TLS: Add crypto locks for older OpenSSL support.

This is really required only for older OpenSSL versions.

Also, at the moment Redis does not use OpenSSL from multiple threads so
this will only be useful if modules end up doing that.

src/tls.c

@@ -93,11 +93,56 @@ static int parseProtocolsConfig(const char *str) {
  * served to the reader yet. */
 static list *pending_list = NULL;
 
+/**
+ * OpenSSL global initialization and locking handling callbacks.
+ * Note that this is only required for OpenSSL < 1.1.0.
+ */
+
+#if OPENSSL_VERSION_NUMBER < 0x10100000L
+#define USE_CRYPTO_LOCKS
+#endif
+
+#ifdef USE_CRYPTO_LOCKS
+
+static pthread_mutex_t *openssl_locks;
+
+static void sslLockingCallback(int mode, int lock_id, const char *f, int line) {
+    pthread_mutex_t *mt = openssl_locks + lock_id;
+
+    if (mode & CRYPTO_LOCK) {
+        pthread_mutex_lock(mt);
+    } else {
+        pthread_mutex_unlock(mt);
+    }
+
+    (void)f;
+    (void)line;
+}
+
+static void initCryptoLocks(void) {
+    unsigned i, nlocks;
+    if (CRYPTO_get_locking_callback() != NULL) {
+        /* Someone already set the callback before us. Don't destroy it! */
+        return;
+    }
+    nlocks = CRYPTO_num_locks();
+    openssl_locks = zmalloc(sizeof(*openssl_locks) * nlocks);
+    for (i = 0; i < nlocks; i++) {
+        pthread_mutex_init(openssl_locks + i, NULL);
+    }
+    CRYPTO_set_locking_callback(sslLockingCallback);
+}
+#endif /* USE_CRYPTO_LOCKS */
+
 void tlsInit(void) {
     ERR_load_crypto_strings();
     SSL_load_error_strings();
     SSL_library_init();
 
+#ifdef USE_CRYPTO_LOCKS
+    initCryptoLocks();
+#endif
+
     if (!RAND_poll()) {
         serverLog(LL_WARNING, "OpenSSL: Failed to seed random number generator.");
     }