setpos(): Set lowercase mark in other buffers (#5753)

Also make setpos("'A", [999, 1, 1, 0]) fail, i.e. return -1 (assuming there is no buffer 999).

Fixes #5713

Background:

`:help setpos()` mentions an argument `"bufnum"` that determines the buffer a mark should be put in.
This argument is respected for uppercase marks, but not for lowercase marks.
This is reasonable (though I personally would like `setpos()` to be able to set marks in other buffers), but the help doesn't mention this anywhere.

It's also strange that attempting to change buffers with `setpos('.', [bufnr('#'), 1, 1, 0])` alerts the user that having a different buffer is an error, while attempting to set a mark with `setpos("'d", [bufnr('#'), 1, 1, 0])` doesn't tell the user that the `"bufnum"` argument is an error.

src/nvim/mark.c

@@ -130,9 +130,15 @@ int setmark_pos(int c, pos_T *pos, int fnum)
     return OK;
   }
 
+  buf_T *buf = buflist_findnr(fnum);
+  // Can't set a mark in a non-existant buffer.
+  if (buf == NULL) {
+    return FAIL;
+  }
+
   if (ASCII_ISLOWER(c)) {
     i = c - 'a';
-    RESET_FMARK(curbuf->b_namedm + i, *pos, curbuf->b_fnum);
+    RESET_FMARK(buf->b_namedm + i, *pos, fnum);
     return OK;
   }
   if (ASCII_ISUPPER(c) || ascii_isdigit(c)) {

test/functional/eval/setpos_spec.lua

@@ -0,0 +1,64 @@
+local helpers = require('test.functional.helpers')(after_each)
+local setpos = helpers.funcs.setpos
+local getpos = helpers.funcs.getpos
+local insert = helpers.insert
+local clear = helpers.clear
+local execute = helpers.execute
+local eval = helpers.eval
+local eq = helpers.eq
+local exc_exec = helpers.exc_exec
+
+
+describe('setpos() function', function()
+  before_each(function()
+    clear()
+    insert([[
+    First line of text
+    Second line of text
+    Third line of text]])
+    execute('new')
+    insert([[
+    Line of text 1
+    Line of text 2
+    Line of text 3]])
+  end)
+  it('can set the current cursor position', function()
+    setpos(".", {0, 2, 1, 0})
+    eq(getpos("."), {0, 2, 1, 0})
+    setpos(".", {2, 1, 1, 0})
+    eq(getpos("."), {0, 1, 1, 0})
+    -- Ensure get an error attempting to set position to another buffer
+    local ret = exc_exec('call setpos(".", [1, 1, 1, 0])')
+    eq('Vim(call):E474: Invalid argument', ret)
+  end)
+  it('can set lowercase marks in the current buffer', function()
+    setpos("'d", {0, 2, 1, 0})
+    eq(getpos("'d"), {0, 2, 1, 0})
+    execute('undo', 'call setpos("\'d", [2, 3, 1, 0])')
+    eq(getpos("'d"), {0, 3, 1, 0})
+  end)
+  it('can set lowercase marks in other buffers', function()
+    local retval = setpos("'d", {1, 2, 1, 0})
+    eq(0, retval)
+    setpos("'d", {1, 2, 1, 0})
+    eq(getpos("'d"), {0, 0, 0, 0})
+    execute('wincmd w')
+    eq(eval('bufnr("%")'), 1)
+    eq(getpos("'d"), {0, 2, 1, 0})
+  end)
+  it("fails when setting a mark in a buffer that doesn't exist", function()
+    local retval = setpos("'d", {3, 2, 1, 0})
+    eq(-1, retval)
+    eq(getpos("'d"), {0, 0, 0, 0})
+    retval = setpos("'D", {3, 2, 1, 0})
+    eq(-1, retval)
+    eq(getpos("'D"), {0, 0, 0, 0})
+  end)
+  it('can set uppercase marks', function()
+    setpos("'D", {2, 2, 3, 0})
+    eq(getpos("'D"), {2, 2, 3, 0})
+    -- Can set a mark in another buffer
+    setpos("'D", {1, 2, 2, 0})
+    eq(getpos("'D"), {1, 2, 2, 0})
+  end)
+end)