Fix FindCompileCommands to handle absolute paths in c_build_dir_names

Use ale#path#GetAbsPath to resolve build directory paths, which correctly
handles both absolute and relative paths. When the dirname is absolute,
derive the project root from the parent of the build directory.

Co-authored-by: w0rp <3518142+w0rp@users.noreply.github.com>

autoload/ale/c.vim

@@ -249,11 +249,18 @@ function! ale#c#FindCompileCommands(buffer) abort
     " Search in build directories if we can't find it in the project.
     for l:path in ale#path#Upwards(expand('#' . a:buffer . ':p:h'))
         for l:dirname in ale#Var(a:buffer, 'c_build_dir_names')
-            let l:c_build_dir = l:path . s:sep . l:dirname
+            let l:c_build_dir = ale#path#GetAbsPath(l:path, l:dirname)
             let l:json_file = l:c_build_dir . s:sep . 'compile_commands.json'
 
             if filereadable(l:json_file)
-                return [l:path, l:json_file]
+                " Use the parent of the build dir for absolute
+                " paths, otherwise use the path found by searching
+                " upwards from the file.
+                let l:root = ale#path#IsAbsolute(l:dirname)
+                \   ? fnamemodify(l:c_build_dir, ':h')
+                \   : l:path
+
+                return [l:root, l:json_file]
             endif
         endfor
     endfor

test/test_c_find_compile_commands.vader

@@ -0,0 +1,33 @@
+Before:
+  Save g:ale_c_build_dir_names
+
+  call ale#test#SetDirectory('/testplugin/test')
+
+After:
+  Restore
+
+  call ale#test#RestoreDirectory()
+
+Execute(FindCompileCommands should find compile_commands.json with relative build dir names):
+  call ale#test#SetFilename('test-files/c/json_project/subdir/dummy')
+
+  let g:ale_c_build_dir_names = ['build']
+
+  AssertEqual
+  \ [
+  \   ale#path#Simplify(g:dir . '/test-files/c/json_project'),
+  \   ale#path#Simplify(g:dir . '/test-files/c/json_project/build/compile_commands.json'),
+  \ ],
+  \ ale#c#FindCompileCommands(bufnr(''))
+
+Execute(FindCompileCommands should find compile_commands.json with absolute build dir names):
+  call ale#test#SetFilename('test-files/c/json_project/subdir/dummy')
+
+  let g:ale_c_build_dir_names = [ale#path#Simplify(g:dir . '/test-files/c/json_project/build')]
+
+  AssertEqual
+  \ [
+  \   ale#path#Simplify(g:dir . '/test-files/c/json_project'),
+  \   ale#path#Simplify(g:dir . '/test-files/c/json_project/build/compile_commands.json'),
+  \ ],
+  \ ale#c#FindCompileCommands(bufnr(''))