patch 9.1.0999: Vim9: leaking finished exception

Problem:  leaking finished exception
          (after v9.1.0984)
Solution: use finish_exception to clean up caught exceptions
          (Yee Cheng Chin)

In Vimscript, v:exception/throwpoint/stacktrace are supposed to reflect
the currently caught exception, and be popped after the exception is
finished (via endtry, finally, or a thrown exception inside catch).
Vim9script does not handle this properly, and leaks them instead. This
is clearly visible when launching GVim with menu enabled.  A caught
exception inside the s:BMShow() in menu.vim would show up when querying
`v:stacktrace` even though the exception was already caught and handled.

To fix this, just use the same functionality as Vimscript by calling
`finish_exception` to properly restore the states. Note that this
assumes `current_exception` is always the same as `caught_stack` which
believe should be the case.

Added tests for this. Also fix up test_stacktrace to properly test the
stack restore behavior where we have nested exceptions in catch blocks
and to also test the vim9script functionality properly.

- Also, remove its dependency on explicitly checking a line number in
  runtest.vim which is a very fragile way to write tests as any minor
  change in runtest.vim (shared among all tests) would require changing
  test_stacktrace.vim. We don't actually need such granularity in the
  test.

closes: #16413

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/vim9execute.c b/src/vim9execute.c
index d696280..c7f0e67 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -3281,7 +3281,15 @@
 		trycmd_T *trycmd = ((trycmd_T *)trystack->ga_data)
 							+ trystack->ga_len - 1;
 		if (trycmd->tcd_frame_idx == ectx->ec_frame_idx)
-		    trycmd->tcd_caught = FALSE;
+		{
+		    if (trycmd->tcd_caught)
+		    {
+			// Inside a "catch" we need to first discard the caught
+			// exception.
+			finish_exception(caught_stack);
+			trycmd->tcd_caught = FALSE;
+		    }
+		}
 	    }
 	}
 
@@ -4972,6 +4980,12 @@
 		    // Reset the index to avoid a return statement jumps here
 		    // again.
 		    trycmd->tcd_finally_idx = 0;
+		    if (trycmd->tcd_caught)
+		    {
+			// discard the exception
+			finish_exception(caught_stack);
+			trycmd->tcd_caught = FALSE;
+		    }
 		    break;
 		}
 
@@ -4986,12 +5000,10 @@
 		    trycmd = ((trycmd_T *)trystack->ga_data) + trystack->ga_len;
 		    if (trycmd->tcd_did_throw)
 			did_throw = TRUE;
-		    if (trycmd->tcd_caught && current_exception != NULL)
+		    if (trycmd->tcd_caught)
 		    {
 			// discard the exception
-			if (caught_stack == current_exception)
-			    caught_stack = caught_stack->caught;
-			discard_current_exception();
+			finish_exception(caught_stack);
 		    }
 
 		    if (trycmd->tcd_return)
@@ -5040,12 +5052,10 @@
 		    {
 			trycmd_T    *trycmd = ((trycmd_T *)trystack->ga_data)
 							+ trystack->ga_len - 1;
-			if (trycmd->tcd_caught && current_exception != NULL)
+			if (trycmd->tcd_caught)
 			{
 			    // discard the exception
-			    if (caught_stack == current_exception)
-				caught_stack = caught_stack->caught;
-			    discard_current_exception();
+			    finish_exception(caught_stack);
 			    trycmd->tcd_caught = FALSE;
 			}
 		    }