vim-patch:9.1.0529: silent! causes following try/catch to not work (#29567)

Problem:  silent! causes following try/catch to not work
          (Malcolm Rowe)
Solution: consider emsg_silent in handle_did_throw() and do not abort
          evaluation flow for :silent! (LemonBoy)

The silent! flag causes the evaluation not to be aborted in case of
uncaught exceptions, adjust handle_did_throw to take this detail into
account.

Fixes the long-standing todo.txt item:
```
Problem that a previous silent ":throw" causes a following try/catch not
to work. (ZyX, 2013 Sep 28) With examples: (Malcolm Rowe, 2015 Dec 24)
Also see vim/vim#8487 for an example.
```

fixes: vim/vim#538
closes: vim/vim#15128

749ba0f6d9

Cherry-pick Test_deeply_nested_source() from patch 8.2.5169.

Co-authored-by: LemonBoy <thatlemon@gmail.com>

src/nvim/ex_docmd.c

@@ -972,8 +972,13 @@ void handle_did_throw(void)
   current_exception->throw_name = NULL;
 
   discard_current_exception();              // uses IObuff if 'verbose'
+
+  // If "silent!" is active the uncaught exception is not fatal.
+  if (emsg_silent == 0) {
     suppress_errthrow = true;
     force_abort = true;
+  }
+
   msg_ext_set_kind("emsg");  // kind=emsg for :throw, exceptions. #9993
 
   if (messages != NULL) {

test/old/testdir/test_vimscript.vim

@@ -7437,6 +7437,49 @@ func Test_for_over_string()
   call assert_equal('', res)
 endfunc
 
+" Test for deeply nested :source command  {{{1
+func Test_deeply_nested_source()
+  throw 'Skipped: Vim9 script is N/A'
+  let lines =<< trim END
+
+      so
+      sil 0scr
+      delete
+      so
+      0
+  END
+  call writefile(["vim9 silent! @0 \n/"] + lines, 'Xnested.vim', 'D')
+
+  " this must not crash
+  let cmd = GetVimCommand() .. " -e -s -S Xnested.vim -c qa!"
+  call system(cmd)
+endfunc
+
+func Test_exception_silent()
+  XpathINIT
+  let lines =<< trim END
+  func Throw()
+    Xpath 'a'
+    throw "Uncaught"
+    " This line is not executed.
+    Xpath 'b'
+  endfunc
+  " The exception is suppressed due to the presence of silent!.
+  silent! call Throw()
+  try
+    call DoesNotExist()
+  catch /E117:/
+    Xpath 'c'
+  endtry
+  Xpath 'd'
+  END
+  let verify =<< trim END
+    call assert_equal('acd', g:Xpath)
+  END
+
+  call RunInNewVim(lines, verify)
+endfunc
+
 "-------------------------------------------------------------------------------
 " Modelines								    {{{1
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker