Add proper logfile support

shared/flags.go

@@ -25,6 +25,11 @@ var (
 		false,
 		"Also start the local debugging server",
 	)
+	FlagLogFile *string = flag.String(
+		"logfile",
+		"/var/log/linstrom/logs",
+		"Set the target logging file. Linstrom auto-rotates these. If it can't be created, it will be ignored",
+	)
 )
 
 func flagUsage() {
@@ -50,5 +55,5 @@ func flagUsage() {
 }
 
 func init() {
-	flag.Usage = flagUsage
+	// flag.Usage = flagUsage
 }

shared/isWritable.go

@@ -0,0 +1,11 @@
+//go:build !windows
+// +build !windows
+
+package shared
+
+import "golang.org/x/sys/unix"
+
+// Copied from https://stackoverflow.com/a/20026945 and https://stackoverflow.com/a/49148866
+func IsWritable(path string) bool {
+	return unix.Access(path, unix.W_OK) == nil
+}

shared/isWritable_windows.go

@@ -0,0 +1,23 @@
+package shared
+
+import "os"
+
+// Copied from https://stackoverflow.com/a/20026945 and https://stackoverflow.com/a/49148866
+func IsWritable(path string) bool {
+	info, err := os.Stat(path)
+	if err != nil {
+		return false
+	}
+
+	err = nil
+	if !info.IsDir() {
+		return false
+	}
+
+	// Check if the user bit is enabled in file permission
+	if info.Mode().Perm()&(1<<(uint(7))) == 0 {
+		return false
+	}
+
+	return true
+}