Bug 1692934 - Record meta.contentEarliestTime in each process - r=canaltinova

This contentEarliestTime is the time when the earliest (and oldest) surviving
chunk was prepared to start receiving data.
It should be a good hint to the front-end about where the profiling data
actually starts.

Differential Revision: https://phabricator.services.mozilla.com/D151355

mozglue/baseprofiler/core/platform.cpp

@@ -1859,6 +1859,18 @@ static void StreamMetaJSCustomObject(PSLockRef aLock,
                                                 CorePS::ProcessStartTime())
                                                    .ToMilliseconds());
 
+  if (const TimeStamp contentEarliestTime =
+          ActivePS::Buffer(aLock)
+              .UnderlyingChunkedBuffer()
+              .GetEarliestChunkStartTimeStamp();
+      !contentEarliestTime.IsNull()) {
+    aWriter.DoubleProperty(
+        "contentEarliestTime",
+        (contentEarliestTime - CorePS::ProcessStartTime()).ToMilliseconds());
+  } else {
+    aWriter.NullProperty("contentEarliestTime");
+  }
+
   if (aIsShuttingDown) {
     aWriter.DoubleProperty("shutdownTime", profiler_time());
   } else {

mozglue/baseprofiler/public/ProfileBufferChunk.h

@@ -161,6 +161,7 @@ class ProfileBufferChunk {
 #endif
     mInternalHeader.mHeader.mOffsetFirstBlock = aTailSize;
     mInternalHeader.mHeader.mOffsetPastLastBlock = aTailSize;
+    mInternalHeader.mHeader.mStartTimeStamp = TimeStamp::Now();
     return SpanOfBytes(&mBuffer, aTailSize);
   }
 
@@ -237,6 +238,7 @@ class ProfileBufferChunk {
     void Reset() {
       mOffsetFirstBlock = 0;
       mOffsetPastLastBlock = 0;
+      mStartTimeStamp = TimeStamp{};
       mDoneTimeStamp = TimeStamp{};
       mBlockCount = 0;
       mRangeStart = 0;
@@ -255,7 +257,9 @@ class ProfileBufferChunk {
     // ProfileBufferChunk. It may be before mBufferBytes if ProfileBufferChunk
     // is marked "Done" before the end is reached.
     Length mOffsetPastLastBlock = 0;
-    // Timestamp when buffer is "Done" (which happens when the last block is
+    // Timestamp when the buffer becomes in-use, ready to record data.
+    TimeStamp mStartTimeStamp;
+    // Timestamp when the buffer is "Done" (which happens when the last block is
     // written). This will be used to find and discard the oldest
     // ProfileBufferChunk.
     TimeStamp mDoneTimeStamp;

mozglue/baseprofiler/public/ProfileChunkedBuffer.h

@@ -254,6 +254,26 @@ class ProfileChunkedBuffer {
             mFailedPutBytes};
   }
 
+  // In in-session, return the start TimeStamp of the earliest chunk.
+  // If out-of-session, return a null TimeStamp.
+  [[nodiscard]] TimeStamp GetEarliestChunkStartTimeStamp() const {
+    baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
+    if (MOZ_UNLIKELY(!mChunkManager)) {
+      // Out-of-session.
+      return {};
+    }
+    return mChunkManager->PeekExtantReleasedChunks(
+        [&](const ProfileBufferChunk* aOldestChunk) -> TimeStamp {
+          if (aOldestChunk) {
+            return aOldestChunk->ChunkHeader().mStartTimeStamp;
+          }
+          if (mCurrentChunk) {
+            return mCurrentChunk->ChunkHeader().mStartTimeStamp;
+          }
+          return {};
+        });
+  }
+
   [[nodiscard]] bool IsEmpty() const {
     baseprofiler::detail::BaseProfilerMaybeAutoLock lock(mMutex);
     return mRangeStart == mRangeEnd;

mozglue/tests/TestBaseProfiler.cpp

@@ -1312,6 +1312,7 @@ static void TestChunk() {
       sizeof(ProfileBufferChunk::Header) ==
           sizeof(ProfileBufferChunk::Header::mOffsetFirstBlock) +
               sizeof(ProfileBufferChunk::Header::mOffsetPastLastBlock) +
+              sizeof(ProfileBufferChunk::Header::mStartTimeStamp) +
               sizeof(ProfileBufferChunk::Header::mDoneTimeStamp) +
               sizeof(ProfileBufferChunk::Header::mBufferBytes) +
               sizeof(ProfileBufferChunk::Header::mBlockCount) +

tools/profiler/core/platform.cpp

@@ -2793,6 +2793,18 @@ static void StreamMetaJSCustomObject(
                                                 CorePS::ProcessStartTime())
                                                    .ToMilliseconds());
 
+  if (const TimeStamp contentEarliestTime =
+          ActivePS::Buffer(aLock)
+              .UnderlyingChunkedBuffer()
+              .GetEarliestChunkStartTimeStamp();
+      !contentEarliestTime.IsNull()) {
+    aWriter.DoubleProperty(
+        "contentEarliestTime",
+        (contentEarliestTime - CorePS::ProcessStartTime()).ToMilliseconds());
+  } else {
+    aWriter.NullProperty("contentEarliestTime");
+  }
+
   if (aIsShuttingDown) {
     aWriter.DoubleProperty("shutdownTime", profiler_time());
   } else {

tools/profiler/tests/gtest/GeckoProfiler.cpp

@@ -1379,6 +1379,7 @@ static void JSONRootCheck(const Json::Value& aRoot,
   EXPECT_HAS_JSON(meta["version"], UInt);
   EXPECT_HAS_JSON(meta["startTime"], Double);
   EXPECT_HAS_JSON(meta["profilingStartTime"], Double);
+  EXPECT_HAS_JSON(meta["contentEarliestTime"], Double);
 
   EXPECT_HAS_JSON(aRoot["pages"], Array);