Add application defined sequential consistency for certification

The before_prepare() takes now as argument a callback which
is called by the provider after it can guarantee sequential
consistency. This allows application threads which wish to
maintain sequential consistency to enter before_prepare()
calls concurrently without waiting the prior call to finish.

include/wsrep/client_state.hpp

@@ -413,7 +413,25 @@ namespace wsrep
 
         /** @name Commit ordering interface */
         /** @{ */
-        int before_prepare();
+
+        /**
+         * This method should be called before the transaction
+         * is prepared. This call certifies the transaction and
+         * assigns write set meta data.
+         *
+         * @param seq_cb Callback which is passed to underlying
+         *               certify() call. See wsrep::provider::certify().
+         *
+         * @return Zero on success, non-zero on failure.
+         */
+        int before_prepare(const wsrep::provider::seq_cb_t* seq_cb);
+
+        /** Same as before_prepare() above, but nullptr is passed
+         * to seq_cb. */
+        int before_prepare()
+        {
+            return before_prepare(nullptr);
+        }
 
         int after_prepare();
 

include/wsrep/provider.hpp

@@ -332,10 +332,37 @@ namespace wsrep
         virtual int append_key(wsrep::ws_handle&, const wsrep::key&) = 0;
         virtual enum status append_data(
             wsrep::ws_handle&, const wsrep::const_buffer&) = 0;
+
+        /**
+         * Callback for application defined sequential consistency.
+         * The provider will call
+         * the callback once it can guarantee sequential consistency. */
+        typedef struct seq_cb {
+            /** Opaque caller context */
+            void *ctx;
+            /** Function to be called by the provider when sequential
+             * consistency is guaranteed. */
+            void (*fn)(void *ctx);
+        } seq_cb_t;
+
+        /**
+         * Certify the write set. 
+         *
+         * @param client_id[in] Id of the client session.
+         * @param ws_handle[in,out] Write set handle associated to the current
+         *                 transaction.
+         * @param flags[in] Flags associated to the write set (see struct flag).
+         * @param ws_meta[out] Write set meta data associated to the
+         *                     replicated write set.
+         * @param seq_cb[in] Optional callback for application defined
+         *                   sequential consistency.
+         *
+         * @return Status code defined in struct status.
+         */
         virtual enum status
-        certify(wsrep::client_id, wsrep::ws_handle&,
-                int,
-                wsrep::ws_meta&) = 0;
+        certify(wsrep::client_id client_id, wsrep::ws_handle& ws_handle,
+                int flags, wsrep::ws_meta& ws_meta, const seq_cb_t* seq_cb)
+            = 0;
         /**
          * BF abort a transaction inside provider.
          *

include/wsrep/transaction.hpp

@@ -175,7 +175,8 @@ namespace wsrep
 
         int after_row();
 
-        int before_prepare(wsrep::unique_lock<wsrep::mutex>&);
+        int before_prepare(wsrep::unique_lock<wsrep::mutex>&,
+                           const wsrep::provider::seq_cb_t*);
 
         int after_prepare(wsrep::unique_lock<wsrep::mutex>&);
 
@@ -248,7 +249,8 @@ namespace wsrep
         bool abort_or_interrupt(wsrep::unique_lock<wsrep::mutex>&);
         int streaming_step(wsrep::unique_lock<wsrep::mutex>&, bool force = false);
         int certify_fragment(wsrep::unique_lock<wsrep::mutex>&);
-        int certify_commit(wsrep::unique_lock<wsrep::mutex>&);
+        int certify_commit(wsrep::unique_lock<wsrep::mutex>&,
+                           const wsrep::provider::seq_cb_t*);
         int append_sr_keys_for_commit();
         int release_commit_order(wsrep::unique_lock<wsrep::mutex>&);
         void remove_fragments_in_storage_service_scope(

src/client_state.cpp

@@ -365,12 +365,12 @@ int wsrep::client_state::next_fragment(const wsrep::ws_meta& meta)
     return transaction_.next_fragment(meta);
 }
 
-int wsrep::client_state::before_prepare()
+int wsrep::client_state::before_prepare(const wsrep::provider::seq_cb_t* seq_cb)
 {
     wsrep::unique_lock<wsrep::mutex> lock(mutex_);
     assert(owning_thread_id_ == wsrep::this_thread::get_id());
     assert(state_ == s_exec);
-    return transaction_.before_prepare(lock);
+    return transaction_.before_prepare(lock, seq_cb);
 }
 
 int wsrep::client_state::after_prepare()

src/transaction.cpp

@@ -273,8 +273,8 @@ int wsrep::transaction::after_row()
     return ret;
 }
 
-int wsrep::transaction::before_prepare(
-    wsrep::unique_lock<wsrep::mutex>& lock)
+int wsrep::transaction::before_prepare(wsrep::unique_lock<wsrep::mutex>& lock,
+                                       const wsrep::provider::seq_cb_t* seq_cb)
 {
     assert(lock.owns_lock());
     int ret(0);
@@ -349,7 +349,7 @@ int wsrep::transaction::before_prepare(
             }
             else
             {
-                ret = certify_commit(lock);
+                ret = certify_commit(lock, seq_cb);
             }
 
             assert((ret == 0 && state() == s_preparing) ||
@@ -465,7 +465,7 @@ int wsrep::transaction::before_commit()
     case wsrep::client_state::m_local:
         if (state() == s_executing)
         {
-            ret = before_prepare(lock) || after_prepare(lock);
+            ret = before_prepare(lock, nullptr) || after_prepare(lock);
             assert((ret == 0 &&
                     (state() == s_committing || state() == s_prepared))
                    ||
@@ -495,7 +495,7 @@ int wsrep::transaction::before_commit()
 
         if (ret == 0 && state() == s_prepared)
         {
-            ret = certify_commit(lock);
+            ret = certify_commit(lock, nullptr);
             assert((ret == 0 && state() == s_committing) ||
                    (state() == s_must_abort ||
                     state() == s_must_replay ||
@@ -543,7 +543,7 @@ int wsrep::transaction::before_commit()
         }
         else if (state() == s_executing || state() == s_replaying)
         {
-            ret = before_prepare(lock) || after_prepare(lock);
+            ret = before_prepare(lock, nullptr) || after_prepare(lock);
         }
         else
         {
@@ -1195,7 +1195,7 @@ int wsrep::transaction::commit_or_rollback_by_xid(const wsrep::xid& xid,
         provider().certify(client_state_.id(),
                            ws_handle_,
                            flags(),
-                           meta));
+                           meta, nullptr));
 
     int ret;
     if (cert_ret == wsrep::provider::success)
@@ -1622,7 +1622,7 @@ int wsrep::transaction::certify_fragment(
             cert_ret = provider().certify(client_state_.id(),
                                           ws_handle_,
                                           flags(),
-                                          sr_ws_meta);
+                                          sr_ws_meta, nullptr);
             client_service_.debug_crash(
                 "crash_replicate_fragment_after_certify");
 
@@ -1744,7 +1744,7 @@ int wsrep::transaction::certify_fragment(
 }
 
 int wsrep::transaction::certify_commit(
-    wsrep::unique_lock<wsrep::mutex>& lock)
+    wsrep::unique_lock<wsrep::mutex>& lock, const provider::seq_cb_t* seq_cb)
 {
     assert(lock.owns_lock());
     assert(active());
@@ -1828,7 +1828,7 @@ int wsrep::transaction::certify_commit(
         cert_ret(provider().certify(client_state_.id(),
                                    ws_handle_,
                                    flags(),
-                                   ws_meta_));
+                                   ws_meta_, seq_cb));
     client_service_.debug_sync("wsrep_after_certification");
 
     lock.lock();

src/wsrep_provider_v26.cpp

@@ -922,7 +922,8 @@ enum wsrep::provider::status
 wsrep::wsrep_provider_v26::certify(wsrep::client_id client_id,
                                    wsrep::ws_handle& ws_handle,
                                    int flags,
-                                   wsrep::ws_meta& ws_meta)
+                                   wsrep::ws_meta& ws_meta,
+                                   const seq_cb_t* /* todo: use wsrep_cert_fn_v1 extension if provider has it */)
 {
     mutable_ws_handle mwsh(ws_handle);
     mutable_ws_meta mmeta(ws_meta, flags);

src/wsrep_provider_v26.hpp

@@ -59,7 +59,7 @@ namespace wsrep
         enum wsrep::provider::status
         certify(wsrep::client_id, wsrep::ws_handle&,
                 int,
-                wsrep::ws_meta&) WSREP_OVERRIDE;
+                wsrep::ws_meta&, const seq_cb_t*) WSREP_OVERRIDE;
         enum wsrep::provider::status
         bf_abort(wsrep::seqno,
                  wsrep::transaction_id,

test/mock_provider.hpp

@@ -79,7 +79,8 @@ namespace wsrep
         certify(wsrep::client_id client_id,
                 wsrep::ws_handle& ws_handle,
                 int flags,
-                wsrep::ws_meta& ws_meta)
+                wsrep::ws_meta& ws_meta,
+                const seq_cb* /* Ignored in unit tests. */)
             WSREP_OVERRIDE
         {
             ws_handle = wsrep::ws_handle(ws_handle.transaction_id(), (void*)1);