imap: fix build on macos

Fix the following build error on mac os:

 worker/imap/worker.go:368:29: undefined: syscall.TCP_KEEPCNT
 worker/imap/worker.go:376:29: undefined: syscall.TCP_KEEPINTVL

These symbols are not defined on darwin.

Fixes: 5dfeff75f3 ("imap: add tcp connection options")
Signed-off-by: Robin Jarry <robin@jarry.cc>

doc/aerc-imap.5.scd

@@ -82,6 +82,9 @@ available:
 	By default, the system tcp socket settings are used.
 	If keepalive-period is specified, this option defaults to 3 probes.
 
+	This option is only supported on linux. On other platforms, it will be
+	ignored.
+
 *keepalive-interval*
 	The interval between subsequential keepalive probes, regardless of what
 	the connection has exchanged in the meantime. Fractional seconds are
@@ -90,6 +93,9 @@ available:
 	By default, the system tcp socket settings are used.
 	If keepalive-period is specified, this option defaults to 3s.
 
+	This option is only supported on linux. On other platforms, it will be
+	ignored.
+
 # SEE ALSO
 
 *aerc*(1) *aerc-config*(5)

lib/keepalive_dummy.go

@@ -0,0 +1,11 @@
+//+build !linux
+
+package lib
+
+func SetTcpKeepaliveProbes(fd, count int) error {
+	return nil
+}
+
+func SetTcpKeepaliveInterval(fd, interval int) error {
+	return nil
+}

lib/keepalive_linux.go

@@ -0,0 +1,17 @@
+//+build linux
+
+package lib
+
+import (
+	"syscall"
+)
+
+func SetTcpKeepaliveProbes(fd, count int) error {
+	return syscall.SetsockoptInt(
+		fd, syscall.IPPROTO_TCP, syscall.TCP_KEEPCNT, count)
+}
+
+func SetTcpKeepaliveInterval(fd, interval int) error {
+	return syscall.SetsockoptInt(
+		fd, syscall.IPPROTO_TCP, syscall.TCP_KEEPINTVL, interval)
+}

worker/imap/worker.go

@@ -7,7 +7,6 @@ import (
 	"net/url"
 	"strconv"
 	"strings"
-	"syscall"
 	"time"
 
 	"github.com/emersion/go-imap"
@@ -364,17 +363,13 @@ func (w *IMAPWorker) setKeepaliveParameters(conn *net.TCPConn) error {
 	err = rawConn.Control(func(fdPtr uintptr) {
 		fd := int(fdPtr)
 		// Max number of probes before failure
-		err := syscall.SetsockoptInt(
-			fd, syscall.IPPROTO_TCP, syscall.TCP_KEEPCNT,
-			w.config.keepalive_probes)
+		err := lib.SetTcpKeepaliveProbes(fd, w.config.keepalive_probes)
 		if err != nil {
 			w.worker.Logger.Printf(
 				"cannot set tcp keepalive probes: %v\n", err)
 		}
 		// Wait time after an unsuccessful probe
-		err = syscall.SetsockoptInt(
-			fd, syscall.IPPROTO_TCP, syscall.TCP_KEEPINTVL,
-			w.config.keepalive_interval)
+		err = lib.SetTcpKeepaliveInterval(fd, w.config.keepalive_interval)
 		if err != nil {
 			w.worker.Logger.Printf(
 				"cannot set tcp keepalive interval: %v\n", err)