(FACT-3149) Fix memory usage reporting on FreeBSD

* In vmstat(8), "free memory" correspond to unused memory that does not
  contain any data, and does not include cache / inactive memory which
  has some data but is immediatly available to the system if memory is
  needed.
  Prefer to get active and wired memory page count form sysctl(3) which
  correpond to the used memory that can (active) and cannot (wired) be
  swapped out, and multiply these by the size of a memory page.  This
  give a better overview of the current memory usage.
* Prefer `vm.stats.vm.v_page_count` over `hw.physmem`:  The available
  memory can be lower that what is physically present in the system and
  the free / used memory is a portion of the usable memory more than a
  portion of the physical memory.

lib/facter/resolvers/freebsd/system_memory.rb

@@ -15,25 +15,33 @@ def post_resolve(fact_name, _options)
 
           def calculate_system_memory(fact_name)
             read_total_memory_in_bytes
-            read_available_memory_in_bytes
+            read_used_memory_in_bytes
 
-            @fact_list[:used_bytes] = @fact_list[:total_bytes] - @fact_list[:available_bytes]
+            @fact_list[:available_bytes] = @fact_list[:total_bytes] - @fact_list[:used_bytes]
             @fact_list[:capacity] = Facter::Util::Resolvers::FilesystemHelper
                                     .compute_capacity(@fact_list[:used_bytes], @fact_list[:total_bytes])
 
             @fact_list[fact_name]
           end
 
-          def read_available_memory_in_bytes
-            output = Facter::Core::Execution.execute('vmstat -H --libxo json', logger: log)
-            data = JSON.parse(output)
-            @fact_list[:available_bytes] = data['memory']['free-memory'] * 1024
+          def pagesize
+            @pagesize ||= Facter::Freebsd::FfiHelper.sysctl_by_name(:long, 'vm.stats.vm.v_page_size')
+          end
+
+          def read_used_memory_in_bytes
+            require_relative 'ffi/ffi_helper'
+
+            @fact_list[:used_bytes] = pagesize * (
+              Facter::Freebsd::FfiHelper.sysctl_by_name(:long, 'vm.stats.vm.v_active_count') +
+              Facter::Freebsd::FfiHelper.sysctl_by_name(:long, 'vm.stats.vm.v_wire_count')
+            )
           end
 
           def read_total_memory_in_bytes
             require_relative 'ffi/ffi_helper'
 
-            @fact_list[:total_bytes] = Facter::Freebsd::FfiHelper.sysctl_by_name(:long, 'hw.physmem')
+            @fact_list[:total_bytes] = pagesize *
+                                       Facter::Freebsd::FfiHelper.sysctl_by_name(:long, 'vm.stats.vm.v_page_count')
           end
         end
       end

spec/facter/resolvers/freebsd/system_memory_spec.rb

@@ -12,12 +12,17 @@
   before do
     system_memory.instance_variable_set(:@log, log_spy)
     allow(Facter::Freebsd::FfiHelper).to receive(:sysctl_by_name)
-      .with(:long, 'hw.physmem')
-      .and_return(17_043_554_304)
-
-    allow(Facter::Core::Execution).to receive(:execute)
-      .with('vmstat -H --libxo json', { logger: log_spy })
-      .and_return(load_fixture('freebsd_vmstat').read)
+      .with(:long, 'vm.stats.vm.v_page_size')
+      .and_return(4096)
+    allow(Facter::Freebsd::FfiHelper).to receive(:sysctl_by_name)
+      .with(:long, 'vm.stats.vm.v_page_count')
+      .and_return(4_161_024)
+    allow(Facter::Freebsd::FfiHelper).to receive(:sysctl_by_name)
+      .with(:long, 'vm.stats.vm.v_active_count')
+      .and_return(2_335_139)
+    allow(Facter::Freebsd::FfiHelper).to receive(:sysctl_by_name)
+      .with(:long, 'vm.stats.vm.v_wire_count')
+      .and_return(1_167_569)
   end
 
   it 'returns available system memory in bytes' do