Android: Add support for TraceEvent before the native library is loaded.

base/android/java/src/org/chromium/base/TraceEvent.java

Index: base/android/java/src/org/chromium/base/TraceEvent.java
diff --git a/base/android/java/src/org/chromium/base/TraceEvent.java b/base/android/java/src/org/chromium/base/TraceEvent.java
index 878275c2b80a60b28a8b763a53cdc5441d3c0a83..9e17ff132d4389a75f9f735c3b71f8989cee0c00 100644
--- a/base/android/java/src/org/chromium/base/TraceEvent.java
+++ b/base/android/java/src/org/chromium/base/TraceEvent.java
@@ -179,6 +179,7 @@ public class TraceEvent {
      */
     @CalledByNative
     public static void setEnabled(boolean enabled) {
+        if (enabled) EarlyTraceEvent.disable();

[pasko, 2015/12/15 17:52:59]

Some observations:

1. we may miss a few early traces this way (early is off, regular tracing is not
on yet), though the flakiness would be rare

2. someone may call this function from Java without realizing that it can stop
tracing _sometimes_. Probably a comment at the top of this method would be
sufficient: "Does not work correctly when called from Java".

3. it makes it quite possible for setEnabled(true) to take a long time so that
in the meantime setEnabled(false) can execute, what a confusion

I think we cannot solve (1) in a robust way, unfortunately.

Something that seems to solve (2) and (3) (and maybe slightly better (1)) is in
setEnabled(true):

* crash if native library is not loaded

* ONCE: atomically (with barriers, etc etc) switch the trace collection
mechanism from early traces to regular ones

* ONCE: _asynchronously_ dump the "EarlyJava" traces. We would not cause any
problems when doing a dump because the new trace events would have all different
categories.

Some racy behavior: when we begin in EarlyJava and end in Java, the events would
be broken. We can probably "fixup" them when writing the output trace somewhere,
not in this change.

WDYT? I might be missing something this evening.

[Benoit L, 2016/01/06 12:50:06]

Per offline discussion, my understanding is that we agree that the current
solution is fine.

[pasko, 2016/01/06 13:50:10]

I do not remember the details :(

Can you outline the concurrency issues at the top of EarlyTraceEvent.java with
quick resolution why it is good enough?

         sEnabled = enabled;
         // Android M+ systrace logs this on its own. Only log it if not writing to Android systrace.
         if (sATraceEnabled) return;
@@ -186,6 +187,10 @@ public class TraceEvent {
                 enabled ? LooperMonitorHolder.sInstance : null);
     }
 
+    public static void enableEarlyTracing() {
+        EarlyTraceEvent.enable();
+    }
+
     /**
      * Enables or disabled Android systrace path of Chrome tracing. If enabled, all Chrome
      * traces will be also output to Android systrace. Because of the overhead of Android
@@ -254,7 +259,7 @@ public class TraceEvent {
      * @param name The name of the event.
      */
     public static void begin(String name) {
-        if (sEnabled) nativeBegin(name, null);
+        begin(name, null);
     }
 
     /**
@@ -263,6 +268,7 @@ public class TraceEvent {
      * @param arg  The arguments of the event.
      */
     public static void begin(String name, String arg) {
+        EarlyTraceEvent.begin(name);
         if (sEnabled) nativeBegin(name, arg);
     }
 
@@ -271,7 +277,7 @@ public class TraceEvent {
      * @param name The name of the event.
      */
     public static void end(String name) {
-        if (sEnabled) nativeEnd(name, null);
+        end(name, null);
     }
 
     /**
@@ -280,6 +286,7 @@ public class TraceEvent {
      * @param arg  The arguments of the event.
      */
     public static void end(String name, String arg) {
+        EarlyTraceEvent.end(name);
         if (sEnabled) nativeEnd(name, arg);
     }