @@ func checkMissingChildren(node *Node) error {
 		return nil
 	}
 
-	if len(missing) > 5 {
-		missing = append(missing[:5], "...")
-	}
-	if len(missing) == 1 {
-		return fmt.Errorf("expected %s", missing[0])
-	}
-	return fmt.Errorf("expected one of %s", strings.Join(missing, ",  "))
+	return newMissingChildError(missing)
 }
 
 // If we're missing any positionals and they're required, return an error.

error.go

@@ -1,5 +1,10 @@
 package kong
 
+import (
+	"fmt"
+	"strings"
+)
+
 // ParseError is the error type returned by Kong.Parse().
 //
 // It contains the parse Context that triggered the error.
@@ -10,3 +15,29 @@ type ParseError struct {
 
 // Cause returns the original cause of the error.
 func (p *ParseError) Cause() error { return p.error }
+
+type missingChildError struct {
+	missing []string
+}
+
+func (m *missingChildError) Error() string {
+	if len(m.missing) > 5 {
+		m.missing = append(m.missing[:5], "...")
+	}
+	if len(m.missing) == 1 {
+		return fmt.Sprintf("expected %s", m.missing[0])
+	}
+	return fmt.Sprintf("expected one of %s", strings.Join(m.missing, ",  "))
+}
+
+func newMissingChildError(missing []string) *missingChildError {
+	return &missingChildError{missing}
+}
+
+func isMissingChildError(err error) bool {
+	if err == nil {
+		return false
+	}
+	_, ok := err.(*missingChildError)
+	return ok
+}

kong.go

@@ -219,7 +219,10 @@ func (k *Kong) Parse(args []string) (ctx *Context, err error) {
 		return nil, &ParseError{error: err, Context: ctx}
 	}
 	if err = ctx.Validate(); err != nil {
-		return nil, &ParseError{error: err, Context: ctx}
+		if !k.usageOnError || !isMissingChildError(err) {
+			return nil, &ParseError{error: err, Context: ctx}
+		}
+		ctx.Error = err
 	}
 	if err = k.applyHook(ctx, "AfterApply"); err != nil {
 		return nil, &ParseError{error: err, Context: ctx}