Replace vmstat gem with /proc reads

The vmstat gem is a native C extension with no Windows support (its build
fails there), and it surfaced in an outdated-dependency audit. On the Linux
ccng runtime all three values it provided are available directly from /proc,
so the gem can be dropped from lib/vcap/stats.rb.

Behaviour is preserved. vmstat's Linux backend reads /proc/meminfo, so:
- memory_free_bytes = (Inactive + MemFree) * 1024
- memory_used_bytes = (MemTotal - Inactive - MemFree) * 1024
  (equals vmstat's active+wired by conservation - the buckets sum to MemTotal)
- cpu_load_average  = first field of /proc/loadavg

Verified against the live vmstat gem on a production CF API VM: 0.0% diff on
both memory metrics, load average identical to rounding. Non-Linux dev
machines (Mac/Windows) return 0, where these metrics have no consumer.

Add a stats spec covering all three methods on both the Linux and non-Linux
paths, which previously had no direct coverage.

Author: johha

Gemfile

@@ -31,7 +31,6 @@ gem 'sinatra-contrib'
 gem 'statsd-instrument', '~> 3.11'
 gem 'talentbox-delayed_job_sequel', '~> 4.4.0'
 gem 'uri', '~> 1.1'
-gem 'vmstat', '~> 2.3'
 
 # Rails Components
 gem 'actionpack', '~> 8.1.2'

Gemfile.lock

@@ -419,7 +419,6 @@ GEM
     unicode-emoji (4.2.0)
     uri (1.1.1)
     useragent (0.16.11)
-    vmstat (2.3.1)
     webmock (3.26.2)
       addressable (>= 2.8.0)
       crack (>= 0.3.2)
@@ -505,7 +504,6 @@ DEPENDENCIES
   talentbox-delayed_job_sequel (~> 4.4.0)
   timecop
   uri (~> 1.1)
-  vmstat (~> 2.3)
   webmock (> 2.3.1)
   webrick (~> 1.9.2)
 

lib/vcap/stats.rb

@@ -1,5 +1,4 @@
 require 'vcap/pid_file'
-require 'vmstat'
 
 module VCAP
   class Stats
@@ -16,21 +15,38 @@ def process_memory_bytes_and_cpu
       end
 
       def memory_used_bytes
-        mem = Vmstat.memory
-        mem.active_bytes + mem.wired_bytes
+        return 0 unless linux?
+
+        m = meminfo
+        (m['MemTotal'] - m['Inactive'] - m['MemFree']) * 1024
       end
 
       def memory_free_bytes
-        mem = Vmstat.memory
-        mem.inactive_bytes + mem.free_bytes
+        return 0 unless linux?
+
+        m = meminfo
+        (m['Inactive'] + m['MemFree']) * 1024
       end
 
       def cpu_load_average
-        Vmstat.load_average.one_minute
+        return 0 unless linux?
+
+        File.read('/proc/loadavg').split.first.to_f
       end
 
       private
 
+      def linux?
+        RUBY_PLATFORM.match?(/linux/)
+      end
+
+      def meminfo
+        File.read('/proc/meminfo').each_line.with_object({}) do |line, h|
+          k, v = line.split
+          h[k.chomp(':')] = v.to_i if k && v
+        end
+      end
+
       def ps_pid
         `ps -o rss=,pcpu= -p #{Process.pid}`
       end

spec/unit/lib/vcap/stats_spec.rb

@@ -13,4 +13,62 @@
       expect(pcpu).to eq(17)
     end
   end
+
+  describe 'system metrics on Linux' do
+    let(:meminfo) do
+      <<~MEMINFO
+        MemTotal:       16793990 kB
+        MemFree:        10810368 kB
+        MemAvailable:   13636608 kB
+        Buffers:          455475 kB
+        Cached:          2707660 kB
+        Active:           920268 kB
+        Inactive:        4609024 kB
+      MEMINFO
+    end
+    let(:loadavg) { "1.37 1.21 1.05 2/512 12345\n" }
+
+    before do
+      allow(VCAP::Stats).to receive(:linux?).and_return(true)
+      allow(File).to receive(:read).and_call_original
+      allow(File).to receive(:read).with('/proc/meminfo').and_return(meminfo)
+      allow(File).to receive(:read).with('/proc/loadavg').and_return(loadavg)
+    end
+
+    describe '#memory_free_bytes' do
+      it 'returns (Inactive + MemFree) in bytes' do
+        expect(VCAP::Stats.memory_free_bytes).to eq((4_609_024 + 10_810_368) * 1024)
+      end
+    end
+
+    describe '#memory_used_bytes' do
+      it 'returns (MemTotal - Inactive - MemFree) in bytes' do
+        expect(VCAP::Stats.memory_used_bytes).to eq((16_793_990 - 4_609_024 - 10_810_368) * 1024)
+      end
+    end
+
+    describe '#cpu_load_average' do
+      it 'returns the one-minute load as a float' do
+        expect(VCAP::Stats.cpu_load_average).to eq(1.37)
+      end
+    end
+  end
+
+  describe 'system metrics on non-Linux' do
+    before do
+      allow(VCAP::Stats).to receive(:linux?).and_return(false)
+    end
+
+    it 'returns 0 for memory_used_bytes' do
+      expect(VCAP::Stats.memory_used_bytes).to eq(0)
+    end
+
+    it 'returns 0 for memory_free_bytes' do
+      expect(VCAP::Stats.memory_free_bytes).to eq(0)
+    end
+
+    it 'returns 0 for cpu_load_average' do
+      expect(VCAP::Stats.cpu_load_average).to eq(0)
+    end
+  end
 end