Facts: Use vm_stat instead of sysctl for free memory (#52917)

Signed-off-by: Abhijeet Kasurde <akasurde@redhat.com>

changelogs/fragments/52896-gather_facts-fix_negative_free_value.yml

@@ -0,0 +1,2 @@
+minor_changes:
+- Add better parsing for gathering facts about free memory in Mac OS (https://github.com/ansible/ansible/pull/52917).

lib/ansible/module_utils/facts/hardware/darwin.py

@@ -17,8 +17,8 @@
 from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type
 
+from ansible.module_utils.common.process import get_bin_path
 from ansible.module_utils.facts.hardware.base import Hardware, HardwareCollector
-
 from ansible.module_utils.facts.sysctl import get_sysctl
 
 
@@ -84,13 +84,39 @@ class DarwinHardware(Hardware):
         return cpu_facts
 
     def get_memory_facts(self):
-        memory_facts = {}
-
-        memory_facts['memtotal_mb'] = int(self.sysctl['hw.memsize']) // 1024 // 1024
-
-        rc, out, err = self.module.run_command("sysctl hw.usermem")
+        memory_facts = {
+            'memtotal_mb': int(self.sysctl['hw.memsize']) // 1024 // 1024
+        }
+
+        total_used = 0
+        page_size = 4096
+        vm_stat_command = get_bin_path('vm_stat')
+        rc, out, err = self.module.run_command(vm_stat_command)
         if rc == 0:
-            memory_facts['memfree_mb'] = int(out.splitlines()[-1].split()[1]) // 1024 // 1024
+            # Free = Total - (Wired + active + inactive)
+            # Get a generator of tuples from the command output so we can later
+            # turn it into a dictionary
+            memory_stats = (line.rstrip('.').split(':', 1) for line in out.splitlines())
+
+            # Strip extra left spaces from the value
+            memory_stats = dict((k, v.lstrip()) for k, v in memory_stats)
+
+            for k, v in memory_stats.items():
+                try:
+                    memory_stats[k] = int(v)
+                except ValueError as ve:
+                    # Most values convert cleanly to integer values but if the field does
+                    # not convert to an integer, just leave it alone.
+                    pass
+
+            if memory_stats.get('Pages wired down'):
+                total_used += memory_stats['Pages wired down'] * page_size
+            if memory_stats.get('Pages active'):
+                total_used += memory_stats['Pages active'] * page_size
+            if memory_stats.get('Pages inactive'):
+                total_used += memory_stats['Pages inactive'] * page_size
+
+        memory_facts['memfree_mb'] = memory_facts['memtotal_mb'] - (total_used // 1024 // 1024)
 
         return memory_facts