Allow obtaining BPFPerfBuffer pointer for polling

examples/cpp/FollyRequestContextSwitch.cc

@@ -98,8 +98,11 @@ int main(int argc, char** argv) {
 
   signal(SIGINT, signal_handler);
   std::cout << "Started tracing, hit Ctrl-C to terminate." << std::endl;
-  while (true)
-    bpf->poll_perf_buffer("events");
+  auto perf_buffer = bpf->get_perf_buffer("events");
+  if (perf_buffer)
+    while (true)
+      // 100ms timeout
+      perf_buffer->poll(100);
 
   return 0;
 }

src/cc/api/BPF.cc

@@ -443,6 +443,11 @@ StatusTuple BPF::close_perf_buffer(const std::string& name) {
   return StatusTuple(0);
 }
 
+BPFPerfBuffer* BPF::get_perf_buffer(const std::string& name) {
+  auto it = perf_buffers_.find(name);
+  return (it == perf_buffers_.end()) ? nullptr : it->second;
+}
+
 void BPF::poll_perf_buffer(const std::string& name, int timeout_ms) {
   auto it = perf_buffers_.find(name);
   if (it == perf_buffers_.end())

src/cc/api/BPF.h

@@ -136,11 +136,20 @@ class BPF {
 
   StatusTuple close_perf_event(const std::string& name);
 
+  // Open a Perf Buffer of given name, providing callback and callback cookie
+  // to use when polling. BPF class owns the opened Perf Buffer and will free
+  // it on-demand or on destruction.
   StatusTuple open_perf_buffer(const std::string& name, perf_reader_raw_cb cb,
                                perf_reader_lost_cb lost_cb = nullptr,
                                void* cb_cookie = nullptr,
                                int page_cnt = DEFAULT_PERF_BUFFER_PAGE_CNT);
+  // Close and free the Perf Buffer of given name.
   StatusTuple close_perf_buffer(const std::string& name);
+  // Obtain an pointer to the opened BPFPerfBuffer instance of given name.
+  // Will return nullptr if such open Perf Buffer doesn't exist.
+  BPFPerfBuffer* get_perf_buffer(const std::string& name);
+  // Poll an opened Perf Buffer of given name with given timeout, using callback
+  // provided when opening. Do nothing if such open Perf Buffer doesn't exist.
   void poll_perf_buffer(const std::string& name, int timeout_ms = -1);
 
   StatusTuple load_func(const std::string& func_name, enum bpf_prog_type type,