api: Reserve more numbers for internal calls

Reasoning; currently INTERNAL_CALL is mostly used to determine whether it is 
needed to deal with NL-used-as-NUL problem. This code is useful for nvim_… API 
calls done from VimL, but not for API calls done from lua, yet lua needs to 
supply something as channel_id.

scripts/genmsgpack.lua

@@ -422,7 +422,7 @@ local function process_function(fn)
     cparams = cparam .. ', ' .. cparams
   end
   if fn.receives_channel_id then
-    cparams = 'INTERNAL_CALL, ' .. cparams
+    cparams = 'LUA_INTERNAL_CALL, ' .. cparams
   end
   if fn.can_fail then
     cparams = cparams .. '&err'

src/nvim/api/buffer.c

@@ -192,7 +192,7 @@ ArrayOf(String) nvim_buf_get_lines(uint64_t channel_id,
     Object str = STRING_OBJ(cstr_to_string(bufstr));
 
     // Vim represents NULs as NLs, but this may confuse clients.
-    if (channel_id != INTERNAL_CALL) {
+    if (channel_id != VIML_INTERNAL_CALL) {
       strchrsub(str.data.string.data, '\n', '\0');
     }
 
@@ -313,7 +313,7 @@ void nvim_buf_set_lines(uint64_t channel_id,
     // line and convert NULs to newlines to avoid truncation.
     lines[i] = xmallocz(l.size);
     for (size_t j = 0; j < l.size; j++) {
-      if (l.data[j] == '\n' && channel_id != INTERNAL_CALL) {
+      if (l.data[j] == '\n' && channel_id != VIML_INTERNAL_CALL) {
         api_set_error(err, Exception, _("string cannot contain newlines"));
         new_len = i + 1;
         goto end;

src/nvim/api/private/defs.h

@@ -5,6 +5,8 @@
 #include <stdbool.h>
 #include <string.h>
 
+#include "nvim/func_attr.h"
+
 #define ARRAY_DICT_INIT {.size = 0, .capacity = 0, .items = NULL}
 #define STRING_INIT {.data = NULL, .size = 0}
 #define OBJECT_INIT { .type = kObjectTypeNil }
@@ -33,8 +35,29 @@ typedef enum {
 /// Used as the message ID of notifications.
 #define NO_RESPONSE UINT64_MAX
 
-/// Used as channel_id when the call is local.
-#define INTERNAL_CALL UINT64_MAX
+/// Mask for all internal calls
+#define INTERNAL_CALL_MASK (UINT64_MAX ^ (UINT64_MAX >> 1))
+// (1 << 63) in all forms produces “warning: shift count >= width of type
+// [-Wshift-count-overflow]”
+
+/// Internal call from VimL code
+#define VIML_INTERNAL_CALL INTERNAL_CALL_MASK
+
+/// Internal call from lua code
+#define LUA_INTERNAL_CALL (VIML_INTERNAL_CALL + 1)
+
+static inline bool is_internal_call(uint64_t channel_id)
+  REAL_FATTR_ALWAYS_INLINE REAL_FATTR_CONST;
+
+/// Check whether call is internal
+///
+/// @param[in]  channel_id  Channel id.
+///
+/// @return true if channel_id refers to internal channel.
+static inline bool is_internal_call(const uint64_t channel_id)
+{
+  return !!(channel_id & INTERNAL_CALL_MASK);
+}
 
 typedef struct {
   ErrorType type;

src/nvim/eval.c

@@ -7796,7 +7796,7 @@ static void api_wrapper(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   }
 
   Error err = ERROR_INIT;
-  Object result = fn(INTERNAL_CALL, args, &err);
+  Object result = fn(VIML_INTERNAL_CALL, args, &err);
 
   if (err.set) {
     nvim_err_writeln(cstr_as_string(err.msg));