fix: correct parameter name in error log when split fails to open a… (#948)

* fix: correct parameter name in error log when `split` fails to open a library handle

* improve error reporting a bit

* pulls the error up from GetHandle() into Split() where more context exists
* tells the user only to use the --xcode parameter if they haven't done so
* differentiates between a missing path and a failure to dlopen()

Author: supervacuus

pkg/dyld/split.go

@@ -60,7 +60,7 @@ type XCodeAppInfoPlist struct {
 }
 
 // GetHandle returns a handle to a library
-func GetHandle(libs []string) (*LibHandle, error) {
+func GetHandle(libs []string) *LibHandle {
 	for _, name := range libs {
 		libname := C.CString(name)
 		defer C.free(unsafe.Pointer(libname))
@@ -71,10 +71,10 @@ func GetHandle(libs []string) (*LibHandle, error) {
 				Handle:  handle,
 				Libname: name,
 			}
-			return h, nil
+			return h
 		}
 	}
-	return nil, fmt.Errorf("unable to open a handle to the Xcode library (use --xcode-path to specify the path to Xcode)")
+	return nil
 }
 
 // GetSymbolPointer takes a symbol name and returns a pointer to the symbol.
@@ -107,22 +107,40 @@ func (l *LibHandle) Close() error {
 // Split extracts all the dyld_shared_cache libraries
 func Split(dyldSharedCachePath, destinationPath, xcodePath string, xcodeCache bool) error {
 	var xcodeVersion string
+	var xcodePathProvided bool
 
 	if len(xcodePath) == 0 {
 		var err error
 		xcodePath, err = utils.GetXCodePath()
 		if err != nil {
 			return fmt.Errorf("failed to get Xcode path: %v", err)
 		}
+		xcodePathProvided = false
 	} else {
 		if !strings.HasSuffix(xcodePath, "Contents/Developer") {
 			xcodePath = filepath.Join(xcodePath, "Contents/Developer")
 		}
+		xcodePathProvided = true
 	}
 
-	dscExtractor, err := GetHandle([]string{filepath.Join(xcodePath, "Platforms/iPhoneOS.platform/usr/lib/dsc_extractor.bundle")})
+	dscExtractorPath := filepath.Join(xcodePath, "Platforms/iPhoneOS.platform/usr/lib/dsc_extractor.bundle")
+	_, err := os.Stat(dscExtractorPath)
 	if err != nil {
-		return fmt.Errorf("failed to split %s: %v", dyldSharedCachePath, err)
+		err_msg := "failed to find DSC extractor library in %s"
+		if !xcodePathProvided {
+			err_msg += " (use --xcode to specify the path to Xcode)"
+		}
+		return fmt.Errorf(err_msg, dscExtractorPath)
+	}
+
+	dscExtractor := GetHandle([]string{dscExtractorPath})
+	if dscExtractor == nil {
+		err_msg := "unable to open a handle to the DSC extractor library in %s"
+		if !xcodePathProvided {
+			err_msg += " (use --xcode to specify the path to Xcode)"
+		}
+
+		return fmt.Errorf(err_msg, dyldSharedCachePath, dscExtractorPath)
 	}
 
 	if xcodeCache {