refactor: enhance NewClient to support environment variable fallback and add tests

dns.go

@@ -5,6 +5,8 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"net/url"
+	"os"
 	"strings"
 	"time"
 )
@@ -24,8 +26,51 @@ type DNSRecord struct {
 	Value string `json:"value"`
 }
 
-// NewClient constructs a Client. host should be the bare hostname (e.g. "box.example.com").
-func NewClient(host, username, password string) *Client {
+// NewClient constructs a Client. Any empty parameter is filled from environment variables:
+//
+//	host:     MIAB_HOST or hostname parsed from MAILINABOX_BASE_URL
+//	username: MIAB_USERNAME or MAILINABOX_EMAIL
+//	password: MIAB_PASSWORD or MAILINABOX_PASSWORD
+//
+// Returns an error if a value is still empty after the env lookup.
+func NewClient(host, username, password string) (*Client, error) {
+	if host == "" {
+		host = os.Getenv("MIAB_HOST")
+		if host == "" {
+			if base := os.Getenv("MAILINABOX_BASE_URL"); base != "" {
+				if u, err := url.Parse(base); err == nil {
+					host = u.Hostname()
+				}
+			}
+		}
+	}
+	if username == "" {
+		username = os.Getenv("MIAB_USERNAME")
+		if username == "" {
+			username = os.Getenv("MAILINABOX_EMAIL")
+		}
+	}
+	if password == "" {
+		password = os.Getenv("MIAB_PASSWORD")
+		if password == "" {
+			password = os.Getenv("MAILINABOX_PASSWORD")
+		}
+	}
+
+	var missing []string
+	if host == "" {
+		missing = append(missing, "MIAB_HOST or MAILINABOX_BASE_URL")
+	}
+	if username == "" {
+		missing = append(missing, "MIAB_USERNAME or MAILINABOX_EMAIL")
+	}
+	if password == "" {
+		missing = append(missing, "MIAB_PASSWORD or MAILINABOX_PASSWORD")
+	}
+	if len(missing) > 0 {
+		return nil, fmt.Errorf("miab: missing required environment variables: %v", missing)
+	}
+
 	return &Client{
 		host:     host,
 		username: username,
@@ -33,7 +78,7 @@ func NewClient(host, username, password string) *Client {
 		httpClient: &http.Client{
 			Timeout: 30 * time.Second,
 		},
-	}
+	}, nil
 }
 
 // SetRecord replaces all existing records of the given type for name with value (HTTP PUT).