mark zfs arc size as free in memory

src/modules/memory/common.cpp

@@ -15,11 +15,11 @@ auto waybar::modules::Memory::update() -> void {
   unsigned long memfree;
   if (meminfo_.count("MemAvailable")) {
     // New kernels (3.4+) have an accurate available memory field.
-    memfree = meminfo_["MemAvailable"];
+    memfree = meminfo_["MemAvailable"] + meminfo_["zfs_size"];
   } else {
     // Old kernel; give a best-effort approximation of available memory.
     memfree = meminfo_["MemFree"] + meminfo_["Buffers"] + meminfo_["Cached"] +
-              meminfo_["SReclaimable"] - meminfo_["Shmem"];
+              meminfo_["SReclaimable"] - meminfo_["Shmem"] + meminfo_["zfs_size"];
   }
 
   if (memtotal > 0 && memfree >= 0) {

src/modules/memory/linux.cpp

@@ -1,5 +1,26 @@
 #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_);
@@ -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();
 }