From f11f6f978ffe3a1de46e4e6302c5834340b9a496 Mon Sep 17 00:00:00 2001
From: Shlomi Noach <shlomi-noach@github.com>
Date: Fri, 24 Feb 2017 14:48:10 -0700
Subject: [PATCH] mitigating cut-over/write race condition

---
 go/logic/inspect.go  |  4 ++--
 go/logic/migrator.go | 15 ++++++++++++++-
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/go/logic/inspect.go b/go/logic/inspect.go
index 862be16..1d30cb5 100644
--- a/go/logic/inspect.go
+++ b/go/logic/inspect.go
@@ -348,7 +348,7 @@ func (this *Inspector) validateLogSlaveUpdates() error {
 	}
 
 	if this.migrationContext.IsTungsten {
-		log.Warning("log_slave_updates not found on %s:%d, but --tungsten provided, so I'm proceeding", this.connectionConfig.Key.Hostname, this.connectionConfig.Key.Port)
+		log.Warningf("log_slave_updates not found on %s:%d, but --tungsten provided, so I'm proceeding", this.connectionConfig.Key.Hostname, this.connectionConfig.Key.Port)
 		return nil
 	}
 
@@ -357,7 +357,7 @@ func (this *Inspector) validateLogSlaveUpdates() error {
 	}
 
 	if this.migrationContext.InspectorIsAlsoApplier() {
-		log.Warning("log_slave_updates not found on %s:%d, but executing directly on master, so I'm proceeeding", this.connectionConfig.Key.Hostname, this.connectionConfig.Key.Port)
+		log.Warningf("log_slave_updates not found on %s:%d, but executing directly on master, so I'm proceeeding", this.connectionConfig.Key.Hostname, this.connectionConfig.Key.Port)
 		return nil
 	}
 
diff --git a/go/logic/migrator.go b/go/logic/migrator.go
index de3a018..94b5e54 100644
--- a/go/logic/migrator.go
+++ b/go/logic/migrator.go
@@ -1000,11 +1000,13 @@ func (this *Migrator) iterateChunks() error {
 	for {
 		if atomic.LoadInt64(&this.rowCopyCompleteFlag) == 1 {
 			// Done
+			// There's another such check down the line
 			return nil
 		}
 		copyRowsFunc := func() error {
 			if atomic.LoadInt64(&this.rowCopyCompleteFlag) == 1 {
-				// Done
+				// Done.
+				// There's another such check down the line
 				return nil
 			}
 			hasFurtherRange, err := this.applier.CalculateNextIterationRangeEndValues()
@@ -1016,6 +1018,17 @@ func (this *Migrator) iterateChunks() error {
 			}
 			// Copy task:
 			applyCopyRowsFunc := func() error {
+				if atomic.LoadInt64(&this.rowCopyCompleteFlag) == 1 {
+					// No need for more writes.
+					// This is the de-facto place where we avoid writing in the event of completed cut-over.
+					// There could _still_ be a race condition, but that's as close as we can get.
+					// What about the race condition? Well, there's actually no data integrity issue.
+					// when rowCopyCompleteFlag==1 that means **guaranteed** all necessary rows have been copied.
+					// But some are still then collected at the binary log, and these are the ones we're trying to
+					// not apply here. If the race condition wins over us, then we just attempt to apply onto the
+					// _ghost_ table, which no longer exists. So, bothering error messages and all, but no damage.
+					return nil
+				}
 				_, rowsAffected, _, err := this.applier.ApplyIterationInsertQuery()
 				if err != nil {
 					return terminateRowIteration(err)