MDEV-452 Add full support for auto-initialized/updated timestamp and datetime

Generalized support for auto-updated and/or auto-initialized timestamp
and datetime columns. This patch is a reimplementation of MySQL's
"WL#5874: CURRENT_TIMESTAMP as DEFAULT for DATETIME columns". In order to
ease future merges, this implementation reused few function and variable
names from MySQL's patch, however the implementation is quite different.

TODO:
The only unresolved problem in this patch is the semantics of LOAD DATA for
TIMESTAMP and DATETIME columns in the cases when there are missing or NULL
columns. I couldn't fully comprehend the logic behind MySQL's behavior and
its relationship with their own documentation, so I left the results to be
more consistent with all other LOAD cases.

The problematic test cases can be seen by running the test file function_defaults,
and observing the test case differences. Those were left on purpose for discussion.

1 files changed, 69 insertions, 3 deletions

diff --git a/sql/field.h b/sql/field.h
index f22bab0409d..7006e0bfbe8 100644
--- a/sql/field.h
+++ b/sql/field.h
@@ -329,6 +329,37 @@ public:
       *null_ptr= ((*null_ptr & (uchar) ~null_bit) |
 		  (null_ptr[l_offset] & null_bit));
   }
+
+  bool has_insert_default_function() const
+  {
+    return unireg_check == TIMESTAMP_DN_FIELD ||
+      unireg_check == TIMESTAMP_DNUN_FIELD;
+  }
+
+  bool has_update_default_function() const
+  {
+    return unireg_check == TIMESTAMP_UN_FIELD ||
+      unireg_check == TIMESTAMP_DNUN_FIELD;
+  }
+
+  virtual void set_explicit_default(Item *value);
+
+  /**
+     Evaluates the @c INSERT default function and stores the result in the
+     field. If no such function exists for the column, or the function is not
+     valid for the column's data type, invoking this function has no effect.
+  */
+  virtual int evaluate_insert_default_function() { return 0; }
+
+
+  /**
+     Evaluates the @c UPDATE default function, if one exists, and stores the
+     result in the record buffer. If no such function exists for the column,
+     or the function is not valid for the column's data type, invoking this
+     function has no effect.
+  */
+  virtual int evaluate_update_default_function() { return 0; }
+
   virtual bool binary() const { return 1; }
   virtual bool zero_pack() const { return 1; }
   virtual enum ha_base_keytype key_type() const { return HA_KEYTYPE_BINARY; }
@@ -1239,12 +1270,26 @@ public:
   virtual int set_time();
   virtual void set_default()
   {
-    if (table->timestamp_field == this &&
-        unireg_check != TIMESTAMP_UN_FIELD)
+    if (has_insert_default_function())
       set_time();
     else
       Field::set_default();
   }
+  virtual void set_explicit_default(Item *value);
+  virtual int evaluate_insert_default_function()
+  {
+    int res= 0;
+    if (has_insert_default_function())
+      res= set_time();
+    return res;
+  }
+  virtual int evaluate_update_default_function()
+  {
+    int res= 0;
+    if (has_update_default_function())
+      res= set_time();
+    return res;
+  }
   /* Get TIMESTAMP field value as seconds since begging of Unix Epoch */
   virtual my_time_t get_timestamp(ulong *sec_part) const;
   virtual void store_TIME(my_time_t timestamp, ulong sec_part)
@@ -1252,7 +1297,6 @@ public:
     int4store(ptr,timestamp);
   }
   bool get_date(MYSQL_TIME *ltime, ulonglong fuzzydate);
-  timestamp_auto_set_type get_auto_set_type() const;
   uchar *pack(uchar *to, const uchar *from,
               uint max_length __attribute__((unused)))
   {
@@ -1503,6 +1547,28 @@ public:
   void sql_type(String &str) const;
   bool zero_pack() const { return 1; }
   bool get_date(MYSQL_TIME *ltime, ulonglong fuzzydate);
+  virtual int set_time();
+  virtual void set_default()
+  {
+    if (has_insert_default_function())
+      set_time();
+    else
+      Field::set_default();
+  }
+  virtual int evaluate_insert_default_function()
+  {
+    int res= 0;
+    if (has_insert_default_function())
+      res= set_time();
+    return res;
+  }
+  virtual int evaluate_update_default_function()
+  {
+    int res= 0;
+    if (has_update_default_function())
+      res= set_time();
+    return res;
+  }
   uchar *pack(uchar* to, const uchar *from,
               uint max_length __attribute__((unused)))
   {