eval/encode: Make sure that encoder can encode NULL variables

Adds two undocumented v: variables: _null_list and _null_dict because I do not 
know a reproducible way to get such lists (though I think I heard about this) 
and dictionaries (do not remember hearing about them). NULL strings are obtained 
using $XXX_UNEXISTENT_VAR_XXX.

Fixes crash in json_encode($XXX_UNEXISTENT_VAR_XXX). Other added tests worked 
fine before this commit.

7 files changed, 89 insertions, 1 deletions

diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index 204c0fb1ef..aaad9fab34 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -379,6 +379,8 @@ static struct vimvar {
   VV(VV_FALSE,          "false",            VAR_SPECIAL, VV_RO),
   VV(VV_TRUE,           "true",             VAR_SPECIAL, VV_RO),
   VV(VV_NULL,           "null",             VAR_SPECIAL, VV_RO),
+  VV(VV__NULL_LIST,     "_null_list",       VAR_LIST, VV_RO),
+  VV(VV__NULL_DICT,     "_null_dict",       VAR_DICT, VV_RO),
 };
 #undef VV
 
diff --git a/src/nvim/eval.h b/src/nvim/eval.h
index f81eb5d063..d6800afd52 100644
--- a/src/nvim/eval.h
+++ b/src/nvim/eval.h
@@ -122,6 +122,8 @@ typedef enum {
     VV_FALSE,
     VV_TRUE,
     VV_NULL,
+    VV__NULL_LIST,  // List with NULL value. For test purposes only.
+    VV__NULL_DICT,  // Dictionary with NULL value. For test purposes only.
 } VimVarIndex;
 
 /// All recognized msgpack types
diff --git a/src/nvim/eval/encode.c b/src/nvim/eval/encode.c
index 0bde6562b8..88c731e92a 100644
--- a/src/nvim/eval/encode.c
+++ b/src/nvim/eval/encode.c
@@ -287,6 +287,9 @@ int encode_read_from_list(ListReaderState *const state, char *const buf,
       (val)->copyID_attr = copyID; \
     } while (0)
 
+#define TV_STRLEN(tv) \
+    (tv->vval.v_string == NULL ? 0 : STRLEN(tv->vval.v_string))
+
 /// Define functions which convert VimL value to something else
 ///
 /// Creates function `vim_to_{name}(firstargtype firstargname, typval_T *const
@@ -306,7 +309,7 @@ static int name##_convert_one_value(firstargtype firstargname, \
 { \
   switch (tv->v_type) { \
     case VAR_STRING: { \
-      CONV_STRING(tv->vval.v_string, STRLEN(tv->vval.v_string)); \
+      CONV_STRING(tv->vval.v_string, TV_STRLEN(tv)); \
       break; \
     } \
     case VAR_NUMBER: { \
diff --git a/test/functional/eval/json_functions_spec.lua b/test/functional/eval/json_functions_spec.lua
index b788c2cf07..0a7c4cf669 100644
--- a/test/functional/eval/json_functions_spec.lua
+++ b/test/functional/eval/json_functions_spec.lua
@@ -784,4 +784,16 @@ describe('json_encode() function', function()
     eq([["\u0000\u0001\u0002\u0003\u0004\u0005\u0006\u0007\b\t\n\u000B\f\r\u000E\u000F\u0010\u0011\u0012\u0013"]],
        eval('json_encode({"_TYPE": v:msgpack_types.string, "_VAL": ["\n\1\2\3\4\5\6\7\8\9", "\11\12\13\14\15\16\17\18\19"]})'))
   end)
+
+  it('can dump NULL string', function()
+    eq('""', eval('json_encode($XXX_UNEXISTENT_VAR_XXX)'))
+  end)
+
+  it('can dump NULL list', function()
+    eq('[]', eval('json_encode(v:_null_list)'))
+  end)
+
+  it('can dump NULL dictionary', function()
+    eq('{}', eval('json_encode(v:_null_dict)'))
+  end)
 end)
diff --git a/test/functional/eval/msgpack_functions_spec.lua b/test/functional/eval/msgpack_functions_spec.lua
index a602bad86f..9e501353a5 100644
--- a/test/functional/eval/msgpack_functions_spec.lua
+++ b/test/functional/eval/msgpack_functions_spec.lua
@@ -686,4 +686,18 @@ describe('msgpackdump() function', function()
         exc_exec('call msgpackdump(' .. val .. ')'))
     end
   end)
+
+  it('can dump NULL string', function()
+    eq({'\196\n'}, eval('msgpackdump([$XXX_UNEXISTENT_VAR_XXX])'))
+    eq({'\196\n'}, eval('msgpackdump([{"_TYPE": v:msgpack_types.binary, "_VAL": [$XXX_UNEXISTENT_VAR_XXX]}])'))
+    eq({'\160'}, eval('msgpackdump([{"_TYPE": v:msgpack_types.string, "_VAL": [$XXX_UNEXISTENT_VAR_XXX]}])'))
+  end)
+
+  it('can dump NULL list', function()
+    eq({'\144'}, eval('msgpackdump([v:_null_list])'))
+  end)
+
+  it('can dump NULL dictionary', function()
+    eq({'\128'}, eval('msgpackdump([v:_null_dict])'))
+  end)
 end)
diff --git a/test/functional/eval/string_spec.lua b/test/functional/eval/string_spec.lua
index 0c4ff87231..20a800cde0 100644
--- a/test/functional/eval/string_spec.lua
+++ b/test/functional/eval/string_spec.lua
@@ -84,6 +84,18 @@ describe('string() function', function()
       eq('\'\'\'b\'\'\'\'d\'', funcs.string('\'b\'\'d'))
       eq('\'a\'\'b\'\'c\'\'d\'', funcs.string('a\'b\'c\'d'))
     end)
+
+    it('dumps NULL strings', function()
+      eq('\'\'', eval('string($XXX_UNEXISTENT_VAR_XXX)'))
+    end)
+
+    it('dumps NULL lists', function()
+      eq('[]', eval('string(v:_null_list)'))
+    end)
+
+    it('dumps NULL dictionaries', function()
+      eq('{}', eval('string(v:_null_dict)'))
+    end)
   end)
 
   describe('used to represent funcrefs', function()
diff --git a/test/unit/eval/tricks_spec.lua b/test/unit/eval/tricks_spec.lua
new file mode 100644
index 0000000000..4c5184995c
--- /dev/null
+++ b/test/unit/eval/tricks_spec.lua
@@ -0,0 +1,43 @@
+local helpers = require('test.unit.helpers')
+
+local cimport = helpers.cimport
+local to_cstr = helpers.to_cstr
+local ffi = helpers.ffi
+local eq = helpers.eq
+
+local eval = cimport('./src/nvim/eval.h', './src/nvim/memory.h')
+
+local eval_expr = function(expr)
+  return ffi.gc(eval.eval_expr(to_cstr(expr), nil), function(tv)
+    eval.clear_tv(tv)
+    eval.xfree(tv)
+  end)
+end
+
+describe('NULL typval_T', function()
+  it('is produced by $XXX_UNEXISTENT_VAR_XXX', function()
+    -- Required for various tests which need to check whether typval_T with NULL
+    -- string works correctly. This test checks that unexistent environment
+    -- variable produces NULL string, not that some specific environment
+    -- variable does not exist. Last bit is left for the test writers.
+    local unexistent_env = 'XXX_UNEXISTENT_VAR_XXX'
+    while os.getenv(unexistent_env) ~= nil do
+      unexistent_env = unexistent_env .. '_XXX'
+    end
+    local rettv = eval_expr('$' .. unexistent_env)
+    eq(eval.VAR_STRING, rettv.v_type)
+    eq(nil, rettv.vval.v_string)
+  end)
+
+  it('is produced by v:_null_list', function()
+    local rettv = eval_expr('v:_null_list')
+    eq(eval.VAR_LIST, rettv.v_type)
+    eq(nil, rettv.vval.v_list)
+  end)
+
+  it('is produced by v:_null_dict', function()
+    local rettv = eval_expr('v:_null_dict')
+    eq(eval.VAR_DICT, rettv.v_type)
+    eq(nil, rettv.vval.v_dict)
+  end)
+end)