Support OSXFUSE 3.x, fall back to 2.x if needed

Also supports custom bundled OSXFUSE installs.

OSXFUSE 3.x is still currently in beta, approach with caution.

All tests here pass on Yosemite with what should become OSXFUSE 3.0.6,
as discussed in
macfuse/macfuse#232 (comment)

mount_darwin.go

@@ -13,9 +13,11 @@ import (
 	"syscall"
 )
 
-var errNoAvail = errors.New("no available fuse devices")
-
-var errNotLoaded = errors.New("osxfuse is not loaded")
+var (
+	errNoAvail         = errors.New("no available fuse devices")
+	errNotLoaded       = errors.New("osxfuse is not loaded")
+	errOSXFUSENotFound = errors.New("cannot locate OSXFUSE")
+)
 
 func loadOSXFUSE(bin string) error {
 	cmd := exec.Command(bin)
@@ -170,22 +172,36 @@ func callMount(bin string, dir string, conf *mountConfig, f *os.File, ready chan
 }
 
 func mount(dir string, conf *mountConfig, ready chan<- struct{}, errp *error) (*os.File, error) {
-	f, err := openOSXFUSEDev("/dev/osxfuse")
-	if err == errNotLoaded {
-		err = loadOSXFUSE("/Library/Filesystems/osxfusefs.fs/Support/load_osxfusefs")
+	locations := conf.osxfuseLocations
+	if locations == nil {
+		locations = []OSXFUSEPaths{
+			OSXFUSELocationV3,
+			OSXFUSELocationV2,
+		}
+	}
+	for _, loc := range locations {
+		f, err := openOSXFUSEDev(loc.DevicePrefix)
+		if err == errNotLoaded {
+			err = loadOSXFUSE(loc.Load)
+			if err != nil {
+				if os.IsNotExist(err) {
+					// try the other locations
+					continue
+				}
+				return nil, err
+			}
+			// try again
+			f, err = openOSXFUSEDev(loc.DevicePrefix)
+		}
 		if err != nil {
 			return nil, err
 		}
-		// try again
-		f, err = openOSXFUSEDev("/dev/osxfuse")
-	}
-	if err != nil {
-		return nil, err
-	}
-	err = callMount("/Library/Filesystems/osxfusefs.fs/Support/mount_osxfusefs", dir, conf, f, ready, errp)
-	if err != nil {
-		f.Close()
-		return nil, err
+		err = callMount(loc.Mount, dir, conf, f, ready, errp)
+		if err != nil {
+			f.Close()
+			return nil, err
+		}
+		return f, nil
 	}
-	return f, nil
+	return nil, errOSXFUSENotFound
 }

options.go

@@ -12,9 +12,10 @@ func dummyOption(conf *mountConfig) error {
 // mountConfig holds the configuration for a mount operation.
 // Use it by passing MountOption values to Mount.
 type mountConfig struct {
-	options      map[string]string
-	maxReadahead uint32
-	initFlags    InitFlags
+	options          map[string]string
+	maxReadahead     uint32
+	initFlags        InitFlags
+	osxfuseLocations []OSXFUSEPaths
 }
 
 func escapeComma(s string) string {
@@ -168,3 +169,50 @@ func WritebackCache() MountOption {
 		return nil
 	}
 }
+
+// OSXFUSEPaths describes the paths used by an installed OSXFUSE
+// version. See OSXFUSELocationV3 for typical values.
+type OSXFUSEPaths struct {
+	// Prefix for the device file. At mount time, an incrementing
+	// number is suffixed until a free FUSE device is found.
+	DevicePrefix string
+	// Path of the load helper, used to load the kernel extension if
+	// no device files are found.
+	Load string
+	// Path of the mount helper, used for the actual mount operation.
+	Mount string
+}
+
+// Default paths for OSXFUSE. See OSXFUSELocations.
+var (
+	OSXFUSELocationV3 = OSXFUSEPaths{
+		DevicePrefix: "/dev/osxfuse",
+		Load:         "/Library/Filesystems/osxfuse.fs/Contents/Resources/load_osxfuse",
+		Mount:        "/Library/Filesystems/osxfuse.fs/Contents/Resources/mount_osxfuse",
+	}
+	OSXFUSELocationV2 = OSXFUSEPaths{
+		DevicePrefix: "/dev/osxfuse",
+		Load:         "/Library/Filesystems/osxfusefs.fs/Support/load_osxfusefs",
+		Mount:        "/Library/Filesystems/osxfusefs.fs/Support/mount_osxfusefs",
+	}
+)
+
+// OSXFUSELocations sets where to look for OSXFUSE files. The
+// arguments are all the possible locations. The previous locations
+// are replaced.
+//
+// Without this option, OSXFUSELocationV3 and OSXFUSELocationV2 are
+// used.
+//
+// OS X only. Others ignore this option.
+func OSXFUSELocations(paths ...OSXFUSEPaths) MountOption {
+	return func(conf *mountConfig) error {
+		if len(paths) == 0 {
+			return errors.New("must specify at least one location for OSXFUSELocations")
+		}
+		// replace previous values, but make a copy so there's no
+		// worries about caller mutating their slice
+		conf.osxfuseLocations = append(conf.osxfuseLocations[:0], paths...)
+		return nil
+	}
+}