Merge pull request #560 from Snapchat/aof_fixes

* technically possible for child_type == CHILD_TYPE_AOF without active child

* don't release lock on child as it can hang

* need child specific release that doesn't trigger cv

* refactor aeReleaseForkLockChild to capture releaseRead case

* rdb_child_pid isn't the correct value

src/ae.cpp

@@ -859,6 +859,11 @@ void aeReleaseForkLock()
     g_forkLock.downgradeWrite();
 }
 
+void aeForkLockInChild()
+{
+    g_forkLock.setNotify(false);
+}
+
 int aeThreadOwnsLock()
 {
     return fOwnLockOverride || g_lock.fOwnLock();

src/ae.h

@@ -171,6 +171,7 @@ int aeTryAcquireLock(int fWeak);
 void aeThreadOffline();
 void aeReleaseLock();
 void aeReleaseForkLock();
+void aeForkLockInChild();
 int aeThreadOwnsLock();
 void aeSetThreadOwnsLockOverride(int fOverride);
 int aeLockContested(int threshold);

src/aof.cpp

@@ -752,7 +752,7 @@ void feedAppendOnlyFile(struct redisCommand *cmd, int dictid, robj **argv, int a
      * accumulate the differences between the child DB and the current one
      * in a buffer, so that when the child process will do its work we
      * can append the differences to the new append only file. */
-    if (g_pserver->child_type == CHILD_TYPE_AOF)
+    if (hasActiveChildProcess() && g_pserver->child_type == CHILD_TYPE_AOF)
         aofRewriteBufferAppend((unsigned char*)buf,sdslen(buf));
 
     sdsfree(buf);

src/rdb.cpp

@@ -3694,7 +3694,7 @@ void killRDBChild(bool fSynchronous) {
     serverAssert(GlobalLocksAcquired());
 
     if (cserver.fForkBgSave) {
-        kill(g_pserver->rdb_child_pid,SIGUSR1);
+        kill(g_pserver->child_pid,SIGUSR1);
     } else { 
         g_pserver->rdbThreadVars.fRdbThreadCancel = true;
         if (g_pserver->rdb_child_type == RDB_CHILD_TYPE_SOCKET) {

src/readwritelock.h

@@ -8,6 +8,7 @@ class readWriteLock {
     int m_readCount = 0;
     int m_writeCount = 0;
     bool m_writeWaiting = false;
+    bool m_notify = true;
 public:
     readWriteLock(const char *name) : m_readLock(name), m_writeLock(name) {}
 
@@ -65,6 +66,7 @@ public:
     void releaseRead() {
         std::unique_lock<fastlock> rm(m_readLock);
         m_readCount--;
+        if (m_notify)
             m_cv.notify_all();
     }
 
@@ -74,6 +76,7 @@ public:
         if (exclusive)
             m_writeLock.unlock();
         m_writeCount--;
+        if (m_notify)
             m_cv.notify_all();
     }
 
@@ -82,6 +85,10 @@ public:
         acquireRead();
     }
 
+    void setNotify(bool notify) {
+        m_notify = notify;
+    }
+
     bool hasReader() {
         return m_readCount > 0;
     }

src/server.cpp

@@ -6888,6 +6888,7 @@ int redisFork(int purpose) {
     latencyAddSampleIfNeeded("fork-lock",(ustime()-startWriteLock)/1000);
     if ((childpid = fork()) == 0) {
         /* Child */
+        aeForkLockInChild();
         aeReleaseForkLock();
         g_pserver->in_fork_child = purpose;
         setOOMScoreAdj(CONFIG_OOM_BGCHILD);