From 74dab5fc395184254ce0fb29ad813000bc75f1df Mon Sep 17 00:00:00 2001
From: Mechiel Lukkien <mechiel@ueber.net>
Date: Mon, 24 Apr 2023 10:34:19 +0200
Subject: [PATCH] fix sending to address where the domain does not have an mx
 record (but where we should connect directly to the host, or follow cname
 records)

such deliveries would fail because a canceled "context" was reused, so the dns
lookups would fail.

the tests didn't catch it before because they ignored their context parameters.
---
 dns/mock.go    | 30 ++++++++++++++++++++++++++++++
 queue/queue.go |  3 +++
 2 files changed, 33 insertions(+)

diff --git a/dns/mock.go b/dns/mock.go
index 2a8e84b..b0d420c 100644
--- a/dns/mock.go
+++ b/dns/mock.go
@@ -44,6 +44,9 @@ func (r MockResolver) servfail(s string) *net.DNSError {
 }
 
 func (r MockResolver) LookupCNAME(ctx context.Context, name string) (string, error) {
+	if err := ctx.Err(); err != nil {
+		return "", err
+	}
 	if _, ok := r.Fail[Mockreq{"cname", name}]; ok {
 		return "", r.servfail(name)
 	}
@@ -54,6 +57,9 @@ func (r MockResolver) LookupCNAME(ctx context.Context, name string) (string, err
 }
 
 func (r MockResolver) LookupAddr(ctx context.Context, ip string) ([]string, error) {
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
 	if _, ok := r.Fail[Mockreq{"ptr", ip}]; ok {
 		return nil, r.servfail(ip)
 	}
@@ -65,18 +71,30 @@ func (r MockResolver) LookupAddr(ctx context.Context, ip string) ([]string, erro
 }
 
 func (r MockResolver) LookupNS(ctx context.Context, name string) ([]*net.NS, error) {
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
 	return nil, r.servfail("ns not implemented")
 }
 
 func (r MockResolver) LookupPort(ctx context.Context, network, service string) (port int, err error) {
+	if err := ctx.Err(); err != nil {
+		return 0, err
+	}
 	return 0, r.servfail("port not implemented")
 }
 
 func (r MockResolver) LookupSRV(ctx context.Context, service, proto, name string) (string, []*net.SRV, error) {
+	if err := ctx.Err(); err != nil {
+		return "", nil, err
+	}
 	return "", nil, r.servfail("srv not implemented")
 }
 
 func (r MockResolver) LookupIPAddr(ctx context.Context, host string) ([]net.IPAddr, error) {
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
 	if _, ok := r.Fail[Mockreq{"ipaddr", host}]; ok {
 		return nil, r.servfail(host)
 	}
@@ -96,6 +114,9 @@ func (r MockResolver) LookupIPAddr(ctx context.Context, host string) ([]net.IPAd
 }
 
 func (r MockResolver) LookupHost(ctx context.Context, host string) (addrs []string, err error) {
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
 	if _, ok := r.Fail[Mockreq{"host", host}]; ok {
 		return nil, r.servfail(host)
 	}
@@ -111,6 +132,9 @@ func (r MockResolver) LookupHost(ctx context.Context, host string) (addrs []stri
 }
 
 func (r MockResolver) LookupIP(ctx context.Context, network, host string) ([]net.IP, error) {
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
 	if _, ok := r.Fail[Mockreq{"ip", host}]; ok {
 		return nil, r.servfail(host)
 	}
@@ -134,6 +158,9 @@ func (r MockResolver) LookupIP(ctx context.Context, network, host string) ([]net
 }
 
 func (r MockResolver) LookupMX(ctx context.Context, name string) ([]*net.MX, error) {
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
 	if _, ok := r.Fail[Mockreq{"mx", name}]; ok {
 		return nil, r.servfail(name)
 	}
@@ -145,6 +172,9 @@ func (r MockResolver) LookupMX(ctx context.Context, name string) ([]*net.MX, err
 }
 
 func (r MockResolver) LookupTXT(ctx context.Context, name string) ([]string, error) {
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
 	if _, ok := r.Fail[Mockreq{"txt", name}]; ok {
 		return nil, r.servfail(name)
 	}
diff --git a/queue/queue.go b/queue/queue.go
index 0776f65..f345196 100644
--- a/queue/queue.go
+++ b/queue/queue.go
@@ -658,7 +658,10 @@ func gatherHosts(resolver dns.Resolver, m Msg, cid int64, qlog *mlog.Log) (hosts
 			}
 
 			// No MX record. First attempt CNAME lookup. ../rfc/5321:3838 ../rfc/3974:197
+			ctx, cancel = context.WithTimeout(cidctx, 30*time.Second)
+			defer cancel()
 			cname, err := resolver.LookupCNAME(ctx, effectiveDomain.ASCII+".")
+			cancel()
 			if err != nil && !dns.IsNotFound(err) {
 				return nil, effectiveDomain, false, fmt.Errorf("%w: cname lookup for %s: %v", errDNS, effectiveDomain, err)
 			}