mark zfs arc size as free in memory

src/modules/memory/linux.cpp

@@ -1,8 +1,29 @@
 #include "modules/memory.hpp"
 
+static unsigned zfsArcSize() {
+  std::ifstream zfs_arc_stats{"/proc/spl/kstat/zfs/arcstats"};
+
+  if (zfs_arc_stats.is_open()) {
+    std::string   name;
+    std::string   type;
+    unsigned long data{0};
+
+    std::string line;
+    while (std::getline(zfs_arc_stats, line)) {
+      std::stringstream(line) >> name >> type >> data;
+
+      if (name == "size") {
+        return data / 1024;  // convert to kB
+      }
+    }
+  }
+
+  return 0;
+}
+
 void waybar::modules::Memory::parseMeminfo() {
   const std::string data_dir_ = "/proc/meminfo";
-  std::ifstream info(data_dir_);
+  std::ifstream     info(data_dir_);
   if (!info.is_open()) {
     throw std::runtime_error("Can't open " + data_dir_);
   }
@@ -17,4 +38,6 @@ void waybar::modules::Memory::parseMeminfo() {
     int64_t     value = std::stol(line.substr(posDelim + 1));
     meminfo_[name] = value;
   }
+
+  meminfo_["zfs_size"] = zfsArcSize();
 }