ProcStats : Use mutex lock to make fetch() thread safe.

Whilst investigating a possible memory issue using the Memory Auditor, I discovered the information printed was garbled if multiple threads where used.

The issue was the ProcStats::fetch method which was not thread safe. This MR fixes that with a simple mutex lock.

I also did a drive-by cleanup and code modernisation.

FYI @clemenci

GaudiAud/src/ProcStats.cpp

@@ -263,12 +263,11 @@ bool ProcStats::fetch( procInfo& f ) {
 
 #if defined( __linux__ ) or defined( __APPLE__ )
 
-  double     pr_size{0}, pr_rssize{0};
   linux_proc pinfo;
-  int        cnt{0};
-  char       buf[500];
 
   auto read_proc = [&]() {
+    int  cnt{0};
+    char buf[500];
     m_ufd.lseek( 0, SEEK_SET );
     if ( ( cnt = m_ufd.read( buf, sizeof( buf ) ) ) < 0 ) {
       std::cerr << "ProcStats : LINUX Read of Proc file failed:" << std::endl;
@@ -289,10 +288,10 @@ bool ProcStats::fetch( procInfo& f ) {
           &pinfo.kstkesp, &pinfo.kstkeip, &pinfo.signal, &pinfo.blocked, &pinfo.sigignore, &pinfo.sigcatch,
           &pinfo.wchan );
       // resident set size in pages
-      pr_size   = (double)pinfo.vsize;
-      pr_rssize = (double)pinfo.rss;
-      f.vsize   = pr_size / ( 1024 * 1024 );
-      f.rss     = pr_rssize * m_pg_size / ( 1024 * 1024 );
+      const auto pr_size   = (double)pinfo.vsize;
+      const auto pr_rssize = (double)pinfo.rss;
+      f.vsize              = pr_size / ( 1024 * 1024 );
+      f.rss                = pr_rssize * m_pg_size / ( 1024 * 1024 );
     }
     return true;
   };