misc/getfiles: fix infinite recursion loop when given a symlink

This fixes an infinite recursion loop in getFile caused by:

1) `os.Stat(file)` resolves a symlink so that `fileInfo.isDir()` returns True
2) `filepath.Walk()` starts iterating on the root directory (in this case the symlink)
3) `filepath.Walk()` uses `os.Lstat` internally, which does NOT dereference the symlink
4) in the walk func, `f.isDir()` returns False, and the walk func calls `getFile()` on it
5) goto 1

fixes #47

Part-of: https://gitlab.postmarketos.org/postmarketOS/postmarketos-mkinitfs/-/merge_requests/65

internal/misc/getfiles.go

@@ -3,6 +3,7 @@ package misc
 import (
 	"debug/elf"
 	"fmt"
+	"io/fs"
 	"os"
 	"path/filepath"
 
@@ -39,6 +40,24 @@ func getFile(file string, required bool) (files []string, err error) {
 		return RemoveDuplicates(files), nil
 	}
 
+	// If the file is a symlink we need to do this to prevent an infinite recursion
+	// loop:
+	// Symlinks need special handling to prevent infinite recursion:
+	// 1) add the symlink to the list of files
+	// 2) set file to dereferenced target
+	// 4) continue this function to either walk it if the target is a dir or add the
+	// target to the list of files
+	if s, err := os.Lstat(file); err != nil {
+		return files, err
+	} else if s.Mode()&fs.ModeSymlink != 0 {
+		files = append(files, file)
+		if target, err := filepath.EvalSymlinks(file); err != nil {
+			return files, err
+		} else {
+			file = target
+		}
+	}
+
 	fileInfo, err := os.Stat(file)
 	if err != nil {
 		// Check if there is a Zstd-compressed version of the file