fix(fs): make vim.fs.root work for relative paths and unnamed buffers (#28964)

If a buffer does not have a backing file then fall back to the current
working directory.

runtime/doc/lua.txt

@@ -3012,7 +3012,10 @@ vim.fs.parents({start})                                     *vim.fs.parents()*
 
 vim.fs.root({source}, {marker})                                *vim.fs.root()*
     Find the first parent directory containing a specific "marker", relative
-    to a buffer's directory.
+    to a file path or buffer.
+
+    If the buffer is unnamed (has no backing file) or has a non-empty
+    'buftype' then the search begins from Nvim's |current-directory|.
 
     Example: >lua
         -- Find the root of a Python project, starting from file 'main.py'
@@ -3029,7 +3032,8 @@ vim.fs.root({source}, {marker})                                *vim.fs.root()*
 
     Parameters: ~
       • {source}  (`integer|string`) Buffer number (0 for current buffer) or
-                  file path to begin the search from.
+                  file path (absolute or relative to the |current-directory|)
+                  to begin the search from.
       • {marker}  (`string|string[]|fun(name: string, path: string): boolean`)
                   A marker, or list of markers, to search for. If a function,
                   the function is called for each evaluated item and should

runtime/lua/vim/fs.lua

@@ -328,8 +328,11 @@ function M.find(names, opts)
   return matches
 end
 
---- Find the first parent directory containing a specific "marker", relative to a buffer's
---- directory.
+--- Find the first parent directory containing a specific "marker", relative to a file path or
+--- buffer.
+---
+--- If the buffer is unnamed (has no backing file) or has a non-empty 'buftype' then the search
+--- begins from Nvim's |current-directory|.
 ---
 --- Example:
 ---
@@ -346,8 +349,8 @@ end
 --- end)
 --- ```
 ---
---- @param source integer|string Buffer number (0 for current buffer) or file path to begin the
----               search from.
+--- @param source integer|string Buffer number (0 for current buffer) or file path (absolute or
+---               relative to the |current-directory|) to begin the search from.
 --- @param marker (string|string[]|fun(name: string, path: string): boolean) A marker, or list
 ---               of markers, to search for. If a function, the function is called for each
 ---               evaluated item and should return true if {name} and {path} are a match.
@@ -361,14 +364,18 @@ function M.root(source, marker)
   if type(source) == 'string' then
     path = source
   elseif type(source) == 'number' then
+    if vim.bo[source].buftype ~= '' then
+      path = assert(vim.uv.cwd())
+    else
       path = vim.api.nvim_buf_get_name(source)
+    end
   else
     error('invalid type for argument "source": expected string or buffer number')
   end
 
   local paths = M.find(marker, {
     upward = true,
-    path = path,
+    path = vim.fn.fnamemodify(path, ':p:h'),
   })
 
   if #paths == 0 then

test/functional/lua/fs_spec.lua

@@ -310,6 +310,25 @@ describe('vim.fs', function()
     it('works with a filename argument', function()
       eq(test_source_path, exec_lua([[return vim.fs.root(..., '.git')]], nvim_prog))
     end)
+
+    it('works with a relative path', function()
+      eq(
+        test_source_path,
+        exec_lua([[return vim.fs.root(..., '.git')]], vim.fs.basename(nvim_prog))
+      )
+    end)
+
+    it('uses cwd for unnamed buffers', function()
+      command('new')
+      eq(test_source_path, exec_lua([[return vim.fs.root(0, '.git')]]))
+    end)
+
+    it("uses cwd for buffers with non-empty 'buftype'", function()
+      command('new')
+      command('set buftype=nofile')
+      command('file lua://')
+      eq(test_source_path, exec_lua([[return vim.fs.root(0, '.git')]]))
+    end)
   end)
 
   describe('joinpath()', function()