Log redacted email provider errors and diagnose sender configuration (#57)

backend/notifications/email/failure.go

@@ -0,0 +1,95 @@
+package email
+
+import (
+	"encoding/json"
+	"regexp"
+	"sort"
+	"strings"
+	"unicode"
+)
+
+const maxErrorBody = 16 << 10
+const maxErrorMessage = 512
+
+var (
+	errorName = regexp.MustCompile(`^[a-zA-Z0-9_.-]{1,64}$`)
+	addresses = regexp.MustCompile(`[^\s<>"'(),;:]+@[^\s<>"'(),;:]+`)
+	urls      = regexp.MustCompile(`(?i)https?://[^\s<>"']+`)
+	tokens    = regexp.MustCompile(`[a-zA-Z0-9_+/-]{32,}={0,2}`)
+	numbers   = regexp.MustCompile(`[0-9]{4,}`)
+)
+
+// Only the documented human-readable error fields cross the adapter boundary.
+// Full bodies, headers and request data must never reach logs or public failures.
+func providerFailure(body, request []byte, secrets ...string) (string, string) {
+	var dto struct {
+		Name    string `json:"name"`
+		Message string `json:"message"`
+		Errors  []struct {
+			Message string `json:"message"`
+		} `json:"errors"`
+	}
+	if err := json.Unmarshal(body, &dto); err != nil {
+		return "", "Provider error details unavailable: invalid JSON response"
+	}
+	if dto.Message == "" && len(dto.Errors) > 0 {
+		dto.Message = dto.Errors[0].Message
+	}
+	// Providers can echo any scalar string from their request DTOs. Strip those
+	// values before truncation, in addition to credentials and embedded tokens.
+	var payload any
+	if json.Unmarshal(request, &payload) == nil {
+		for _, value := range requestStrings(payload) {
+			secrets = append(secrets, value)
+			// Rendered login/verification codes and numeric account references
+			// can be echoed as fragments of an otherwise longer email body.
+			secrets = append(secrets, numbers.FindAllString(value, -1)...)
+		}
+	}
+	sort.Slice(secrets, func(i, j int) bool { return len(secrets[i]) > len(secrets[j]) })
+	name := safeErrorText(dto.Name, secrets)
+	if !errorName.MatchString(name) {
+		name = ""
+	}
+	message := safeErrorText(dto.Message, secrets)
+	if message == "" {
+		message = "Provider did not include an error message"
+	}
+	return name, message
+}
+
+func requestStrings(value any) []string {
+	var values []string
+	switch value := value.(type) {
+	case string:
+		values = append(values, value)
+	case []any:
+		for _, item := range value {
+			values = append(values, requestStrings(item)...)
+		}
+	case map[string]any:
+		for _, item := range value {
+			values = append(values, requestStrings(item)...)
+		}
+	}
+	return values
+}
+
+func safeErrorText(text string, secrets []string) string {
+	for _, secret := range secrets {
+		if secret != "" {
+			text = strings.ReplaceAll(text, secret, "[redacted]")
+		}
+	}
+	text = addresses.ReplaceAllString(text, "[redacted-address]")
+	text = urls.ReplaceAllString(text, "[redacted-url]")
+	text = tokens.ReplaceAllString(text, "[redacted-token]")
+	text = strings.Join(strings.FieldsFunc(text, func(r rune) bool {
+		return unicode.IsSpace(r) || unicode.IsControl(r) || unicode.Is(unicode.Cf, r)
+	}), " ")
+	runes := []rune(text)
+	if len(runes) > maxErrorMessage {
+		text = string(runes[:maxErrorMessage]) + "…"
+	}
+	return text
+}