Hello, thanks for your patch!

+    for i = 1, select('#', ...) do
+        if type(results[i] == 'cdata') then
+            results[i] = msgpack.decode(msgpack.encode(results[i]))

+    if not ... then
+        return ...
+    end

What do you want to achieve with it?

This condition can't distinguish what it's a function without arguments or has nil as first argument.

I think this check should be dropped or replaced to something as "select('#', ...) == 0".

+    local results = {...}

This transformation drops trailing nils that should be evaluated to box.NULL.

Please, add tests to check that all nils you passed will be converted to box.NULLs after transformation.

--
Oleg Babin

Despite what was stated in the documentation, netbox.connect was not always
equivalent to netbox.self. In particular, they converted tuple to different
types - table and cdata respectively.
Netbox.self also allowed to modify source objects after transfer, which was
not an example of expected behavior either.

The patch fixes both those issues and covers all cases where netbox.self and
connect perform conversion of types - e.g., for box.error.

Closes #4513, #4602
---
Issues:
https://github.com/tarantool/tarantool/issues/4513
https://github.com/tarantool/tarantool/issues/4602
Branch:
https://github.com/tarantool/tarantool/compare/eljashm/gh-4513-netbox.self-convert-tuples-to-table-type

 src/box/lua/net_box.lua            | 11 +++-
 test/box/net.box.result            |  4 +-
 test/box/net_connect_self.result   | 81 ++++++++++++++++++++++++++++++
 test/box/net_connect_self.test.lua | 33 ++++++++++++
 4 files changed, 125 insertions(+), 4 deletions(-)
 create mode 100644 test/box/net_connect_self.result
 create mode 100644 test/box/net_connect_self.test.lua

diff --git a/src/box/lua/net_box.lua b/src/box/lua/net_box.lua
index c2e1bb9c4..aa36ba7d1 100644
--- a/src/box/lua/net_box.lua
+++ b/src/box/lua/net_box.lua
@@ -1523,7 +1523,16 @@ local function handle_eval_result(status, ...)
         rollback()
         return box.error(E_PROC_LUA, (...))
     end
-    return ...
+    if not ... then
+        return ...
+    end
+    local results = {...}
+    for i = 1, select('#', ...) do
+        if type(results[i] == 'cdata') then
+            results[i] = msgpack.decode(msgpack.encode(results[i]))
+        end
+    end
+    return unpack(results)
 end
 
 this_module.self = {
diff --git a/test/box/net.box.result b/test/box/net.box.result
index e3dabf7d9..836a4fe8c 100644
--- a/test/box/net.box.result
+++ b/test/box/net.box.result
@@ -626,9 +626,7 @@ box.schema.func.drop('pause')
 -- call
 remote.self:call('test_foo', {'a', 'b', 'c'})
 ---
-- - - a: 1
-  - - b: 2
-  - c
+- [[{'a': 1}], [{'b': 2}], 'c']
 ...
 cn:call('test_foo', {'a', 'b', 'c'})
 ---
diff --git a/test/box/net_connect_self.result b/test/box/net_connect_self.result
new file mode 100644
index 000000000..b6ec38fb8
--- /dev/null
+++ b/test/box/net_connect_self.result
@@ -0,0 +1,81 @@
+-- test-run result file version 2
+remote = require('net.box')
+ | ---
+ | ...
+--
+-- gh-4513 netbox.connect and netbox.self should be interchangeable
+--
+space = box.schema.space.create('gh4513')
+ | ---
+ | ...
+box.schema.user.grant('guest','read, write, execute','universe')
+ | ---
+ | ...
+idx = box.space.gh4513:create_index('primary')
+ | ---
+ | ...
+box.space.gh4513:insert({1})
+ | ---
+ | - [1]
+ | ...
+
+type(remote.connect(box.cfg.listen):eval("return box.space.gh4513:get({1})"))
+ | ---
+ | - table
+ | ...
+type(remote.self:eval("return box.space.gh4513:get{1}"))
+ | ---
+ | - table
+ | ...
+
+type(remote.connect(box.cfg.listen):eval('return box.error.new(1, "test error")'))
+ | ---
+ | - string
+ | ...
+type(remote.self:eval('return box.error.new(1, "test error")'))
+ | ---
+ | - string
+ | ...
+
+type(remote.self:eval("return box.NULL"))
+ | ---
+ | - cdata
+ | ...
+type(remote.connect(box.cfg.listen):eval("return box.NULL"))
+ | ---
+ | - cdata
+ | ...
+
+--
+-- gh-4602 net.box:self allows to modify source object after transfer
+--
+x = {}
+ | ---
+ | ...
+function test() return x end
+ | ---
+ | ...
+box.schema.func.create('test')
+ | ---
+ | ...
+box.schema.user.grant('guest', 'execute', 'function', 'test')
+ | ---
+ | ...
+
+y = remote.connect(box.cfg.listen):call('test')
+ | ---
+ | ...
+-- should be false
+y == x
+ | ---
+ | - false
+ | ...
+
+z = remote.self:call('test')
+ | ---
+ | ...
+-- should be false as well
+z == x
+ | ---
+ | - false
+ | ...
diff --git a/test/box/net_connect_self.test.lua b/test/box/net_connect_self.test.lua
new file mode 100644
index 000000000..76b16704f
--- /dev/null
+++ b/test/box/net_connect_self.test.lua
@@ -0,0 +1,33 @@
+remote = require('net.box')
+--
+-- gh-4513 netbox.connect and netbox.self should be interchangeable
+--
+space = box.schema.space.create('gh4513')
+box.schema.user.grant('guest','read, write, execute','universe')
+idx = box.space.gh4513:create_index('primary')
+box.space.gh4513:insert({1})
+
+type(remote.connect(box.cfg.listen):eval("return box.space.gh4513:get({1})"))
+type(remote.self:eval("return box.space.gh4513:get{1}"))
+
+type(remote.connect(box.cfg.listen):eval('return box.error.new(1, "test error")'))
+type(remote.self:eval('return box.error.new(1, "test error")'))
+
+type(remote.self:eval("return box.NULL"))
+type(remote.connect(box.cfg.listen):eval("return box.NULL"))
+
+--
+-- gh-4602 net.box:self allows to modify source object after transfer
+--
+x = {}
+function test() return x end
+box.schema.func.create('test')
+box.schema.user.grant('guest', 'execute', 'function', 'test')
+
+y = remote.connect(box.cfg.listen):call('test')
+-- should be false
+y == x
+
+z = remote.self:call('test')
+-- should be false as well
+z == x