guard against permission denied errors

reading from /proc/<pid>/smaps is not available inside of Docker container

script/memstats.rb

@@ -86,31 +86,41 @@ def consume_mapping( map_lines, totals )
   return m
 end
 
+def create_memstats_not_available( totals )
+  Mapping::FIELDS.each do |field|
+    totals[field] += Float::NAN
+  end
+end
+
 abort 'usage: memstats [pid]' unless ARGV.first
 pid = ARGV.shift.to_i
 totals = Hash.new(0)
 mappings = []
 
-File.open( "/proc/#{pid}/smaps" ) do |smaps|
+begin
+  File.open( "/proc/#{pid}/smaps" ) do |smaps|
 
-  map_lines = []
+    map_lines = []
 
-  loop do
-    break if smaps.eof?
-    line = smaps.readline.strip
-    case line
-    when /\w+:\s+/
-      map_lines << line
-    when /[0-9a-f]+:[0-9a-f]+\s+/
-      if map_lines.size > 0 then
-        mappings << consume_mapping( map_lines, totals )
+    loop do
+      break if smaps.eof?
+      line = smaps.readline.strip
+      case line
+      when /\w+:\s+/
+        map_lines << line
+      when /[0-9a-f]+:[0-9a-f]+\s+/
+        if map_lines.size > 0 then
+          mappings << consume_mapping( map_lines, totals )
+        end
+        map_lines.clear
+        map_lines << line
+      else
+        break
       end
-      map_lines.clear
-      map_lines << line
-    else
-      break
     end
   end
+rescue
+  create_memstats_not_available( totals )
 end
 
 # http://rubyforge.org/snippet/download.php?type=snippet&id=511