Fix the return value of Concurrent.available_processor_count when cpu.cfs_quota_us is -1

I tried to use `Concurrent.available_processor_count` in `parallel`
gem, but we got some feedback `Concurrent.available_processor_count`
returned a negative value.
grosser/parallel#348 (comment)
grosser/parallel#349 (comment)

According to the https://docs.kernel.org/scheduler/sched-bwc.html#management,
The default value of `cpu.cfs_quota_us` is -1. In that case,
cgroup does not adhere to any CPU time restrictions.

This PR adds the case of `cpu.cfs_quota_us` is -1 to `#cpu_quota`
to return processor count from `Concurrent.available_processor_count`
in that case.

Author: y-yagi

lib/concurrent-ruby/concurrent/utility/processor_counter.rb

@@ -83,7 +83,7 @@ def compute_physical_processor_count
                   # Bail out if both commands returned something unexpected
                   processor_count
                 else
-                  # powershell: "\nNumberOfCores\n-------------\n            4\n\n\n" 
+                  # powershell: "\nNumberOfCores\n-------------\n            4\n\n\n"
                   # wmic:       "NumberOfCores  \n\n4              \n\n\n\n"
                   result.scan(/\d+/).map(&:to_i).reduce(:+)
                 end
@@ -112,7 +112,9 @@ def compute_cpu_quota
           elsif File.exist?("/sys/fs/cgroup/cpu,cpuacct/cpu.cfs_quota_us")
             # cgroups v1: https://kernel.googlesource.com/pub/scm/linux/kernel/git/glommer/memcg/+/cpu_stat/Documentation/cgroups/cpu.txt
             max = File.read("/sys/fs/cgroup/cpu,cpuacct/cpu.cfs_quota_us").to_i
-            return nil if max == 0
+            # If the cpu.cfs_quota_us is -1, cgroup does not adhere to any CPU time restrictions
+            # https://docs.kernel.org/scheduler/sched-bwc.html#management
+            return nil if max == 0 || max == -1
             period = File.read("/sys/fs/cgroup/cpu,cpuacct/cpu.cfs_period_us").to_f
             max / period
           end

spec/concurrent/utility/processor_count_spec.rb

@@ -56,6 +56,15 @@ module Concurrent
       expect(counter.cpu_quota).to be_nil
     end
 
+    it 'returns nil if cgroups v1 and cpu.cfs_quota_us is -1' do
+      expect(RbConfig::CONFIG).to receive(:[]).with("target_os").and_return("linux")
+      expect(File).to receive(:exist?).with("/sys/fs/cgroup/cpu.max").and_return(false)
+      expect(File).to receive(:exist?).with("/sys/fs/cgroup/cpu,cpuacct/cpu.cfs_quota_us").and_return(true)
+
+      expect(File).to receive(:read).with("/sys/fs/cgroup/cpu,cpuacct/cpu.cfs_quota_us").and_return("-1\n")
+      expect(counter.cpu_quota).to be_nil
+    end
+
     it 'returns a float if cgroups v1 sets a limit' do
       expect(RbConfig::CONFIG).to receive(:[]).with("target_os").and_return("linux")
       expect(File).to receive(:exist?).with("/sys/fs/cgroup/cpu.max").and_return(false)