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

Initial plan
Fix FindCompileCommands to handle absolute paths in cbuilddir_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 cbuilddir_names in FindCompileCommands

Mirror the existing s:CanParseMakefile pattern to ensure the
g:alecbuilddirnames 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>

2 files changed, 52 insertions, 2 deletions

diff --git a/autoload/ale/c.vim b/autoload/ale/c.vim
index bc955d0ea..329637b5c 100644
--- a/autoload/ale/c.vim
+++ b/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
diff --git a/test/test_c_find_compile_commands.vader b/test/test_c_find_compile_commands.vader
new file mode 100644
index 000000000..c24f26c0a
--- /dev/null
+++ b/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(''))