Don't refresh already gone PIDs

Currently we check if `ProcSyms` needs refresh for every symboling call.
However if the `ProcSyms` was loaded, but the Process terminated later,
we will see it as a "need refresh" and effectively clear the symbol
table and not able to resovle anything anymore.

This commit changes so that we only refresh when the PID still exists
(but differnt from what loaded).

src/cc/bcc_syms.cc

@@ -33,6 +33,11 @@ ino_t ProcStat::getinode_() {
   return (!stat(procfs_.c_str(), &s)) ? s.st_ino : -1;
 }
 
+bool ProcStat::is_stale() {
+  ino_t cur_inode = getinode_();
+  return (cur_inode > 0) &&  (cur_inode != inode_);
+}
+
 ProcStat::ProcStat(int pid)
     : procfs_(tfm::format("/proc/%d/exe", pid)), inode_(getinode_()) {}
 

src/cc/syms.h

@@ -30,7 +30,7 @@ class ProcStat {
 
 public:
   ProcStat(int pid);
-  bool is_stale() { return inode_ != getinode_(); }
+  bool is_stale();
   void reset() { inode_ = getinode_(); }
 };