Handle offline CPUs and CPU hotplug

First of all in case when the number CPUs change
prevent out-of-bound index access in
waybar::modules::CpuUsage::getCpuUsage()

Secondly on Linux when updating CPU usage
read /sys/devices/system/cpu/present
and use it to detect the offline CPUs missing from /proc/stat

For offline CPUs report 0 usage and "offline" in the tooltip

Fixes issue #3498

On Linux one can test this functionality with:
echo 0 > /sys/devices/system/cpu/cpu1/online
echo 1 > /sys/devices/system/cpu/cpu1/online

On non-Linux OSes I'm not sure how to detect offline CPUs,
so I didn't add the offline CPU detection there
but at least CPU number change should not cause a crash there anymore
or cause memory safety issues after this fix

src/modules/cpu_usage/common.cpp

@@ -61,9 +61,36 @@ std::tuple<std::vector<uint16_t>, std::string> waybar::modules::CpuUsage::getCpu
   std::vector<std::tuple<size_t, size_t>> curr_times = CpuUsage::parseCpuinfo();
   std::string tooltip;
   std::vector<uint16_t> usage;
+
+  if (curr_times.size() != prev_times.size()) {
+    // The number of CPUs has changed, eg. due to CPU hotplug
+    // We don't know which CPU came up or went down
+    // so only give total usage (if we can)
+    if (!curr_times.empty() && !prev_times.empty()) {
+      auto [curr_idle, curr_total] = curr_times[0];
+      auto [prev_idle, prev_total] = prev_times[0];
+      const float delta_idle = curr_idle - prev_idle;
+      const float delta_total = curr_total - prev_total;
+      uint16_t tmp = 100 * (1 - delta_idle / delta_total);
+      tooltip = fmt::format("Total: {}%\nCores: (pending)", tmp);
+      usage.push_back(tmp);
+    } else {
+      tooltip = "(pending)";
+      usage.push_back(0);
+    }
+    prev_times = curr_times;
+    return {usage, tooltip};
+  }
+
   for (size_t i = 0; i < curr_times.size(); ++i) {
     auto [curr_idle, curr_total] = curr_times[i];
     auto [prev_idle, prev_total] = prev_times[i];
+    if (i > 0 && (curr_total == 0 || prev_total == 0)) {
+      // This CPU is offline
+      tooltip = tooltip + fmt::format("\nCore{}: offline", i - 1);
+      usage.push_back(0);
+      continue;
+    }
     const float delta_idle = curr_idle - prev_idle;
     const float delta_total = curr_total - prev_total;
     uint16_t tmp = 100 * (1 - delta_idle / delta_total);

src/modules/cpu_usage/linux.cpp

@@ -3,6 +3,23 @@
 #include "modules/cpu_usage.hpp"
 
 std::vector<std::tuple<size_t, size_t>> waybar::modules::CpuUsage::parseCpuinfo() {
+  // Get the "existing CPU count" from /sys/devices/system/cpu/present
+  // Probably this is what the user wants the offline CPUs accounted from
+  // For further details see:
+  // https://www.kernel.org/doc/html/latest/core-api/cpu_hotplug.html
+  const std::string sys_cpu_present_path = "/sys/devices/system/cpu/present";
+  size_t cpu_present_last = 0;
+  std::ifstream cpu_present_file(sys_cpu_present_path);
+  std::string cpu_present_text;
+  if (cpu_present_file.is_open()) {
+    getline(cpu_present_file, cpu_present_text);
+    // This is a comma-separated list of ranges, eg. 0,2-4,7
+    size_t last_separator = cpu_present_text.find_last_of("-,");
+    if (last_separator < cpu_present_text.size()) {
+      std::stringstream(cpu_present_text.substr(last_separator + 1)) >> cpu_present_last;
+    }
+  }
+
   const std::string data_dir_ = "/proc/stat";
   std::ifstream info(data_dir_);
   if (!info.is_open()) {
@@ -10,14 +27,23 @@ std::vector<std::tuple<size_t, size_t>> waybar::modules::CpuUsage::parseCpuinfo(
   }
   std::vector<std::tuple<size_t, size_t>> cpuinfo;
   std::string line;
+  size_t current_cpu_number = -1;  // First line is total, second line is cpu 0
   while (getline(info, line)) {
     if (line.substr(0, 3).compare("cpu") != 0) {
       break;
     }
+    size_t line_cpu_number;
+    if (current_cpu_number >= 0) {
+      std::stringstream(line.substr(3)) >> line_cpu_number;
+      while (line_cpu_number > current_cpu_number) {
+        // Fill in 0 for offline CPUs missing inside the lines of /proc/stat
+        cpuinfo.emplace_back(0, 0);
+        current_cpu_number++;
+      }
+    }
     std::stringstream sline(line.substr(5));
     std::vector<size_t> times;
-    for (size_t time = 0; sline >> time; times.push_back(time))
-      ;
+    for (size_t time = 0; sline >> time; times.push_back(time));
 
     size_t idle_time = 0;
     size_t total_time = 0;
@@ -27,6 +53,14 @@ std::vector<std::tuple<size_t, size_t>> waybar::modules::CpuUsage::parseCpuinfo(
       total_time = std::accumulate(times.begin(), times.end(), 0);
     }
     cpuinfo.emplace_back(idle_time, total_time);
+    current_cpu_number++;
   }
+
+  while (cpu_present_last >= current_cpu_number) {
+    // Fill in 0 for offline CPUs missing after the lines of /proc/stat
+    cpuinfo.emplace_back(0, 0);
+    current_cpu_number++;
+  }
+
   return cpuinfo;
 }