fix(completion): recover from panics in the application completer

completion.go

@@ -227,11 +227,24 @@ func (rl *Shell) startMenuComplete(completer completion.Completer) {
 }
 
 // commandCompletion generates the completions for commands/args/flags.
-func (rl *Shell) commandCompletion() completion.Values {
+//
+// The application-provided completer is user code that readline calls on nearly
+// every keystroke (menu completion and as-you-type autocomplete both funnel
+// through here). A panic in it — a bad completer or transient command-tree
+// state — is recovered and surfaced as a completion message, so a single faulty
+// completion degrades into a visible error instead of crashing the shell.
+func (rl *Shell) commandCompletion() (values completion.Values) {
 	if rl.Completer == nil {
 		return completion.Values{}
 	}
 
+	defer func() {
+		if r := recover(); r != nil {
+			msg := CompleteMessage("completion error: %v", r)
+			values = msg.convert()
+		}
+	}()
+
 	line, cursor := rl.completer.Line()
 	comps := rl.Completer(*line, cursor.Pos())
 

completion_test.go

@@ -0,0 +1,54 @@
+package readline
+
+import (
+	"strings"
+	"testing"
+)
+
+// TestCommandCompletionRecoversFromPanic ensures a panic in the
+// application-provided completer is recovered and surfaced as a completion
+// message instead of propagating and crashing the shell. readline calls the
+// completer on nearly every keystroke, so a single bad completer or transient
+// state must not take down the process.
+func TestCommandCompletionRecoversFromPanic(t *testing.T) {
+	rl := NewShell()
+	rl.Completer = func([]rune, int) Completions {
+		panic("boom")
+	}
+
+	defer func() {
+		if r := recover(); r != nil {
+			t.Fatalf("commandCompletion did not recover, panicked with: %v", r)
+		}
+	}()
+
+	values := rl.commandCompletion()
+
+	if values.Messages.IsEmpty() {
+		t.Fatal("recovered completion produced no message, want a completion error message")
+	}
+
+	var found bool
+	for _, msg := range values.Messages.Get() {
+		if strings.Contains(msg, "completion error") && strings.Contains(msg, "boom") {
+			found = true
+		}
+	}
+
+	if !found {
+		t.Fatalf("recovered messages = %v, want one containing the panic value", values.Messages.Get())
+	}
+}
+
+// TestCommandCompletionNilCompleter verifies the no-completer case stays a clean
+// no-op (empty values, no message).
+func TestCommandCompletionNilCompleter(t *testing.T) {
+	rl := NewShell()
+	rl.Completer = nil
+
+	values := rl.commandCompletion()
+
+	if !values.Messages.IsEmpty() {
+		t.Fatalf("nil completer produced messages %v, want none", values.Messages.Get())
+	}
+}