fix(message): concatenate multi-chunk nvim_echo({err}) for exception message #38131

Problem:  Exception error message only prints the first chunk of a
          multi-chunk nvim_echo() message.
Solution: Concatenate consecutive message chunks in the exception
          message list.

src/nvim/ex_eval.c

@@ -155,7 +155,7 @@ bool aborted_in_try(void)
 /// When several messages appear in the same command, the first is usually the
 /// most specific one and used as the exception value.  The "severe" flag can be
 /// set to true, if a later but severer message should be used instead.
-bool cause_errthrow(const char *mesg, bool multiline, bool severe, bool *ignore)
+bool cause_errthrow(const char *mesg, bool multiline, bool concat, bool severe, bool *ignore)
   FUNC_ATTR_NONNULL_ALL
 {
   msglist_T *elem;
@@ -241,6 +241,12 @@ bool cause_errthrow(const char *mesg, bool multiline, bool severe, bool *ignore)
     if (msg_list != NULL) {
       msglist_T **plist = msg_list;
       while (*plist != NULL) {
+        // Concatenate (a multihl message) instead.
+        if ((*plist)->next == NULL && concat) {
+          (*plist)->msg = xrealloc((*plist)->msg, strlen((*plist)->msg) + strlen(mesg) + 1);
+          (*plist)->throw_msg = strcat((*plist)->msg, mesg);
+          return true;
+        }
         plist = &(*plist)->next;
       }
 

src/nvim/message.c

@@ -292,8 +292,8 @@ void msg_multiline(String str, int hl_id, bool check_int, bool hist, bool *need_
   }
 }
 
-// Avoid starting a new message for each chunk and adding message to history in msg_keep().
+// Ensure a single msg_show event, msg_list and history entry for entire multihl message.
-static bool is_multihl = false;
+static int is_multihl = 0;
 
 /// Format a progress message, adding title and percent if given.
 ///
@@ -360,7 +360,6 @@ MsgID msg_multihl(MsgID id, HlMessage hl_msg, const char *kind, bool history, bo
   if (kind != NULL) {
     msg_ext_set_kind(kind);
   }
-  is_multihl = true;
   msg_ext_skip_flush = true;
 
   // provide a new id if not given
@@ -384,6 +383,7 @@ MsgID msg_multihl(MsgID id, HlMessage hl_msg, const char *kind, bool history, bo
 
   for (uint32_t i = 0; i < kv_size(hl_msg); i++) {
     HlMessageChunk chunk = kv_A(hl_msg, i);
+    is_multihl++;
     if (err) {
       emsg_multiline(chunk.text.data, kind, chunk.hl_id, true);
     } else {
@@ -397,7 +397,7 @@ MsgID msg_multihl(MsgID id, HlMessage hl_msg, const char *kind, bool history, bo
   }
 
   msg_ext_skip_flush = false;
-  is_multihl = false;
+  is_multihl = 0;
   no_wait_return--;
   msg_end();
 
@@ -776,7 +776,7 @@ bool emsg_multiline(const char *s, const char *kind, int hl_id, bool multiline)
     // be found, the message will be displayed later on.)  "ignore" is set
     // when the message should be ignored completely (used for the
     // interrupt message).
-    if (cause_errthrow(s, multiline, severe, &ignore)) {
+    if (cause_errthrow(s, multiline, is_multihl > 1, severe, &ignore)) {
       if (!ignore) {
         did_emsg++;
       }

test/functional/api/vim_spec.lua

@@ -445,6 +445,13 @@ describe('API', function()
       api.nvim_exec2('hi VisualNC', { output = true })
       eq('VisualNC       xxx cleared', api.nvim_exec2('hi VisualNC', { output = true }).output)
     end)
+
+    it('captures multi-chunk err nvim_echo() #36883', function()
+      eq(
+        'nvim_exec2(), line 1: Vim(call):abc',
+        pcall_err(request, 'nvim_exec2', 'call nvim_echo([["a"],["b"],["c"] ], 0, #{err:1})', {})
+      )
+    end)
   end)
 
   describe('nvim_command', function()