fix(path): check return value of append_path() (#28309)

If the filename passed to vim_FullName() is a relative directory, and
does not exist, it is appended to the current working directory. Since
the return value of append_path() was ignored, and if the buffer length
was too small to fit getcwd() + dirname(filename), it would still try to
append the basename(filename).

This was manifesting as a failure in test/unit/path_spec.lua in:
    itp('fails and uses filename if given filename contains non-existing directory', ..

This failure occurs when running the tests from directory with a short
path such as: /work/src/nv

    test/unit/path_spec.lua:420: Expected objects to be the same.
    Passed in:
    (string) '/work/src/nv/test.file'
    Expected:
    (string) 'non_existing_dir/test.file'

This return value for the second call to append_path() to append
basename(filename) was checked, and this is where it would fail for
normal / longer getcwd()s.

1 files changed, 9 insertions, 6 deletions

diff --git a/test/unit/path_spec.lua b/test/unit/path_spec.lua
index 8df6a52cd1..44919368bf 100644
--- a/test/unit/path_spec.lua
+++ b/test/unit/path_spec.lua
@@ -413,12 +413,15 @@ describe('path.c', function()
     end)
 
     itp('fails and uses filename if given filename contains non-existing directory', function()
-      local filename = 'non_existing_dir/test.file'
-      local buflen = string.len(filename) + 1
-      local do_expand = 1
-      local buf, result = vim_FullName(filename, buflen, do_expand)
-      eq(filename, ffi.string(buf))
-      eq(FAIL, result)
+      -- test with different filename lengths
+      for rep = 1, 10 do
+        local filename = ('non_existing_'):rep(rep) .. 'dir/test.file'
+        local buflen = string.len(filename) + 1
+        local do_expand = 1
+        local buf, result = vim_FullName(filename, buflen, do_expand)
+        eq(filename, ffi.string(buf))
+        eq(FAIL, result)
+      end
     end)
 
     itp('concatenates filename if it does not contain a slash', function()