Re: [PATCH 03/12] alter: move dictionary update from ModifySpace::alter_def to alter

[Vladimir Davydov <vdavydov.dev@gmail.com>]

On Mon, Apr 09, 2018 at 11:32:40PM +0300, Konstantin Osipov wrote:
> * Vladimir Davydov <vdavydov.dev@gmail.com> [18/04/09 10:33]:
> > -	struct tuple_dictionary *new_dict;
> > -	/**
> > -	 * Old tuple dictionary stored to rollback in destructor,
> > -	 * if an exception had been raised after alter_def(), but
> > -	 * before alter().
> > -	 */
> > -	struct tuple_dictionary *old_dict;
> > +	struct tuple_dictionary *dict;
> 
> Please provide a comment. 
> 
> >  	/*
> > -	 * Move new names into an old dictionary, which already is
> > -	 * referenced by existing tuple formats. New dictionary
> > -	 * object is deleted later, in destructor.
> > +	 * Use the old dictionary for the new space, because
> > +	 * it is already referenced by existing tuple formats.
> > +	 * We will update it in place in ModifySpace::alter.
> >  	 */
> 
> I don't understand this comment. Naming the variable "dict" didn't
> help. Not having a comment for the variable where it was declared
> didn't help :(
> 
> I do understand that your strategy is to swap the new space' dict
> with the old space dict and hold the new space dict in a temp
> variable until possible rollback. But I don't understand why you
> had to change it. The changeset comment says nothing about
> changing the way you modify the format, it only mentions changes
> in the timing.
> 
> > -	new_dict = def->dict;
> > -	old_dict = alter->old_space->def->dict;
> > -	tuple_dictionary_swap(new_dict, old_dict);
> > -	def->dict = old_dict;
> > -	tuple_dictionary_ref(old_dict);
> > +	dict = def->dict;
> > +	def->dict = alter->old_space->def->dict;
> > +	tuple_dictionary_ref(def->dict);
> >  
> >  	space_def_delete(alter->space_def);
> >  	alter->space_def = def;

Fixed as discussed f2f. Please take a look. Note, the patch hasn't been
updated on the branch yet, don't push it.

From 71e9dff0d3666a9ac02739d5754bd8d028134c9f Mon Sep 17 00:00:00 2001
From: Vladimir Davydov <vdavydov.dev@gmail.com>
Date: Wed, 4 Apr 2018 16:02:07 +0300
Subject: [PATCH] alter: move dictionary update from ModifySpace::alter_def to
 alter

We may yield while rebuilding a vinyl index hence we must not modify
the old space before we are done building all indexes, otherwise the
user might see an inconsistent state while a space is being altered.
Currently, this requirement doesn't hold - we both modify the old space
and build the new indexes in AlterSpaceOp::alter. We need to introduce
AlterSpaceOp::prepare method that would perform all yielding operations
and forbid yielding in AlterSpaceOp::alter.

Dictionary update, which is currently done by ModifySpace::alter_def,
modifies the old space hence it should live in ModifySpace::alter.
Let's move it there.

While we are at it, let's also rename ModifySpace::def to new_def,
because it denotes the new space definition.

diff --git a/src/box/alter.cc b/src/box/alter.cc
index 0af4ac09..6e351cf5 100644
--- a/src/box/alter.cc
+++ b/src/box/alter.cc
@@ -916,24 +916,19 @@ CheckSpaceFormat::alter(struct alter_space *alter)
 class ModifySpace: public AlterSpaceOp
 {
 public:
-	ModifySpace(struct alter_space *alter, struct space_def *def_arg)
-		:AlterSpaceOp(alter), def(def_arg) {}
+	ModifySpace(struct alter_space *alter, struct space_def *def)
+		:AlterSpaceOp(alter), new_def(def) {}
 	/**
 	 * Newely created field dictionary. When new space_def is
 	 * created, it allocates new dictionary. Alter moves new
 	 * names into an old dictionary and deletes new one.
 	 */
 	struct tuple_dictionary *new_dict;
-	/**
-	 * Old tuple dictionary stored to rollback in destructor,
-	 * if an exception had been raised after alter_def(), but
-	 * before alter().
-	 */
-	struct tuple_dictionary *old_dict;
 	/* New space definition. */
-	struct space_def *def;
+	struct space_def *new_def;
 	virtual void alter_def(struct alter_space *alter);
-	virtual void commit(struct alter_space *alter, int64_t signature);
+	virtual void alter(struct alter_space *alter);
+	virtual void rollback(struct alter_space *alter);
 	virtual ~ModifySpace();
 };
 
@@ -942,40 +937,43 @@ void
 ModifySpace::alter_def(struct alter_space *alter)
 {
 	/*
-	 * Move new names into an old dictionary, which already is
-	 * referenced by existing tuple formats. New dictionary
-	 * object is deleted later, in destructor.
+	 * Use the old dictionary for the new space, because
+	 * it is already referenced by existing tuple formats.
+	 * We will update it in place in ModifySpace::alter.
 	 */
-	new_dict = def->dict;
-	old_dict = alter->old_space->def->dict;
-	tuple_dictionary_swap(new_dict, old_dict);
-	def->dict = old_dict;
-	tuple_dictionary_ref(old_dict);
+	new_dict = new_def->dict;
+	new_def->dict = alter->old_space->def->dict;
+	tuple_dictionary_ref(new_def->dict);
 
 	space_def_delete(alter->space_def);
-	alter->space_def = def;
+	alter->space_def = new_def;
 	/* Now alter owns the def. */
-	def = NULL;
+	new_def = NULL;
+}
+
+void
+ModifySpace::alter(struct alter_space *alter)
+{
+	/*
+	 * Move new names into an old dictionary, which already is
+	 * referenced by existing tuple formats. New dictionary
+	 * object is deleted later, in destructor.
+	 */
+	tuple_dictionary_swap(alter->new_space->def->dict, new_dict);
 }
 
 void
-ModifySpace::commit(struct alter_space *alter, int64_t signature)
+ModifySpace::rollback(struct alter_space *alter)
 {
-	(void) alter;
-	(void) signature;
-	old_dict = NULL;
+	tuple_dictionary_swap(alter->new_space->def->dict, new_dict);
 }
 
 ModifySpace::~ModifySpace()
 {
-	if (new_dict != NULL) {
-		/* Return old names into the old dict. */
-		if (old_dict != NULL)
-			tuple_dictionary_swap(new_dict, old_dict);
+	if (new_dict != NULL)
 		tuple_dictionary_unref(new_dict);
-	}
-	if (def != NULL)
-		space_def_delete(def);
+	if (new_def != NULL)
+		space_def_delete(new_def);
 }
 
 /** DropIndex - remove an index from space. */
diff --git a/src/box/tuple_format.c b/src/box/tuple_format.c
index d3a7702d..11481473 100644
--- a/src/box/tuple_format.c
+++ b/src/box/tuple_format.c
@@ -262,8 +262,6 @@ tuple_format_new(struct tuple_format_vtab *vtab, struct key_def * const *keys,
 		 const struct field_def *space_fields,
 		 uint32_t space_field_count, struct tuple_dictionary *dict)
 {
-	assert((dict == NULL && space_field_count == 0) ||
-	       (dict != NULL && space_field_count == dict->name_count));
 	struct tuple_format *format =
 		tuple_format_alloc(keys, key_count, space_field_count, dict);
 	if (format == NULL)