additions:

- act on error in walk function (OS can detect symbolic links loops (lnk -> lnk))
- add a deepness detector for non trivial loops (dir/lnk -> ../..)

Author: d-a-v

arduino/builder/sketch.go

@@ -22,6 +22,7 @@ import (
 	"path/filepath"
 	"regexp"
 	"strings"
+	"fmt"
 
 	"github.com/arduino/arduino-cli/arduino/globals"
 	"github.com/arduino/arduino-cli/arduino/sketch"
@@ -59,7 +60,7 @@ func SketchSaveItemCpp(item *sketch.Item, destPath string) error {
 }
 
 // SimpleLocalWalk locally replaces filepath.Walk and/but goes through symlinks
-func SimpleLocalWalk(root string, walkFn func(path string, info os.FileInfo, err error) error) error {
+func SimpleLocalWalkRecursive(root string, maxDepth int, walkFn func(path string, info os.FileInfo, err error) error) error {
 
 	info, err := os.Stat(root)
 
@@ -73,10 +74,14 @@ func SimpleLocalWalk(root string, walkFn func(path string, info os.FileInfo, err
 	}
 
 	if info.IsDir() {
+		if maxDepth <= 0 {
+			return walkFn(root, info, errors.New("Filesystem bottom is too deep (directory recursion or filesystem really deep): " + root))
+		}
+		maxDepth--
 		files, err := ioutil.ReadDir(root)
 		if err == nil {
 			for _, file := range files {
-				err = SimpleLocalWalk(root+string(os.PathSeparator)+file.Name(), walkFn)
+				err = SimpleLocalWalkRecursive(root+string(os.PathSeparator)+file.Name(), maxDepth, walkFn)
 				if err == filepath.SkipDir {
 					return nil
 				}
@@ -87,6 +92,11 @@ func SimpleLocalWalk(root string, walkFn func(path string, info os.FileInfo, err
 	return nil
 }
 
+func SimpleLocalWalk(root string, walkFn func(path string, info os.FileInfo, err error) error) error {
+	// see discussion in https://github.com/arduino/arduino-cli/pull/421
+	return SimpleLocalWalkRecursive(root, 40, walkFn)
+}
+
 // SketchLoad collects all the files composing a sketch.
 // The parameter `sketchPath` holds a path pointing to a single sketch file or a sketch folder,
 // the path must be absolute.
@@ -124,6 +134,12 @@ func SketchLoad(sketchPath, buildPath string) (*sketch.Sketch, error) {
 	// collect all the sketch files
 	var files []string
 	err = SimpleLocalWalk(sketchFolder, func(path string, info os.FileInfo, err error) error {
+
+		if err != nil {
+			fmt.Printf("\nerror: %+v\n\n", err)
+			return filepath.SkipDir;
+		}
+
 		// ignore hidden files and skip hidden directories
 		if strings.HasPrefix(info.Name(), ".") {
 			if info.IsDir() {