API: return non-generic VimL errors

- Return VimL errors instead of generic errors for:
  - nvim_call_function
  - nvim_call_dict_function
- Fix tests which were silently broken before this change.

This violates #6150 where we agreed not to translate API errors.  But
that can be fixed later.

4 files changed, 44 insertions, 23 deletions

diff --git a/src/nvim/api/vim.c b/src/nvim/api/vim.c
index 5b9ae141b9..2d1e2bbe94 100644
--- a/src/nvim/api/vim.c
+++ b/src/nvim/api/vim.c
@@ -46,8 +46,7 @@
 
 /// Executes an ex-command.
 ///
-/// On parse error: forwards the Vim error; does not update v:errmsg.
-/// On runtime error: forwards the Vim error; does not update v:errmsg.
+/// On execution error: fails with VimL error, does not update v:errmsg.
 ///
 /// @param command  Ex-command string
 /// @param[out] err Error details (Vim error), if any
@@ -103,7 +102,8 @@ Dictionary nvim_get_hl_by_id(Integer hl_id, Boolean rgb, Error *err)
 }
 
 /// Passes input keys to Nvim.
-/// On VimL error: Does not fail, but updates v:errmsg.
+///
+/// On execution error: does not fail, but updates v:errmsg.
 ///
 /// @param keys         to be typed
 /// @param mode         mapping options
@@ -169,7 +169,8 @@ void nvim_feedkeys(String keys, String mode, Boolean escape_csi)
 }
 
 /// Passes keys to Nvim as raw user-input.
-/// On VimL error: Does not fail, but updates v:errmsg.
+///
+/// On execution error: does not fail, but updates v:errmsg.
 ///
 /// Unlike `nvim_feedkeys`, this uses a lower-level input buffer and the call
 /// is not deferred. This is the most reliable way to send real user input.
@@ -213,8 +214,7 @@ String nvim_replace_termcodes(String str, Boolean from_part, Boolean do_lt,
 /// Executes an ex-command and returns its (non-error) output.
 /// Shell |:!| output is not captured.
 ///
-/// On parse error: forwards the Vim error; does not update v:errmsg.
-/// On runtime error: forwards the Vim error; does not update v:errmsg.
+/// On execution error: fails with VimL error, does not update v:errmsg.
 ///
 /// @param command  Ex-command string
 /// @param[out] err Error details (Vim error), if any
@@ -259,7 +259,8 @@ theend:
 
 /// Evaluates a VimL expression (:help expression).
 /// Dictionaries and Lists are recursively expanded.
-/// On VimL error: Returns a generic error; v:errmsg is not updated.
+///
+/// On execution error: fails with generic error; v:errmsg is not updated.
 ///
 /// @param expr     VimL expression string
 /// @param[out] err Error details, if any
@@ -331,18 +332,23 @@ static Object _call_function(String fn, Array args, dict_T *self, Error *err)
   }
 
   try_start();
-  // Call the function
+  msg_first_ignored_err = NULL;
+
   typval_T rettv;
   int dummy;
   int r = call_func((char_u *)fn.data, (int)fn.size, &rettv, (int)args.size,
                     vim_args, NULL, curwin->w_cursor.lnum,
                     curwin->w_cursor.lnum, &dummy, true, NULL, self);
-  if (r == FAIL) {
-    api_set_error(err, kErrorTypeException, "Error calling function");
+  // call_func() retval is deceptive; must also check did_emsg et al.
+  if (msg_first_ignored_err
+      && (r == FAIL || (did_emsg && force_abort && !current_exception))) {
+    api_set_error(err, kErrorTypeException, msg_first_ignored_err);
   }
   if (!try_end(err)) {
     rv = vim_to_object(&rettv);
   }
+  xfree(msg_first_ignored_err);
+  msg_first_ignored_err = NULL;
   tv_clear(&rettv);
 
 free_vim_args:
@@ -355,7 +361,7 @@ free_vim_args:
 
 /// Calls a VimL function with the given arguments.
 ///
-/// On VimL error: Returns a generic error; v:errmsg is not updated.
+/// On execution error: fails with VimL error, does not update v:errmsg.
 ///
 /// @param fn       Function to call
 /// @param args     Function arguments packed in an Array
@@ -369,6 +375,8 @@ Object nvim_call_function(String fn, Array args, Error *err)
 
 /// Calls a VimL |Dictionary-function| with the given arguments.
 ///
+/// On execution error: fails with VimL error, does not update v:errmsg.
+///
 /// @param dict Dictionary, or String evaluating to a VimL |self| dict
 /// @param fn Name of the function defined on the VimL dict
 /// @param args Function arguments packed in an Array
diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index cc29496968..3c408a4381 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -6241,20 +6241,21 @@ bool set_ref_in_func(char_u *name, ufunc_T *fp_in, int copyID)
 /// invoked function uses them. It is called like this:
 ///   new_argcount = argv_func(current_argcount, argv, called_func_argcount)
 ///
-/// Return FAIL when the function can't be called,  OK otherwise.
-/// Also returns OK when an error was encountered while executing the function.
+/// @return FAIL if function cannot be called, else OK (even if an error
+///         occurred while executing the function! Use `msg_first_ignored_err`
+///         to get the error)
 int
 call_func(
     const char_u *funcname,         // name of the function
     int len,                        // length of "name"
-    typval_T *rettv,                // return value goes here
+    typval_T *rettv,                // [out] value goes here
     int argcount_in,                // number of "argvars"
     typval_T *argvars_in,           // vars for arguments, must have "argcount"
                                     // PLUS ONE elements!
     ArgvFunc argv_func,             // function to fill in argvars
     linenr_T firstline,             // first line of range
     linenr_T lastline,              // last line of range
-    int *doesrange,                 // return: function handled range
+    int *doesrange,                 // [out] function handled range
     bool evaluate,
     partial_T *partial,             // optional, can be NULL
     dict_T *selfdict_in             // Dictionary for "self"
@@ -6428,21 +6429,25 @@ call_func(
   return ret;
 }
 
-/*
- * Give an error message with a function name.  Handle <SNR> things.
- * "ermsg" is to be passed without translation, use N_() instead of _().
- */
+/// Give an error message with a function name.  Handle <SNR> things.
+///
+/// @param ermsg must be passed without translation (use N_() instead of _()).
+/// @param name function name
 static void emsg_funcname(char *ermsg, char_u *name)
 {
-  char_u      *p;
+  char_u *p;
 
-  if (*name == K_SPECIAL)
+  if (*name == K_SPECIAL) {
     p = concat_str((char_u *)"<SNR>", name + 3);
-  else
+  } else {
     p = name;
+  }
+
   EMSG2(_(ermsg), p);
-  if (p != name)
+
+  if (p != name) {
     xfree(p);
+  }
 }
 
 /*
diff --git a/src/nvim/message.c b/src/nvim/message.c
index 7ca82c2878..bcfff90d7d 100644
--- a/src/nvim/message.c
+++ b/src/nvim/message.c
@@ -67,6 +67,7 @@ static char_u   *confirm_msg_tail;              /* tail of confirm_msg */
 
 MessageHistoryEntry *first_msg_hist = NULL;
 MessageHistoryEntry *last_msg_hist = NULL;
+char *msg_first_ignored_err = NULL;
 static int msg_hist_len = 0;
 
 static FILE *verbose_fd = NULL;
@@ -504,6 +505,9 @@ int emsg(const char_u *s_)
     if (cause_errthrow((char_u *)s, severe, &ignore) == true) {
       if (!ignore) {
         did_emsg = true;
+        if (msg_first_ignored_err == NULL) {
+          msg_first_ignored_err = xstrdup(s);
+        }
       }
       return true;
     }
diff --git a/src/nvim/message.h b/src/nvim/message.h
index 82935a36a9..eede0ed9b1 100644
--- a/src/nvim/message.h
+++ b/src/nvim/message.h
@@ -85,6 +85,10 @@ extern MessageHistoryEntry *first_msg_hist;
 /// Last message
 extern MessageHistoryEntry *last_msg_hist;
 
+/// Abort-causing non-exception error ignored by emsg(), needed by callers
+/// (RPC API) of call_func() to get error details when messages are disabled.
+extern char *msg_first_ignored_err;
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "message.h.generated.h"
 #endif