Avoid redundant IM transcript reconstruction

Compare normalized historical overlays before replacing the visible transcript, and retain the current bounded history when durable live rows only roll the oldest entries out of the 200-message window.

Let an active history read observe newer archive generations and restart itself instead of scheduling overlapping reads from snapshot callbacks.

Validated with the RelWithDebInfo secondlife-bin build and the LLChatServiceHistoryCore test suite.

1 files changed, 54 insertions, 4 deletions

diff --git a/indra/newview/llimview.cpp b/indra/newview/llimview.cpp
index a4383b2f0d..e27ad96a63 100644
--- a/indra/newview/llimview.cpp
+++ b/indra/newview/llimview.cpp
@@ -38,6 +38,7 @@
 #include "llrect.h"
 #include "llerror.h"
 #include "llbutton.h"
+#include "llsdutil.h"
 #include "llsdutil_math.h"
 #include "llstring.h"
 #include "lltextutil.h"
@@ -89,6 +90,31 @@ const S32 XL8_PADDING = 3;  // XL8_START_TAG.size() + XL8_END_TAG.size()
 // Tokens are process-unique so a stale completion cannot match a recreated P2P session.
 static U64 sChatHistoryLoadToken = 0;
 
+namespace
+{
+bool sameHistoryRow(LLSD left, LLSD right)
+{
+    // Positional indexes are rebuilt with the overlay and do not change presentation.
+    left.erase("index");
+    right.erase("index");
+    return llsd_equals(left, right);
+}
+
+bool sameHistory(const LLIMModel::chat_message_list_t& left,
+                 const LLIMModel::chat_message_list_t& right)
+{
+    return left.size() == right.size() &&
+        std::equal(left.begin(), left.end(), right.begin(), sameHistoryRow);
+}
+
+bool isStrictHistoryPrefix(const LLIMModel::chat_message_list_t& prefix,
+                           const LLIMModel::chat_message_list_t& complete)
+{
+    return prefix.size() < complete.size() &&
+        std::equal(prefix.begin(), prefix.end(), complete.begin(), sameHistoryRow);
+}
+}
+
 /** Timeout of outgoing session initialization (in seconds) */
 const static U32 SESSION_INITIALIZATION_TIMEOUT = 30;
 
@@ -1458,17 +1484,22 @@ void LLIMModel::LLIMSession::loadHistory()
 
 void LLIMModel::LLIMSession::replaceHistoricalMessages(const chat_message_list_t& history)
 {
-    // Preserve every live row in its current order, replacing only rows tagged historical.
+    // Separate the append-only live tail from the currently presented historical baseline.
     chat_message_list_t live;
+    chat_message_list_t current_historical;
     for (const LLSD& message : mMsgs)
     {
         if (!message["is_history"].asBoolean())
         {
             live.push_back(message);
         }
+        else
+        {
+            current_historical.push_back(message);
+        }
     }
 
-    // An authoritative refresh may contain rows that this open session already
+    // A refresh may contain rows that this open session already
     // displays live. Consume those exact occurrences before rebuilding the overlay.
     const chat_message_list_t filtered = isP2P()
         ? LLChatServiceHistory::filterLiveDuplicates(history, live) : history;
@@ -1504,6 +1535,22 @@ void LLIMModel::LLIMSession::replaceHistoricalMessages(const chat_message_list_t
         historical.push_front(message);
     }
 
+    const bool consumed_live = filtered.size() < history.size();
+    LLFloaterIMSession* floater = LLFloaterIMSession::findInstance(mSessionID);
+    const bool capped_live_rollover =
+        floater && floater->isInVisibleChain() &&
+        consumed_live &&
+        !historical.empty() &&
+        history.size() == current_historical.size() &&
+        isStrictHistoryPrefix(historical, current_historical);
+
+    // Re-publishing the same bounded history, including eviction of its oldest
+    // rows after live messages become durable, must not reconstruct a visible transcript.
+    if (sameHistory(current_historical, historical) || capped_live_rollover)
+    {
+        return;
+    }
+
     live.insert(live.end(), historical.begin(), historical.end());
     mMsgs.swap(live);
 
@@ -1514,7 +1561,7 @@ void LLIMModel::LLIMSession::replaceHistoricalMessages(const chat_message_list_t
         message["index"] = --index;
     }
 
-    if (LLFloaterIMSession* floater = LLFloaterIMSession::findInstance(mSessionID))
+    if (floater)
     {
         floater->reloadMessages(false);
     }
@@ -1589,7 +1636,10 @@ void LLIMModel::LLIMSession::applyChatServiceSnapshot(
         replaceHistoricalMessages(mChatServiceHistoricalValue);
     }
 
-    if (presentation_changed || snapshot.archive_serial != mChatHistoryArchiveSerial)
+    // The active read will restart itself if its captured presentation or archive
+    // generation is stale, so status snapshots do not need to supersede it.
+    if (!mChatHistoryLocalLoading &&
+        (presentation_changed || snapshot.archive_serial != mChatHistoryArchiveSerial))
     {
         loadHistory();
     }