fix(api): correctly pass f-args for nvim_create_user_command (#18098)

Skip runs of whitespace and do not include `\` characters when
followed by another `\` or whitespace. This matches the behavior
of <f-args> when used with `:command`.

src/nvim/ex_docmd.c

@@ -5774,27 +5774,45 @@ static void ex_delcommand(exarg_T *eap)
 /// Split a string by unescaped whitespace (space & tab), used for f-args on Lua commands callback.
 /// Similar to uc_split_args(), but does not allocate, add quotes, add commas and is an iterator.
 ///
-/// @note  If no separator is found start = 0 and end = length - 1
 /// @param[in]  arg String to split
-/// @param[in]  iter Iteration counter
-/// @param[out]  start Start of the split
-/// @param[out]  end End of the split
-/// @param[in]  length Length of the string
+/// @param[in]  arglen Length of {arg}
+/// @param[inout] end Index of last character of previous iteration
+/// @param[out] buf Buffer to copy string into
+/// @param[out] len Length of string in {buf}
 ///
-/// @return  false if it's the last split (don't call again), true otherwise (call again).
-bool uc_split_args_iter(const char_u *arg, int iter, int *start, int *end, int length)
+/// @return true if iteration is complete, else false
+bool uc_split_args_iter(const char_u *arg, size_t arglen, size_t *end, char *buf, size_t *len)
 {
-  int pos;
-  *start = *end + (iter > 1 ? 2 : 0);  // Skip whitespace after the first split
-  for (pos = *start; pos < length - 2; pos++) {
-    if (arg[pos] != '\\' && ascii_iswhite(arg[pos + 1])) {
-      *end = pos;
+  if (!arglen) {
     return true;
   }
+
+  size_t pos = *end;
+  while (pos < arglen && ascii_iswhite(arg[pos])) {
+    pos++;
   }
-  *end = length - 1;
+
+  size_t l = 0;
+  for (; pos < arglen - 1; pos++) {
+    if (arg[pos] == '\\' && (arg[pos + 1] == '\\' || ascii_iswhite(arg[pos + 1]))) {
+      buf[l++] = arg[++pos];
+    } else {
+      buf[l++] = arg[pos];
+      if (ascii_iswhite(arg[pos + 1])) {
+        *end = pos + 1;
+        *len = l;
         return false;
       }
+    }
+  }
+
+  if (pos < arglen && !ascii_iswhite(arg[pos])) {
+    buf[l++] = arg[pos];
+  }
+
+  *len = l;
+  return true;
+}
 
 /// split and quote args for <f-args>
 static char_u *uc_split_args(char_u *arg, size_t *lenp)

src/nvim/lua/executor.c

@@ -1869,18 +1869,22 @@ void nlua_do_ucmd(ucmd_T *cmd, exarg_T *eap)
   } else {
     // Commands with more than one possible argument we split
     lua_pop(lstate, 1);  // Pop the reference of opts.args
-    int length = (int)STRLEN(eap->arg);
-    int start = 0;
-    int end = 0;
+    size_t length = STRLEN(eap->arg);
+    size_t end = 0;
+    size_t len = 0;
     int i = 1;
-    bool res = true;
-    while (res) {
-      res = uc_split_args_iter(eap->arg, i, &start, &end, length);
-      lua_pushlstring(lstate, (const char *)eap->arg + start, (size_t)(end - start + 1));
+    char *buf = xcalloc(length, sizeof(char));
+    bool done = false;
+    while (!done) {
+      done = uc_split_args_iter(eap->arg, length, &end, buf, &len);
+      if (len > 0) {
+        lua_pushlstring(lstate, buf, len);
         lua_rawseti(lstate, -2, i);
         i++;
       }
     }
+    xfree(buf);
+  }
   lua_setfield(lstate, -2, "fargs");
 
   lua_pushstring(lstate, (const char *)&eap->regname);

test/functional/api/command_spec.lua

@@ -114,8 +114,8 @@ describe('nvim_create_user_command', function()
     ]]
 
     eq({
-      args = [[hello my\ friend how\ are\ you?]],
-      fargs = {[[hello]], [[my\ friend]], [[how\ are\ you?]]},
+      args = [[this is    a\ test]],
+      fargs = {"this", "is", "a test"},
       bang = false,
       line1 = 1,
       line2 = 1,
@@ -124,7 +124,37 @@ describe('nvim_create_user_command', function()
       count = 2,
       reg = "",
     }, exec_lua [=[
-      vim.api.nvim_command([[CommandWithLuaCallback hello my\ friend how\ are\ you?]])
+      vim.api.nvim_command([[CommandWithLuaCallback this is    a\ test]])
+      return result
+    ]=])
+
+    eq({
+      args = [[this   includes\ a backslash: \\]],
+      fargs = {"this", "includes a", "backslash:", "\\"},
+      bang = false,
+      line1 = 1,
+      line2 = 1,
+      mods = "",
+      range = 0,
+      count = 2,
+      reg = "",
+    }, exec_lua [=[
+      vim.api.nvim_command([[CommandWithLuaCallback this   includes\ a backslash: \\]])
+      return result
+    ]=])
+
+    eq({
+      args = "a\\b",
+      fargs = {"a\\b"},
+      bang = false,
+      line1 = 1,
+      line2 = 1,
+      mods = "",
+      range = 0,
+      count = 2,
+      reg = "",
+    }, exec_lua [=[
+      vim.api.nvim_command('CommandWithLuaCallback a\\b')
       return result
     ]=])
 
@@ -160,7 +190,7 @@ describe('nvim_create_user_command', function()
 
     eq({
       args = "",
-      fargs = {""},  -- fargs works without args
+      fargs = {},  -- fargs works without args
       bang = false,
       line1 = 1,
       line2 = 1,
@@ -186,8 +216,8 @@ describe('nvim_create_user_command', function()
     ]]
 
     eq({
-      args = "hello I'm one argmuent",
-      fargs = {"hello I'm one argmuent"},  -- Doesn't split args
+      args = "hello I'm one argument",
+      fargs = {"hello I'm one argument"},  -- Doesn't split args
       bang = false,
       line1 = 1,
       line2 = 1,
@@ -196,7 +226,7 @@ describe('nvim_create_user_command', function()
       count = 2,
       reg = "",
     }, exec_lua [[
-      vim.api.nvim_command('CommandWithOneArg hello I\'m one argmuent')
+      vim.api.nvim_command('CommandWithOneArg hello I\'m one argument')
       return result
     ]])