Centralize link URL validation

internal/link/url.go

@@ -0,0 +1,26 @@
+package link
+
+import (
+	"fmt"
+	"net/url"
+)
+
+func ValidateHTTPURL(value string) error {
+	parsed, err := url.Parse(value)
+	if err != nil {
+		return fmt.Errorf("must be a valid URL")
+	}
+	if parsed.Scheme != "http" && parsed.Scheme != "https" {
+		return fmt.Errorf("must use http or https")
+	}
+	if parsed.Host == "" {
+		return fmt.Errorf("must include a host")
+	}
+	if parsed.RawQuery != "" {
+		return fmt.Errorf("must not include a query string")
+	}
+	if parsed.Fragment != "" {
+		return fmt.Errorf("must not include a fragment")
+	}
+	return nil
+}

internal/link/url_test.go

@@ -0,0 +1,39 @@
+package link
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestValidateHTTPURL(t *testing.T) {
+	tests := []struct {
+		name    string
+		value   string
+		wantErr string
+	}{
+		{name: "http", value: "http://reports.example.com/archive"},
+		{name: "https", value: "https://reports.example.com/archive"},
+		{name: "missing host", value: "https:///archive", wantErr: "must include a host"},
+		{name: "unsupported scheme", value: "ftp://reports.example.com/archive", wantErr: "must use http or https"},
+		{name: "query string", value: "https://reports.example.com/archive?preview=1", wantErr: "must not include a query string"},
+		{name: "fragment", value: "https://reports.example.com/archive#top", wantErr: "must not include a fragment"},
+		{name: "parse failure", value: "http://[::1", wantErr: "must be a valid URL"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := ValidateHTTPURL(tt.value)
+			if tt.wantErr == "" {
+				if err != nil {
+					t.Fatalf("ValidateHTTPURL() error = %v", err)
+				}
+				return
+			}
+			if err == nil {
+				t.Fatal("ValidateHTTPURL() error = nil, want error")
+			}
+			if !strings.Contains(err.Error(), tt.wantErr) {
+				t.Fatalf("ValidateHTTPURL() error = %q, want %q", err, tt.wantErr)
+			}
+		})
+	}
+}