go/packages: take into account cfg.Dir while querying relative files

Otherwise, relative file queries are resolved based on the current
working directory, rather than the configured cfg.Dir, which causes the
adhocPackage fallback to be used in such cases.

Change-Id: Id6fee3942e8d02d54c4f442a27424ba8ba8460bc
Reviewed-on: https://go-review.googlesource.com/c/tools/+/680817
Reviewed-by: Alan Donovan <[email protected]>
Commit-Queue: Alan Donovan <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: mateusz834

go/packages/golist.go

@@ -224,13 +224,22 @@ extractQueries:
 	return response.dr, nil
 }
 
+// abs returns an absolute representation of path, based on cfg.Dir.
+func (cfg *Config) abs(path string) (string, error) {
+	if filepath.IsAbs(path) {
+		return path, nil
+	}
+	// In case cfg.Dir is relative, pass it to filepath.Abs.
+	return filepath.Abs(filepath.Join(cfg.Dir, path))
+}
+
 func (state *golistState) runContainsQueries(response *responseDeduper, queries []string) error {
 	for _, query := range queries {
 		// TODO(matloob): Do only one query per directory.
 		fdir := filepath.Dir(query)
 		// Pass absolute path of directory to go list so that it knows to treat it as a directory,
 		// not a package path.
-		pattern, err := filepath.Abs(fdir)
+		pattern, err := state.cfg.abs(fdir)
 		if err != nil {
 			return fmt.Errorf("could not determine absolute path of file= query path %q: %v", query, err)
 		}

go/packages/packages_test.go

@@ -3400,3 +3400,27 @@ func writeTree(t *testing.T, archive string) string {
 	}
 	return root
 }
+
+func TestLoadFileRelativePath(t *testing.T) {
+	exported := packagestest.Export(t, packagestest.Modules, []packagestest.Module{
+		{
+			Name: "fake/pkg",
+			Files: map[string]any{
+				"pkg.go": "package pkg; type A int",
+			},
+		},
+	})
+	t.Cleanup(exported.Cleanup)
+
+	exported.Config.Mode = packages.LoadSyntax
+	pkgs, err := packages.Load(exported.Config, "file=pkg.go")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(pkgs) != 1 {
+		t.Fatalf("len(pkgs) = %v; want = 1", len(pkgs))
+	}
+	if pkgs[0].ID != "fake/pkg" {
+		t.Fatalf(`pkgs[0].Id = %q; want "fake/pkg"`, pkgs[0].ID)
+	}
+}