daemon: ipfs daemon --offline --mount should fail with nice message

ipfs daemon --offline;
ipfs mount;
fails. This uniforms this behaviour.

License: MIT
Signed-off-by: Jakub Sztandera <kubuxu@protonmail.ch>

cmd/ipfs/daemon.go

 package main
 
 import (
+	"errors"
 	_ "expvar"
 	"fmt"
 	"net"
@@ -338,6 +339,11 @@ func daemonFunc(req cmds.Request, res cmds.Response) {
 		res.SetError(err, cmds.ErrNormal)
 		return
 	}
+	if mount && offline {
+		res.SetError(errors.New("mount is not supported in offline mode"),
+			cmds.ErrClient)
+		return
+	}
 	if mount {
 		if err := mountFuse(req); err != nil {
 			res.SetError(err, cmds.ErrNormal)

test/sharness/t0600-issues-and-regressions-online.sh

@@ -29,5 +29,10 @@ test_expect_success "no panic traces on daemon" '
 
 test_kill_ipfs_daemon
 
+test_expect_success "ipfs daemon --offline --mount fails - #2995" '
+	(test_must_fail ipfs daemon --offline --mount 2>daemon_err) &&
+	grep "mount is not supported in offline mode" daemon_err
+'
+
 test_done