login: ask which account, so a bare login can add one

README.md

@@ -259,7 +259,10 @@ pass, and an existing configuration is carried over the first time dwshell saves
 anything.
 
 ```sh
-$ dwshell login --user [email protected]     # the email is the account's identity
+$ dwshell login                             # the email is the account's identity
+User (email) [[email protected]]: [email protected]
+Password: …
+
 $ dwshell account list
 [email protected]  (default)
 [email protected]
@@ -269,6 +272,11 @@ $ dwshell list --account [email protected]   # the other one
 $ [email protected] dwshell list      # for a whole session
 ```
 
+`login` asks which email to use, offering the account you already have: press
+Enter to refresh it, or type another to register a second. `--user` answers the
+question up front, and without a terminal — a script — the default account is
+used and nothing is asked.
+
 The first account you log in with becomes the default; change it with
 `dwshell account default <email>`. Logging in again with an email already
 registered just refreshes that account.

cmd/dwshell/login_test.go

@@ -0,0 +1,70 @@
+package main
+
+import "testing"
+
+// asking records what the prompt was offered as a default, and answers with a
+// canned reply, standing in for a person at a terminal.
+func asking(answer string, seen *string) func(string) string {
+	return func(suggested string) string {
+		*seen = suggested
+		return answer
+	}
+}
+
+func TestLoginUserPrefersTheFlag(t *testing.T) {
+	var asked string
+	got := loginUser("typed@x", "default@x", true, asking("ignored@x", &asked))
+	if got != "typed@x" {
+		t.Fatalf("user = %q, want the --user value", got)
+	}
+	if asked != "" {
+		t.Error("with --user given there is nothing to ask")
+	}
+}
+
+// The whole point: at a terminal the email is asked, so a bare `dwshell login`
+// can register an account that is not the default one.
+func TestLoginUserAsksSoAnAccountCanBeAdded(t *testing.T) {
+	var asked string
+	got := loginUser("", "default@x", true, asking("second@x", &asked))
+	if got != "second@x" {
+		t.Fatalf("user = %q: a new email typed at the prompt must be honoured", got)
+	}
+	if asked != "default@x" {
+		t.Errorf("the prompt should offer the default account, offered %q", asked)
+	}
+}
+
+// Answering nothing accepts the default, so refreshing the account you already
+// have stays a single keystroke.
+func TestLoginUserEmptyAnswerKeepsTheDefault(t *testing.T) {
+	var asked string
+	got := loginUser("", "default@x", true, asking("", &asked))
+	if got != "default@x" {
+		t.Fatalf("user = %q, want the default", got)
+	}
+}
+
+// With no terminal there is nobody to ask, so a script refreshing the default
+// account keeps working exactly as before.
+func TestLoginUserFallsBackToTheDefaultWithoutATerminal(t *testing.T) {
+	var asked string
+	got := loginUser("", "default@x", false, asking("unused@x", &asked))
+	if got != "default@x" {
+		t.Fatalf("user = %q, want the default", got)
+	}
+	if asked != "" {
+		t.Error("nothing may be asked when there is no terminal")
+	}
+}
+
+func TestLoginUserAsksWithNothingConfigured(t *testing.T) {
+	var asked string
+	got := loginUser("", "", true, asking("first@x", &asked))
+	if got != "first@x" {
+		t.Fatalf("user = %q", got)
+	}
+	if asked != "" {
+		t.Errorf("with no account there is no default to offer, offered %q", asked)
+	}
+}

cmd/dwshell/main.go

@@ -196,14 +196,7 @@ func cmdLogin(ctx context.Context, args []string) int {
 	if err != nil {
 		return fail("%v", err)
 	}
-	if user == "" {
-		// Re-logging in with no --user refreshes the default account.
-		user = c.Config().Default
-	}
-	if user == "" {
-		fmt.Fprint(os.Stderr, "User (email): ")
-		fmt.Scanln(&user)
-	}
+	user = loginUser(user, c.Config().Default, term.IsTTY(), promptUser)
 	if user == "" {
 		return fail("a user is required")
 	}
@@ -223,6 +216,41 @@ func cmdLogin(ctx context.Context, args []string) int {
 	return 0
 }
 
+// loginUser decides which account `login` authenticates.
+//
+// The email is asked for whenever there is someone to ask, because it is the
+// account's identity: skipping the question when an account already exists
+// would make a bare `dwshell login` able only ever to refresh that one, never
+// to register a second. The account already configured is offered as the
+// answer, so refreshing it stays one keystroke.
+//
+// With no terminal — a script — there is nobody to ask, so the default account
+// is used, which is what such a script has always done.
+func loginUser(flagUser, defaultAccount string, interactive bool, prompt func(suggested string) string) string {
+	if flagUser != "" {
+		return flagUser
+	}
+	if !interactive {
+		return defaultAccount
+	}
+	if answer := strings.TrimSpace(prompt(defaultAccount)); answer != "" {
+		return answer
+	}
+	return defaultAccount
+}
+
+// promptUser asks for the account email, offering the one already configured.
+func promptUser(suggested string) string {
+	if suggested != "" {
+		fmt.Fprintf(os.Stderr, "User (email) [%s]: ", suggested)
+	} else {
+		fmt.Fprint(os.Stderr, "User (email): ")
+	}
+	var answer string
+	fmt.Scanln(&answer)
+	return answer
+}
+
 func readPassword() (string, error) {
 	if p := os.Getenv("DWSHELL_PASSWORD"); p != "" {
 		return p, nil