refactor(formmailer): production-readiness + dataset.View compatibility

- Blacklist is now a CohortSource interface (Value() *ipcohort.Cohort).
  *dataset.View[ipcohort.Cohort] satisfies it directly; callers with
  an atomic.Pointer can wrap. Drops the atomic/sync import from the
  public API.
- SMTP send now uses net.Dialer.DialContext with a bounded SMTPTimeout
  (default 15s) and conn deadline, so a slow/hung relay no longer holds
  the request goroutine for WriteTimeout. Opportunistic STARTTLS added.
- MX lookup uses net.DefaultResolver.LookupMX with a bounded MXTimeout
  (default 3s), cancellable via r.Context().
- clientIP uses net.SplitHostPort (was LastIndex(":"), broken for IPv6).
- Per-IP limiter map now has a 10-minute TTL with opportunistic sweep
  every 1024 requests — previously grew unbounded.
- Sentinel errors switched to errors.New; fmt.Errorf was unused.

net/formmailer/formmailer.go

@@ -3,8 +3,9 @@
 //
 // Typical setup:
 //
-//	var blacklist atomic.Pointer[ipcohort.Cohort]
+//	// Blacklist can be any snapshot source — e.g. *dataset.View[ipcohort.Cohort]
-//	// ... caller loads blacklist and hot-swaps on a timer ...
+//	// satisfies CohortSource directly via its Value() method.
+//	blacklist := dataset.Add(set, func() (*ipcohort.Cohort, error) { ... })
 //
 //	fm := &formmailer.FormMailer{
 //	    SMTPHost:    "smtp.example.com:587",
@@ -15,7 +16,7 @@
 //	    Subject:     "Contact from {.Email}",
 //	    SuccessBody: successHTML,
 //	    ErrorBody:   errorHTML,
-//	    Blacklist:   &blacklist,
+//	    Blacklist:   blacklist,
 //	    AllowedCountries: []string{"US", "CA", "MX"},
 //	}
 //	http.Handle("POST /contact", fm)
@@ -23,6 +24,8 @@ package formmailer
 
 import (
 	"bytes"
+	"context"
+	"errors"
 	"fmt"
 	"log"
 	"net"
@@ -34,7 +37,6 @@ import (
 	"slices"
 	"strings"
 	"sync"
-	"sync/atomic"
 	"time"
 
 	"github.com/phuslu/iploc"
@@ -53,18 +55,29 @@ const (
 
 	defaultRPM         = 5
 	defaultBurst       = 3
+	defaultSMTPTimeout = 15 * time.Second
+	defaultMXTimeout   = 3 * time.Second
+
+	limiterTTL        = 10 * time.Minute
+	limiterSweepEvery = 1024 // sweep once every N handler invocations
 )
 
 var (
-	ErrInvalidEmail   = fmt.Errorf("email address doesn't look like an email address")
+	ErrInvalidEmail    = errors.New("email address doesn't look like an email address")
-	ErrInvalidMX      = fmt.Errorf("email address isn't deliverable")
+	ErrInvalidMX       = errors.New("email address isn't deliverable")
-	ErrInvalidPhone   = fmt.Errorf("phone number is not properly formatted")
+	ErrInvalidPhone    = errors.New("phone number is not properly formatted")
-	ErrContentTooLong = fmt.Errorf("one or more field values was too long")
+	ErrContentTooLong  = errors.New("one or more field values was too long")
-	ErrInvalidNewlines = fmt.Errorf("invalid use of newlines or carriage returns")
+	ErrInvalidNewlines = errors.New("invalid use of newlines or carriage returns")
 
 	phoneRe = regexp.MustCompile(`^[0-9+\-\(\) ]{7,20}$`)
 )
 
+// CohortSource returns the current cohort snapshot, or nil if not yet loaded.
+// *dataset.View[ipcohort.Cohort] satisfies this interface directly.
+type CohortSource interface {
+	Value() *ipcohort.Cohort
+}
+
 // FormFields maps logical field names to the HTML form field names.
 // Zero values use GravityForms-compatible defaults (input_1, input_3, etc.).
 type FormFields struct {
@@ -93,6 +106,11 @@ type FormMailer struct {
 	SMTPPass string
 	Subject  string // may contain {.Email}
 
+	// SMTPTimeout bounds the entire connect+auth+send cycle. Zero uses 15s.
+	SMTPTimeout time.Duration
+	// MXTimeout bounds the per-submission MX lookup. Zero uses 3s.
+	MXTimeout time.Duration
+
 	// SuccessBody and ErrorBody are the response bodies sent to the client.
 	// ErrorBody may contain {.Error} and {.SupportEmail} placeholders.
 	// Load from files before use: fm.SuccessBody, _ = os.ReadFile("success.html")
@@ -101,7 +119,8 @@ type FormMailer struct {
 	ContentType string // inferred from SuccessBody if empty
 
 	// Blacklist — if set, matching IPs are rejected before any other processing.
-	Blacklist *atomic.Pointer[ipcohort.Cohort]
+	// *dataset.View[ipcohort.Cohort] satisfies this interface.
+	Blacklist CohortSource
 
 	// AllowedCountries — if non-nil, only requests from listed ISO codes are
 	// accepted. Unknown country ("") is always allowed.
@@ -117,11 +136,17 @@ type FormMailer struct {
 
 	once     sync.Once
 	mu       sync.Mutex
-	limiters map[string]*rate.Limiter
+	limiters map[string]*limiterEntry
+	reqCount uint64
+}
+
+type limiterEntry struct {
+	lim      *rate.Limiter
+	lastUsed time.Time
 }
 
 func (fm *FormMailer) init() {
-	fm.limiters = make(map[string]*rate.Limiter)
+	fm.limiters = make(map[string]*limiterEntry)
 }
 
 func (fm *FormMailer) contentType() string {
@@ -154,8 +179,8 @@ func (fm *FormMailer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Blocklist check — fail before any other processing.
-	if bl := fm.Blacklist; bl != nil {
+	if fm.Blacklist != nil {
-		if c := bl.Load(); c != nil && c.ContainsAddr(ip) {
+		if c := fm.Blacklist.Value(); c != nil && c.ContainsAddr(ip) {
 			fm.writeError(w, fmt.Errorf("automated requests are not accepted"), false)
 			return
 		}
@@ -194,7 +219,7 @@ func (fm *FormMailer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		fm.writeError(w, err, true)
 		return
 	}
-	if err := validateEmailAndMX(email); err != nil {
+	if err := fm.validateEmailAndMX(r.Context(), email); err != nil {
 		fm.writeError(w, err, true)
 		return
 	}
@@ -213,16 +238,77 @@ func (fm *FormMailer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		strings.Join(fm.SMTPTo, ", "), fm.SMTPFrom, email, subject, body,
 	)
 
-	hostname := strings.Split(fm.SMTPHost, ":")[0]
+	if err := fm.sendMail(r.Context(), msg); err != nil {
-	auth := smtp.PlainAuth("", fm.SMTPUser, fm.SMTPPass, hostname)
-	if err := smtp.SendMail(fm.SMTPHost, auth, fm.SMTPFrom, fm.SMTPTo, msg); err != nil {
 		log.Printf("contact form: smtp error: %v", err)
 		http.Error(w, "failed to send — please try again later", http.StatusInternalServerError)
 		return
 	}
 
 	w.Header().Set("Content-Type", fm.contentType())
-	w.Write(fm.SuccessBody)
+	_, _ = w.Write(fm.SuccessBody)
+}
+
+// sendMail dials SMTPHost with a bounded timeout and writes the message.
+// Uses smtp.NewClient directly so the dial respects ctx; stdlib smtp.SendMail
+// has no context plumbing.
+func (fm *FormMailer) sendMail(ctx context.Context, msg []byte) error {
+	timeout := fm.SMTPTimeout
+	if timeout == 0 {
+		timeout = defaultSMTPTimeout
+	}
+	ctx, cancel := context.WithTimeout(ctx, timeout)
+	defer cancel()
+
+	d := net.Dialer{}
+	conn, err := d.DialContext(ctx, "tcp", fm.SMTPHost)
+	if err != nil {
+		return fmt.Errorf("dial: %w", err)
+	}
+	if dl, ok := ctx.Deadline(); ok {
+		_ = conn.SetDeadline(dl)
+	}
+	hostname, _, err := net.SplitHostPort(fm.SMTPHost)
+	if err != nil {
+		hostname = fm.SMTPHost
+	}
+	c, err := smtp.NewClient(conn, hostname)
+	if err != nil {
+		_ = conn.Close()
+		return fmt.Errorf("smtp client: %w", err)
+	}
+	defer func() { _ = c.Close() }()
+
+	if ok, _ := c.Extension("STARTTLS"); ok {
+		if err := c.StartTLS(nil); err != nil {
+			return fmt.Errorf("starttls: %w", err)
+		}
+	}
+	if fm.SMTPUser != "" {
+		auth := smtp.PlainAuth("", fm.SMTPUser, fm.SMTPPass, hostname)
+		if err := c.Auth(auth); err != nil {
+			return fmt.Errorf("auth: %w", err)
+		}
+	}
+	if err := c.Mail(fm.SMTPFrom); err != nil {
+		return fmt.Errorf("mail from: %w", err)
+	}
+	for _, to := range fm.SMTPTo {
+		if err := c.Rcpt(to); err != nil {
+			return fmt.Errorf("rcpt to %s: %w", to, err)
+		}
+	}
+	wc, err := c.Data()
+	if err != nil {
+		return fmt.Errorf("data: %w", err)
+	}
+	if _, err := wc.Write(msg); err != nil {
+		_ = wc.Close()
+		return fmt.Errorf("write: %w", err)
+	}
+	if err := wc.Close(); err != nil {
+		return fmt.Errorf("close data: %w", err)
+	}
+	return c.Quit()
 }
 
 func (fm *FormMailer) writeError(w http.ResponseWriter, err error, showSupport bool) {
@@ -234,7 +320,7 @@ func (fm *FormMailer) writeError(w http.ResponseWriter, err error, showSupport b
 	}
 	b := bytes.ReplaceAll(fm.ErrorBody, []byte("{.Error}"), []byte(err.Error()))
 	b = bytes.ReplaceAll(b, []byte("{.SupportEmail}"), []byte(support))
-	w.Write(b)
+	_, _ = w.Write(b)
 }
 
 func (fm *FormMailer) allow(ipStr string) bool {
@@ -247,12 +333,25 @@ func (fm *FormMailer) allow(ipStr string) bool {
 		burst = defaultBurst
 	}
 
+	now := time.Now()
 	fm.mu.Lock()
-	lim, ok := fm.limiters[ipStr]
+	e, ok := fm.limiters[ipStr]
 	if !ok {
-		lim = rate.NewLimiter(rate.Every(time.Minute/time.Duration(rpm)), burst)
+		e = &limiterEntry{
-		fm.limiters[ipStr] = lim
+			lim: rate.NewLimiter(rate.Every(time.Minute/time.Duration(rpm)), burst),
 		}
+		fm.limiters[ipStr] = e
+	}
+	e.lastUsed = now
+	fm.reqCount++
+	if fm.reqCount%limiterSweepEvery == 0 {
+		for k, v := range fm.limiters {
+			if now.Sub(v.lastUsed) > limiterTTL {
+				delete(fm.limiters, k)
+			}
+		}
+	}
+	lim := e.lim
 	fm.mu.Unlock()
 
 	if !lim.Allow() {
@@ -271,15 +370,21 @@ func validateLengths(name, email, phone, company, message string) error {
 	return nil
 }
 
-func validateEmailAndMX(email string) error {
+func (fm *FormMailer) validateEmailAndMX(ctx context.Context, email string) error {
 	if _, err := mail.ParseAddress(email); err != nil {
 		return ErrInvalidEmail
 	}
-	parts := strings.Split(email, "@")
+	_, domain, ok := strings.Cut(email, "@")
-	if len(parts) != 2 {
+	if !ok {
 		return ErrInvalidEmail
 	}
-	if _, err := net.LookupMX(parts[1]); err != nil {
+	timeout := fm.MXTimeout
+	if timeout == 0 {
+		timeout = defaultMXTimeout
+	}
+	lookupCtx, cancel := context.WithTimeout(ctx, timeout)
+	defer cancel()
+	if _, err := net.DefaultResolver.LookupMX(lookupCtx, domain); err != nil {
 		return ErrInvalidMX
 	}
 	return nil
@@ -307,14 +412,11 @@ func validateNoHeaderInjection(fields ...string) error {
 // clientIP returns the originating IP, preferring X-Forwarded-For.
 func clientIP(r *http.Request) string {
 	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
-		if parts := strings.SplitN(xff, ",", 2); len(parts) > 0 {
+		first, _, _ := strings.Cut(xff, ",")
-			return strings.TrimSpace(parts[0])
+		return strings.TrimSpace(first)
 	}
+	if host, _, err := net.SplitHostPort(r.RemoteAddr); err == nil {
+		return host
 	}
-	ip := r.RemoteAddr
+	return r.RemoteAddr
-	if idx := strings.LastIndex(ip, ":"); idx > 0 {
-		return ip[:idx]
 }
-	return ip
-}
-