Use cache as long as a trx only performes reads

As before, the cache will be used if there is no ongoing transaction
(includes autocommit being on), or if there is an explicitly read-
only transaction.

In addition, the cache will be used and populated during any other
transaction as long as only pure read statements are executed. After
first non-read statement, the use of the cache is disabled.

Documentation/Filters/Cache.md

@@ -26,9 +26,18 @@ Currently there is **no** cache invalidation, apart from _time-to-live_.
 Resultsets of prepared statements are **not** cached.
 
 ### Transactions
-The cache will be used and populated **only** if there is _no_ on-going
-transaction or if an on-going transaction is _explicitly_ read-only (that is,
-`START TRANSACTION READ ONLY`).
+The cache will be used and populated in the following circumstances:
+
+* There is _no_ on-going transaction, that is, _autocommit_ is used,
+* there is an _explicitly_ read-only transaction (that is,`START TRANSACTION
+  READ ONLY`) active, or
+* there is a transaction active and _no_ statement that modify the database
+  has been performed.
+
+In practice, the last bullet point basically means that if a transaction has
+been started with `BEGIN` or `START TRANSACTION READ WRITE`, then the cache
+will be used and populated until the first `UPDATE`, `INSERT` or `DELETE`
+statement is encountered.
 
 ### Variables
 The cache key is effectively the entire _SELECT_ statement. However, the

Documentation/Release-Notes/MaxScale-2.1.1-Release-Notes.md

@@ -8,6 +8,14 @@ release 2.1.0.
 For any problems you encounter, please consider submitting a bug
 report at [Jira](https://jira.mariadb.org).
 
+## Changed Features
+
+### Cache
+
+The cache will now _also_ be used and populated in a transaction that is
+_not_ explicitly read only, but only until the first statement that modifies
+the database is encountered.
+
 ## Dropped Features
 
 ### MaxAdmin

server/modules/filter/cache/cachefiltersession.cc

@@ -41,6 +41,7 @@ CacheFilterSession::CacheFilterSession(MXS_SESSION* pSession, Cache* pCache, cha
     , m_zDefaultDb(zDefaultDb)
     , m_zUseDb(NULL)
     , m_refreshing(false)
+    , m_is_read_only(true)
 {
     memset(m_key.data, 0, CACHE_KEY_MAXLEN);
 
@@ -130,7 +131,59 @@ int CacheFilterSession::routeQuery(GWBUF* pPacket)
 
     case MYSQL_COM_QUERY:
         {
-            if (!session_trx_is_active(m_pSession) || session_trx_is_read_only(m_pSession))
+            bool consult_cache = false;
+
+            uint32_t type_mask = qc_get_type_mask(pPacket);
+
+            if (type_mask & QUERY_TYPE_BEGIN_TRX)
+            {
+                // When a transaction is started, we initially assume it is read-only.
+                m_is_read_only = true;
+            }
+            else if (!(type_mask & QUERY_TYPE_READ))
+            {
+                // Thereafter, if there's any non-read statement we mark it as non-readonly.
+                // Note that the state of m_is_read_only is not consulted if there is no
+                // on-going transaction of if there is an explicitly read-only transaction.
+                m_is_read_only = false;
+            }
+
+            if (!session_trx_is_active(m_pSession))
+            {
+                if (log_decisions())
+                {
+                    MXS_NOTICE("Cache can be used and stored to, since there is no transaction.");
+                }
+                consult_cache = true;
+            }
+            else if (session_trx_is_read_only(m_pSession))
+            {
+                if (log_decisions())
+                {
+                    MXS_NOTICE("Cache can be used and stored to since there is an explicitly "
+                               "read-only transaction.");
+                }
+                consult_cache = true;
+            }
+            else if (m_is_read_only)
+            {
+                if (log_decisions())
+                {
+                    MXS_NOTICE("Cache can be used and stored to, since the current transaction "
+                               "(not explicitly read-only) has so far been read-only.");
+                }
+                consult_cache = true;
+            }
+            else
+            {
+                if (log_decisions())
+                {
+                    MXS_NOTICE("Cache can not be used, since a not explicitly read-only transaction "
+                               "is active and the transaction has executed non-read statements.");
+                }
+            }
+
+            if (consult_cache)
             {
                 // We do not care whether the query was fully parsed or not.
                 // If a query cannot be fully parsed, the worst thing that can
@@ -215,16 +268,6 @@ int CacheFilterSession::routeQuery(GWBUF* pPacket)
                     }
                 }
             }
-            else
-            {
-                if (log_decisions())
-                {
-                    MXS_NOTICE("autocommit = %s and transaction state %s => Not using or "
-                               "storing to cache.",
-                               session_is_autocommit(m_pSession) ? "ON" : "OFF",
-                               session_trx_state_to_string(session_get_trx_state(m_pSession)));
-                }
-            }
         }
         break;
 

server/modules/filter/cache/cachefiltersession.hh

@@ -119,5 +119,6 @@ private:
     char*                 m_zDefaultDb;  /**< The default database. */
     char*                 m_zUseDb;      /**< Pending default database. Needs server response. */
     bool                  m_refreshing;  /**< Whether the session is updating a stale cache entry. */
+    bool                  m_is_read_only;/**< Whether the current trx has been read-only in pratice. */
 };