libfuse:- create mountdir if it does not exist on linux

Move creation logic out of kbfsfuse/main.
See
- keybase/kbfs#1669
- keybase/kbfs#1655

go/kbfs/kbfsfuse/main.go

@@ -12,7 +12,6 @@ import (
 	"flag"
 	"fmt"
 	"os"
-	"runtime"
 
 	"bazil.org/fuse"
 
@@ -76,27 +75,16 @@ func start() *libfs.Error {
 		if err != nil {
 			return libfs.InitError(err.Error())
 		}
+		// If a mountdir was not set by keybase config set mountdir,
+		// the service returns a default value which may or may not
+		// exist yet.
 		if len(mountDir) == 0 {
 			fmt.Print(getUsageString(ctx))
 			return libfs.InitError("no mount specified")
 		}
 	} else {
 		mountDir = flag.Arg(0)
 	}
-	err := os.MkdirAll(mountDir, os.ModePerm)
-	if err != nil {
-		fmt.Println(err)
-		switch os := runtime.GOOS; os {
-		case "linux":
-			fmt.Println("failed to create mount directory")
-		case "darwin":
-			// If the user did not override the defaults this isn't expected to work.
-			// The default is in a directory only writable by root.
-			fmt.Println("mount directory does not exist and should have been created by system service")
-		case "windows":
-			fmt.Println("I honestly have no idea what to say here, maybe someone can tell me what is supposed to create this on Windows")
-		}
-	}
 
 	if len(flag.Args()) > 1 {
 		fmt.Print(getUsageString(ctx))

go/kbfs/libfuse/mounter.go

@@ -17,7 +17,9 @@ import (
 
 	"bazil.org/fuse"
 	"github.com/keybase/client/go/kbconst"
+	"github.com/keybase/client/go/libkb"
 	"github.com/keybase/client/go/logger"
+	keybase1 "github.com/keybase/client/go/protocol/keybase1"
 )
 
 type mounter struct {
@@ -52,6 +54,17 @@ func (m *mounter) Mount() (err error) {
 }
 
 func fuseMountDir(dir string, platformParams PlatformParams) (*fuse.Conn, error) {
+	// Create mountdir directory on Linux
+	switch libkb.RuntimeGroup() {
+	case keybase1.RuntimeGroup_UNIXLIKE:
+		// Inherit permissions from containing directory and umask
+		err := os.MkdirAll(dir, os.ModeDir|os.ModePerm)
+		if err != nil {
+			return nil, err
+		}
+	default:
+	}
+
 	fi, err := os.Stat(dir)
 	if err != nil {
 		return nil, err