fix(startup): crash in read_stdin #35699

Problem:
Crash on startup in some situations due to call to `set_curbuf`
with the current value of `cur_buf`.

Solution:
Switch buffers before doing the wipeout of the stdin buffer.
Use cmdline cmds instead of raw buffer pointers to avoid lifetime issues.

src/nvim/main.c

@@ -1625,39 +1625,49 @@ static void read_stdin(void)
   swap_exists_action = SEA_DIALOG;
   no_wait_return = true;
   bool save_msg_didany = msg_didany;
-  buf_T *prev_buf = NULL;
 
   if (curbuf->b_ffname) {
     // curbuf is already opened for a file, create a new buffer for stdin. #35269
-    buf_T *newbuf = buflist_new(NULL, NULL, 0, BLN_LISTED);
+    buf_T *stdin_buf = buflist_new(NULL, NULL, 0, BLN_LISTED);
-    if (newbuf == NULL) {
+    if (stdin_buf == NULL) {
       semsg("Failed to create buffer for stdin");
       return;
     }
 
-    // remember the current buffer so we can go back to it
+    // remember the current buffer number so we can go back to it
-    prev_buf = curbuf;
+    handle_T initial_buf_handle = curbuf->handle;
-    set_curbuf(newbuf, 0, false);
+
+    // set the buffer we just created as curbuf so we can read stdin into it
+    set_curbuf(stdin_buf, 0, false);
     readfile(NULL, NULL, 0, 0, (linenr_T)MAXLNUM, NULL, READ_NEW + READ_STDIN, true);
+
+    // remember stdin_buf_handle so we can close it if stdin_buf ends up empty
+    handle_T stdin_buf_handle = stdin_buf->handle;
+    bool stdin_buf_empty = buf_is_empty(curbuf);
+
+    // switch back to the original starting buffer
+    char buf[100];
+    vim_snprintf(buf, sizeof(buf), "silent! buffer %d", initial_buf_handle);
+    do_cmdline_cmd(buf);
+
+    if (stdin_buf_empty) {
+      // stdin buffer may be first or last ("echo foo | nvim file1 -"). #35269
+      // only wipe buffer after having switched to original starting buffer. #35681
+      vim_snprintf(buf, sizeof(buf), "silent! bwipeout! %d", stdin_buf_handle);
+      do_cmdline_cmd(buf);
+    }
   } else {
+    // stdin buffer is first so we can just use curbuf
     set_buflisted(true);
     // Create memfile and read from stdin.
     open_buffer(true, NULL, 0);
-  }
-
-  if (buf_is_empty(curbuf)) {
     // stdin was empty so we should wipe it (e.g. "echo file1 | xargs nvim"). #8561
-    // stdin buffer may be first or last ("echo foo | nvim file1 -"). #35269
+    if (buf_is_empty(curbuf) && curbuf->b_next != NULL) {
-    if ((curbuf->b_next != NULL) || (curbuf->b_prev != NULL)) {
+      do_cmdline_cmd("silent! bnext");
-      do_bufdel(DOBUF_WIPE, NULL, 0, 0, 0, 1);
+      do_cmdline_cmd("silent! bwipeout 1");
     }
   }
 
-  // we had to switch buffers to load stdin, switch back
-  if (prev_buf) {
-    set_curbuf(prev_buf, 0, false);
-  }
-
   no_wait_return = false;
   msg_didany = save_msg_didany;
   TIME_MSG("reading stdin");

test/functional/core/startup_spec.lua

@@ -536,7 +536,7 @@ describe('startup', function()
 
   it('if stdin is empty and - is last: selects buffer 1, deletes buffer 3 #35269', function()
     eq(
-      '\r\n  1 %a   "file1"                        line 0\r\n  2 #h   "file2"                        line 1',
+      '\r\n  1 %a   "file1"                        line 0\r\n  2      "file2"                        line 0',
       fn.system({
         nvim_prog,
         '-n',
@@ -554,6 +554,30 @@ describe('startup', function()
     )
   end)
 
+  it("empty stdin with terminal split doesn't crash #35681", function()
+    eq(
+      'nocrash',
+      fn.system({
+        nvim_prog,
+        '-n',
+        '-u',
+        'NONE',
+        '-i',
+        'NONE',
+        '--headless',
+        '--cmd',
+        'term',
+        '+split',
+        '+quit!',
+        '+bw!',
+        '+bw!',
+        '+echo "nocrash"',
+        "+call timer_start(1, { -> execute('qa') })", -- need to let event handling happen
+        '-',
+      }, { '' })
+    )
+  end)
+
   it('stdin with -es/-Es #7679', function()
     local input = { 'append', 'line1', 'line2', '.', '%print', '' }
     local inputstr = table.concat(input, '\n')