alter: fix modification of primary key definition

If pk_def passed to index_def_new() is not NULL, the function will merge
it with the given key_def to create index cmp_def, no matter if the
index is primary or secondary. When an index is altered, we call
index_def_new() to create the new definition, passing the primary key
definition of the altered space for pk_def. If it is the primary index
that is altered, we will pass the definition of the old primary index
and index_def_new() will happily merge it with the new index definition,
resulting in invalid index_def::cmp_def. This doesn't affect memtx, as
memtx doesn't use cmp_def for unique indexes, but it does affect vinyl
in a peculiar way:

  tarantool> _ = box.schema.space.create('test', {engine = 'vinyl'})
  ---
  ...

  tarantool> _ = box.space.test:create_index('pk')
  ---
  ...

  tarantool> _ = box.space.test.index.pk:alter{parts = {2, 'unsigned'}}
  ---
  ...

  tarantool> _ = box.space.test:replace{1, 1}
  ---
  ...

  tarantool> _ = box.space.test:replace{2, 1}
  ---
  ...

  tarantool> box.space.test:select()
  ---
  - - [1, 1]
    - [2, 1]
  ...

(expected: [2, 1])

Fix this by making index_def_new() merge key_def with pk_def only for
secondary indexes.

Closes #3508

src/box/index_def.c

@@ -86,7 +86,7 @@ index_def_new(uint32_t space_id, uint32_t iid, const char *name,
 		return NULL;
 	}
 	def->key_def = key_def_dup(key_def);
-	if (pk_def != NULL) {
+	if (iid != 0) {
 		def->cmp_def = key_def_merge(key_def, pk_def);
 		if (! opts->is_unique) {
 			def->cmp_def->unique_part_count =

test/vinyl/ddl.result

@@ -120,9 +120,14 @@ space:replace{1, 2}
 ---
 - [1, 2]
 ...
-space:get(2)
+-- gh-3508 - Altering primary index of a vinyl space doesn't work as expected
+space:replace{2, 2}
 ---
-- [1, 2]
+- [2, 2]
+...
+space:select()
+---
+- - [2, 2]
 ...
 space:drop()
 ---

test/vinyl/ddl.test.lua

@@ -39,7 +39,9 @@ box.snapshot()
 while pk:stat().disk.compact.count == 0 do fiber.sleep(0.01) end
 pk:alter{parts = {2, 'unsigned'}} -- success: space is empty now
 space:replace{1, 2}
-space:get(2)
+-- gh-3508 - Altering primary index of a vinyl space doesn't work as expected
+space:replace{2, 2}
+space:select()
 space:drop()
 
 --