Don't always reload empty Modules

Currently we think the Module's symbol table is not loaed as long as its
empty. But some times we were just not able to read the symbols from the
binary / perf map, or didn't get symbol from the read. This commit makes
so that we don't end up trying to do a read on each lookup.

src/cc/bcc_syms.cc

@@ -190,7 +190,7 @@ bool ProcSyms::resolve_name(const char *module, const char *name,
 }
 
 ProcSyms::Module::Module(const char *name, int pid, bool in_ns)
-  : name_(name), pid_(pid), in_ns_(in_ns) {
+  : name_(name), pid_(pid), in_ns_(in_ns), loaded_(false) {
   struct ns_cookie nsc;
 
   bcc_procutils_enter_mountns(pid_, &nsc);
@@ -213,8 +213,9 @@ bool ProcSyms::Module::is_perf_map() const {
 void ProcSyms::Module::load_sym_table() {
   struct ns_cookie nsc = {-1, -1};
 
-  if (syms_.size())
+  if (loaded_)
     return;
+  loaded_ = true;
 
   if (is_perf_map()) {
     if (in_ns_)

src/cc/syms.h

@@ -91,6 +91,7 @@ class ProcSyms : SymbolCache {
     bool is_so_;
     int pid_;
     bool in_ns_;
+    bool loaded_;
     std::unordered_set<std::string> symnames_;
     std::vector<Symbol> syms_;