Fix FindCompileCommands to handle absolute paths in c_build_dir_names (#5096)

* Initial plan

* 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>

* Improve comment clarity in FindCompileCommands

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

* Add defensive ale#Set for c_build_dir_names in FindCompileCommands

Mirror the existing s:CanParseMakefile pattern to ensure the
g:ale_c_build_dir_names variable always exists when ale#Var is
called, even if test Save/Restore cycles delete it.

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

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: w0rp <3518142+w0rp@users.noreply.github.com>

autoload/ale/c.vim

@@ -246,14 +246,31 @@ function! ale#c#FindCompileCommands(buffer) abort
         return [fnamemodify(l:json_file, ':h'), l:json_file]
     endif
 
+    " Something somewhere seems to delete this setting in tests, so ensure
+    " we always have a default value.
+    call ale#Set('c_build_dir_names', [
+    \   'build',
+    \   'build/Debug',
+    \   'build/Release',
+    \   'bin',
+    \])
+
     " 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]
+                " For absolute build dir paths, use the parent
+                " of the build dir as the project root. For
+                " relative paths, use the directory 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(''))