Switch trigger timer on p28 boundary

docs/metrics.md

@@ -201,7 +201,7 @@ scp.timing.externalized                   | timer     | time spent in ballot pro
 scp.timing.first-to-self-externalize-lag  | timer     | delay between first externalize message and local node externalizing
 scp.timing.self-to-others-externalize-lag | timer     | delay between local node externalizing and later externalize messages from other nodes
 scp.timing.ballot-blocked-on-txset        | timer     | time balloting was blocked waiting for a txset download (milliseconds)
-scp.trigger.prepare-start-fallback        | meter     | experimental trigger timer fell back from the network-close-time anchor to the local prepare-start anchor
+scp.trigger.prepare-start-fallback        | meter     | trigger timer fell back from the network-close-time anchor to the local prepare-start anchor
 scp.value.invalid                         | meter     | SCP value is invalid
 scp.value.valid                           | meter     | SCP value is valid
 scp.slot.values-referenced                | histogram | number of values referenced per consensus round

src/herder/HerderImpl.cpp

@@ -1447,8 +1447,18 @@ HerderImpl::setupTriggerNextLedger()
 
     auto now = mApp.getClock().now();
 
+    // Starting from protocol 28 the trigger timer is anchored on the
+    // network-agreed consensus close time.
+    // FORCE_OLD_STYLE_PREPARE_START_TRIGGER_TIMER forces the older
+    // prepare-start anchor as an emergency fallback.
+    bool const useConsensusCloseTimeAnchor =
+        protocolVersionStartsFrom(
+            lcl.header.ledgerVersion,
+            CONSENSUS_CLOSE_TIME_TRIGGER_PROTOCOL_VERSION) &&
+        !mApp.getConfig().FORCE_OLD_STYLE_PREPARE_START_TRIGGER_TIMER;
+
     auto lastLedgerStartingPoint =
-        mApp.getConfig().EXPERIMENTAL_TRIGGER_TIMER
+        useConsensusCloseTimeAnchor
             ? triggerAnchorFromConsensusCloseTime(lastIndex, now, milliseconds)
             : triggerAnchorFromPrepareStart(lastIndex, now, milliseconds);
 

src/herder/HerderImpl.h

@@ -358,7 +358,7 @@ class HerderImpl : public Herder
         medida::Meter& mEnvelopeValidSig;
         medida::Meter& mEnvelopeInvalidSig;
 
-        // Marked when the experimental trigger timer falls back from the
+        // Marked when the trigger timer falls back from the
         // network-close-time anchor to the local prepare-start anchor.
         medida::Meter& mTriggerPrepareStartFallback;