Regression test for PSYNC2 issue #3899 added.

Experimentally verified that it can trigger the issue reverting the fix.
At least on my system... Being the bug time/backlog dependant, it is
very hard to tell if this test will be able to trigger the problem
consistently, however even if it triggers the problem once in a while,
we'll see it in the CI environment at http://ci.redis.io.

tests/integration/psync2-reg.tcl

@@ -0,0 +1,61 @@
+# Issue 3899 regression test.
+# We create a chain of three instances: master -> slave -> slave2
+# and continuously break the link while traffic is generated by
+# redis-benchmark. At the end we check that the data is the same
+# everywhere.
+
+start_server {tags {"psync2"}} {
+start_server {} {
+start_server {} {
+    set start_time [clock seconds]  ; # Test start time
+
+    # Config
+    set debug_msg 0                 ; # Enable additional debug messages
+
+    set duration 20                 ; # Total test seconds
+
+    for {set j 0} {$j < 3} {incr j} {
+        set R($j) [srv [expr 0-$j] client]
+        set R_host($j) [srv [expr 0-$j] host]
+        set R_port($j) [srv [expr 0-$j] port]
+        if {$debug_msg} {puts "Log file: [srv [expr 0-$j] stdout]"}
+    }
+
+    # Setup the replication and backlog parameters
+    $R(1) slaveof $R_host(0) $R_port(0)
+    $R(2) slaveof $R_host(0) $R_port(0)
+    $R(0) set foo bar
+    wait_for_condition 50 1000 {
+        [$R(1) dbsize] == 1 && [$R(2) dbsize] == 1
+    } else {
+        fail "Slaves not replicating from master"
+    }
+
+    $R(0) config set repl-backlog-size 10mb
+    $R(1) config set repl-backlog-size 10mb
+
+    set cycle_start_time [clock milliseconds]
+    set bench_pid [exec src/redis-benchmark -p $R_port(0) -n 10000000 -r 1000 incr __rand_int__ > /dev/null &]
+    while 1 {
+        set elapsed [expr {[clock milliseconds]-$cycle_start_time}]
+        if {$elapsed > $duration*1000} break
+        if {rand() < .05} {
+            test "Kill first slave link with the master" {
+                $R(1) client kill type master
+            }
+        }
+        if {rand() < .05} {
+            test "Kill chained slave link with the master" {
+                $R(2) client kill type master
+            }
+        }
+        after 100
+    }
+
+    wait_for_condition 50 1000 {
+        ([$R(0) debug digest] eq [$R(1) debug digest]) &&
+        ([$R(1) debug digest] eq [$R(2) debug digest])
+    } else {
+        fail "The three instances have different data sets"
+    }
+}}}

tests/test_helper.tcl

@@ -42,6 +42,7 @@ set ::all_tests {
     integration/convert-zipmap-hash-on-load
     integration/logging
     integration/psync2
+    integration/psync2-reg
     unit/pubsub
     unit/slowlog
     unit/scripting