[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..0cce94c0c039e05efba0223ad27205855b7be708 100644
--- a/base/trace_event/heap_profiler_allocation_context_tracker.cc
+++ b/base/trace_event/heap_profiler_allocation_context_tracker.cc
@@ -8,9 +8,14 @@
 #include <iterator>
 
 #include "base/atomicops.h"
+#include "base/threading/platform_thread.h"
 #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 {
 
@@ -33,6 +38,26 @@ void DestructAllocationContextTracker(void* alloc_ctx_tracker) {
   delete static_cast<AllocationContextTracker*>(alloc_ctx_tracker);
 }
 
+// Gets the thread name from kernel if available or returns a string with id.
+// This function intenionally leaks the allocated strings since they are used to
+// tag allocations even after the thread dies.
+const char* GetThreadName() {
+  char name[50];

[Dmitry Skiba, 2016/05/16 21:11:10]

Why 50? PRCTL needs 16 and longest thing %d (%lld) can print is 22 (with null).

[ssid, 2016/05/16 23:22:41]

Used unsigned long - maximum is 12 chars.

+#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.
+  int err = prctl(PR_GET_NAME, name);
+  if (!err) {
+    return strdup(name);
+  }
+#endif  // defined(OS_LINUX) || defined(OS_ANDROID)
+
+  // Use tid if we don't have a thread name.
+  snprintf(name, 50, "%x", PlatformThread::CurrentId());

[Dmitry Skiba, 2016/05/16 21:11:10]

Better use sizeof(name) instead of 50. Also, I think the convention for TIDs is
that they are decimal, I've never seen them formatted as hex. Also I would
explicitly cast CurrentId() to some certain type, like long. Right now you're
fine only on platforms where CurrentId() is int.

[ssid, 2016/05/16 23:22:41]

I was thinking dword was int32 so %x should still work. Fixed.

I thought %x is more compact, so used.

+  return strdup(name);
+}
+
 }  // namespace
 
 // static
@@ -142,7 +167,15 @@ 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 (!thread_name_) {
+    // Ignore the string allocation made by GetThreadName to avoid reentrancy.
+    ignore_scope_depth_++;
+    thread_name_ = GetThreadName();
+    DCHECK(thread_name_);
+    ignore_scope_depth_--;
+  }
+
+  // Add the thread name as the first entry in pseudo stack.
   if (thread_name_) {
     *backtrace++ = StackFrame::FromThreadName(thread_name_);
   }