[tracing] Fix mising thread names in heap profiler

base/trace_event/heap_profiler_allocation_context_tracker.cc

Index: base/trace_event/heap_profiler_allocation_context_tracker.cc
diff --git a/base/trace_event/heap_profiler_allocation_context_tracker.cc b/base/trace_event/heap_profiler_allocation_context_tracker.cc
index fd4e21abe5d62d6594e6fd5bf56544228cce92f1..7f3dfcc1a4635c729b4b4ae44512a7b9377bfe0c 100644
--- a/base/trace_event/heap_profiler_allocation_context_tracker.cc
+++ b/base/trace_event/heap_profiler_allocation_context_tracker.cc
@@ -11,6 +11,10 @@
 #include "base/threading/thread_local_storage.h"
 #include "base/trace_event/heap_profiler_allocation_context.h"
 
+#if defined(OS_LINUX) || defined(OS_ANDROID)
+#include <sys/prctl.h>
+#endif
+
 namespace base {
 namespace trace_event {
 
@@ -142,7 +146,30 @@ AllocationContext AllocationContextTracker::GetContextSnapshot() {
   auto backtrace = std::begin(ctx.backtrace.frames);
   auto backtrace_end = std::end(ctx.backtrace.frames);
 
-  // Add the thread name as the first entry
+#if defined(OS_LINUX) || defined(OS_ANDROID)
+  // If the thread name is not set, try to get it from prctl. Thread name might
+  // not be set in cases where the thread started before heap profiling was
+  // enabled.
+  if (!thread_name_) {
+    // Ignore the string allocation and allocations from prctl to avoid

[Dmitry Skiba, 2016/05/13 18:34:06]

prctl is a syscall, there can be no reentrancy.

[ssid, 2016/05/13 20:48:08]

Ah yes, I thought it was causing reentrancy too. Fixed comment

+    // reentrancy.
+    ignore_scope_depth_++;
+    char* leaked_str = new char[16];
+    int err = prctl(PR_GET_NAME, leaked_str);
+    ignore_scope_depth_--;
+
+    if (err) {
+      // Set thread name to "Unknown" so that we call prctl only once per
+      // thread.
+      thread_name_ = "Unknown";

[Dmitry Skiba, 2016/05/13 18:34:06]

Let's format thread id instead. Hmm, and I think snprintf() can actually
allocate, so you might need reentrancy check anyway.

[ssid, 2016/05/13 20:48:08]

Hm, do you think it is useful to see a thread id instead of unknown? This also
causes an extra string leak and branching in the view.

+      free(leaked_str);

[Dmitry Skiba, 2016/05/13 18:34:06]

Use 'delete []' here since you allocated with new. Better approach might be to
start with stack array and then strdup() it on assignment to thread_name_.

[ssid, 2016/05/13 20:48:09]

Yes, fixed.

+    } else {
+      thread_name_ = leaked_str;
+    }
+  }
+#endif  // defined(OS_LINUX) || defined(OS_ANDROID)
+
+  // Add the thread name as the first entry in pseudo stack.
   if (thread_name_) {
     *backtrace++ = StackFrame::FromThreadName(thread_name_);
   }