2 files changed, 84 insertions, 1 deletions

diff --git a/src/nvim/api/private/helpers.c b/src/nvim/api/private/helpers.c
index 1ed2bc013e..e736e29e2d 100644
--- a/src/nvim/api/private/helpers.c
+++ b/src/nvim/api/private/helpers.c
@@ -37,7 +37,72 @@ typedef struct {
 # include "api/private/ui_events_metadata.generated.h"
 #endif
 
+/// Start block that may cause VimL exceptions while evaluating another code
+///
+/// Used when caller is supposed to be operating when other VimL code is being
+/// processed and that “other VimL code” must not be affected.
+///
+/// @param[out]  tstate  Location where try state should be saved.
+void try_enter(TryState *const tstate)
+{
+  *tstate = (TryState) {
+    .current_exception = current_exception,
+    .msg_list = (const struct msglist *const *)msg_list,
+    .private_msg_list = NULL,
+    .trylevel = trylevel,
+    .got_int = got_int,
+    .did_throw = did_throw,
+    .need_rethrow = need_rethrow,
+    .did_emsg = did_emsg,
+  };
+  msg_list = &tstate->private_msg_list;
+  current_exception = NULL;
+  trylevel = 1;
+  got_int = false;
+  did_throw = false;
+  need_rethrow = false;
+  did_emsg = false;
+}
+
+/// End try block, set the error message if any and restore previous state
+///
+/// @warning Return is consistent with most functions (false on error), not with
+///          try_end (true on error).
+///
+/// @param[in]  tstate  Previous state to restore.
+/// @param[out]  err  Location where error should be saved.
+///
+/// @return false if error occurred, true otherwise.
+bool try_leave(const TryState *const tstate, Error *const err)
+  FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
+{
+  const bool ret = !try_end(err);
+  assert(trylevel == 0);
+  assert(!need_rethrow);
+  assert(!got_int);
+  assert(!did_throw);
+  assert(!did_emsg);
+  assert(msg_list == &tstate->private_msg_list);
+  assert(*msg_list == NULL);
+  assert(current_exception == NULL);
+  msg_list = (struct msglist **)tstate->msg_list;
+  current_exception = tstate->current_exception;
+  trylevel = tstate->trylevel;
+  got_int = tstate->got_int;
+  did_throw = tstate->did_throw;
+  need_rethrow = tstate->need_rethrow;
+  did_emsg = tstate->did_emsg;
+  return ret;
+}
+
 /// Start block that may cause vimscript exceptions
+///
+/// Each try_start() call should be mirrored by try_end() call.
+///
+/// To be used as a replacement of `:try … catch … endtry` in C code, in cases
+/// when error flag could not already be set. If there may be pending error
+/// state at the time try_start() is executed which needs to be preserved,
+/// try_enter()/try_leave() pair should be used instead.
 void try_start(void)
 {
   ++trylevel;
@@ -50,7 +115,9 @@ void try_start(void)
 /// @return true if an error occurred
 bool try_end(Error *err)
 {
-  --trylevel;
+  // Note: all globals manipulated here should be saved/restored in
+  // try_enter/try_leave.
+  trylevel--;
 
   // Without this it stops processing all subsequent VimL commands and
   // generates strange error messages if I e.g. try calling Test() in a
diff --git a/src/nvim/api/private/helpers.h b/src/nvim/api/private/helpers.h
index 159b9d5c2a..87f334ac30 100644
--- a/src/nvim/api/private/helpers.h
+++ b/src/nvim/api/private/helpers.h
@@ -6,6 +6,7 @@
 #include "nvim/api/private/defs.h"
 #include "nvim/vim.h"
 #include "nvim/memory.h"
+#include "nvim/ex_eval.h"
 #include "nvim/lib/kvec.h"
 
 #define OBJECT_OBJ(o) o
@@ -82,6 +83,21 @@
 #define api_free_window(value)
 #define api_free_tabpage(value)
 
+/// Structure used for saving state for :try
+///
+/// Used when caller is supposed to be operating when other VimL code is being
+/// processed and that “other VimL code” must not be affected.
+typedef struct {
+  except_T *current_exception;
+  struct msglist *private_msg_list;
+  const struct msglist *const *msg_list;
+  int trylevel;
+  int got_int;
+  int did_throw;
+  int need_rethrow;
+  int did_emsg;
+} TryState;
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "api/private/helpers.h.generated.h"
 #endif