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`.

2 files changed, 48 insertions, 26 deletions

diff --git a/src/nvim/ex_docmd.c b/src/nvim/ex_docmd.c
index 482bf69f67..cbfe6e3789 100644
--- a/src/nvim/ex_docmd.c
+++ b/src/nvim/ex_docmd.c
@@ -5774,26 +5774,44 @@ 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]  arg String to split
+/// @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)
-{
-  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;
-      return true;
+/// @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)
+{
+  if (!arglen) {
+    return true;
+  }
+
+  size_t pos = *end;
+  while (pos < arglen && ascii_iswhite(arg[pos])) {
+    pos++;
+  }
+
+  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;
+      }
     }
   }
-  *end = length - 1;
-  return false;
+
+  if (pos < arglen && !ascii_iswhite(arg[pos])) {
+    buf[l++] = arg[pos];
+  }
+
+  *len = l;
+  return true;
 }
 
 /// split and quote args for <f-args>
diff --git a/src/nvim/lua/executor.c b/src/nvim/lua/executor.c
index 21625854fb..81396f1715 100644
--- a/src/nvim/lua/executor.c
+++ b/src/nvim/lua/executor.c
@@ -1869,17 +1869,21 @@ 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));
-      lua_rawseti(lstate, -2, i);
-      i++;
+    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");