Improved macOS VSS checks

VSS support now determined by:
1) Checking that repository filesystem is APFS rather than by inspecting macOS version
2) Checking that repository resides on local device (as external APFS formatted drives are not supported) rather than crudely by path name

src/duplicacy_shadowcopy_osx.go

@@ -16,31 +16,39 @@ import (
 	"errors"
 	"time"
 	"syscall"
-	"strconv"
 )
 
 var snapshotPath string
 var snapshotDate string
 
-func GetKernelVersion() (major int, minor int, component int, err error) {
+// Converts char array to string
+func CharsToString(ca []int8) string {
 	
-	versionString, err := syscall.Sysctl("kern.osrelease")
-	if err != nil {
-		return 0, 0, 0, err;
+	len := len(ca)
+	ba := make([]byte, len) 
+    
+    for i, v := range ca {
+        ba[i] = byte(v)
+		if ba[i] == 0 {
+			len = i
+			break
+		}
 	}
 	
-	versionSplit := strings.Split(versionString, ".")
-	if len(versionSplit) != 3 {
-		return 0, 0, 0, errors.New("Sysctl returned invalid kernel version string")
-	}
-	
-	major, _ = strconv.Atoi(versionSplit[0])
-	minor, _ = strconv.Atoi(versionSplit[1])
-	component, _ = strconv.Atoi(versionSplit[2])
-	
-	return major, minor, component, nil;
+    return string(ba[:len])
 }
 
+// Get ID of device containing path
+func GetPathDeviceId(path string) (deviceId int32, err error) {
+	stat := syscall.Stat_t{}
+	err = syscall.Stat(path, &stat)
+	if err != nil {
+		return 0, err
+	}
+	return stat.Dev, nil
+}
+
+// Executes shell command with timeout and returns stdout
 func CommandWithTimeout(timeoutInSeconds int, name string, arg ...string) (output string, err error) {
 
     ctx, cancel := context.WithTimeout(context.Background(), time.Duration(timeoutInSeconds) * time.Second)
@@ -88,19 +96,31 @@ func CreateShadowCopy(top string, shadowCopy bool, timeoutInSeconds int) (shadow
 		return top
 	}
 	
-	major, _, _, err := GetKernelVersion()
+	// Check repository filesystem is APFS
+	stat := syscall.Statfs_t{}
+	err := syscall.Statfs(top, &stat)
 	if err != nil {
-		LOG_ERROR("VSS_INIT", "Failed to get kernel version: " + err.Error())
+		LOG_ERROR("VSS_INIT", "Unable to determine filesystem of repository path")
+		return top
+	}
+	if CharsToString(stat.Fstypename[:]) != "apfs" {
+		LOG_WARN("VSS_INIT", "VSS requires APFS filesystem")
 		return top
 	}
 	
-	if major < 17 {
-		LOG_WARN("VSS_INIT", "VSS requires macOS 10.13 High Sierra or higher")
+	// Check path is local as tmutil snapshots will not support APFS formatted external drives
+	deviceIdLocal, err := GetPathDeviceId("/")
+	if err != nil {
+		LOG_ERROR("VSS_INIT", "Unable to get device ID of path: /")
 		return top
 	}
-	
-	if top == "/Volumes" || strings.HasPrefix(top, "/Volumes/") {
-		LOG_ERROR("VSS_PATH", "Invalid repository path: %s", top)
+	deviceIdRepository, err := GetPathDeviceId(top)
+	if err != nil {
+		LOG_ERROR("VSS_INIT", "Unable to get device ID of path: ", top)
+		return top
+	}
+	if deviceIdLocal != deviceIdRepository {
+		LOG_WARN("VSS_PATH", "VSS not supported for non-local repository path: ", top)
 		return top 
 	}
 
@@ -108,29 +128,32 @@ func CreateShadowCopy(top string, shadowCopy bool, timeoutInSeconds int) (shadow
         timeoutInSeconds = 60
 	}
 	
+	// Create mount point
     snapshotPath, err = ioutil.TempDir("/tmp/", "snp_")
 	if err != nil {
 	    LOG_ERROR("VSS_CREATE", "Failed to create temporary mount directory")
 		return top
 	}
 	
+	// Use tmutil to create snapshot
 	tmutilOutput, err := CommandWithTimeout(timeoutInSeconds, "tmutil", "snapshot")
     if err != nil {
-	    LOG_ERROR("VSS_CREATE", "Error while calling tmutil: " + err.Error())
+	    LOG_ERROR("VSS_CREATE", "Error while calling tmutil: ", err)
 		return top
 	}
 	
 	colonPos := strings.IndexByte(tmutilOutput, ':')
 	if colonPos < 0 {
-	    LOG_ERROR("VSS_CREATE", "Snapshot creation failed: " + tmutilOutput)
+	    LOG_ERROR("VSS_CREATE", "Snapshot creation failed: ", tmutilOutput)
 		return top
 	}
 	snapshotDate = strings.TrimSpace(tmutilOutput[colonPos+1:])
 
+	// Mount snapshot as readonly and hide from GUI i.e. Finder
 	_, err = CommandWithTimeout(timeoutInSeconds, 
 		"mount", "-t", "apfs", "-o", "nobrowse,-r,-s=com.apple.TimeMachine." + snapshotDate, "/", snapshotPath)
 	if err != nil {
-	    LOG_ERROR("VSS_CREATE", "Error while mounting snapshot: " + err.Error())
+	    LOG_ERROR("VSS_CREATE", "Error while mounting snapshot: ", err)
         return top
 	}