rego: Add new function to list files using a glob pattern

This makes it easy for us to write policies that take `yaml` or `yml`
extensions into account without much hassle.

internal/engine/eval/rego/lib.go

@@ -19,10 +19,12 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"os"
 	"path/filepath"
 
 	"github.com/go-git/go-billy/v5"
+	billyutil "github.com/go-git/go-billy/v5/util"
 	"github.com/open-policy-agent/opa/ast"
 	"github.com/open-policy-agent/opa/rego"
 	"github.com/open-policy-agent/opa/types"
@@ -36,6 +38,7 @@ import (
 var MinderRegoLib = []func(res *engif.Result) func(*rego.Rego){
 	FileExists,
 	FileLs,
+	FileLsGlob,
 	FileRead,
 	ListGithubActions,
 }
@@ -202,6 +205,68 @@ func FileLs(res *engif.Result) func(*rego.Rego) {
 	)
 }
 
+// FileLsGlob is a rego function that lists the files matching a glob in a directory
+// in the filesystem being evaluated (which comes from the ingester).
+// It takes one argument, the path to the pattern to match. It's exposed
+// as `file.ls_glob`.
+func FileLsGlob(res *engif.Result) func(*rego.Rego) {
+	return rego.Function1(
+		&rego.Function{
+			Name: "file.ls_glob",
+			Decl: types.NewFunction(types.Args(types.S), types.A),
+		},
+		func(bctx rego.BuiltinContext, op1 *ast.Term) (*ast.Term, error) {
+			var path string
+			if err := ast.As(op1.Value, &path); err != nil {
+				return nil, err
+			}
+
+			if res.Fs == nil {
+				return nil, fmt.Errorf("cannot walk file without a filesystem")
+			}
+
+			rfs := res.Fs
+
+			// Let's get the base directory and the glob pattern
+			base := filepath.Dir(path)
+
+			pattern := filepath.Base(path)
+			files := []*ast.Term{}
+
+			err := billyutil.Walk(rfs, base, func(path string, info fs.FileInfo, err error) error {
+				// If we got an error, skip the file
+				if err != nil {
+					return nil
+				}
+
+				// If we gave no pattern, we're listing all files
+				if len(pattern) == 0 {
+					files = append(files, ast.NewTerm(ast.String(path)))
+					return nil
+				}
+
+				// list all files matching the pattern
+				matched, merr := filepath.Match(pattern, info.Name())
+				if merr != nil {
+					return merr
+				}
+
+				if matched {
+					files = append(files, ast.NewTerm(ast.String(path)))
+				}
+
+				return nil
+			})
+			if err != nil {
+				return nil, err
+			}
+
+			return ast.NewTerm(
+				ast.NewArray(files...)), nil
+		},
+	)
+}
+
 func fileLsHandleError(err error) (*ast.Term, error) {
 	// If the file does not exist return null
 	if errors.Is(err, os.ErrNotExist) {

internal/engine/eval/rego/lib_test.go

@@ -554,3 +554,80 @@ allow {
 	})
 	require.NoError(t, err, "could not evaluate")
 }
+
+func TestListYamlUsingLSGlob(t *testing.T) {
+	t.Parallel()
+
+	fs := memfs.New()
+
+	require.NoError(t, fs.MkdirAll(".github", 0755))
+
+	_, err := fs.Create(".github/dependabot.yaml")
+	require.NoError(t, err, "could not create dependabot file")
+
+	e, err := rego.NewRegoEvaluator(
+		&minderv1.RuleType_Definition_Eval_Rego{
+			Type: rego.DenyByDefaultEvaluationType.String(),
+			Def: `
+package minder
+
+default allow = false
+
+allow {
+	files := file.ls_glob(".github/dependabot.y*ml")
+	count(files) == 1
+}`,
+		},
+	)
+	require.NoError(t, err, "could not create evaluator")
+
+	emptyPol := map[string]any{}
+
+	err = e.Eval(context.Background(), emptyPol, &engif.Result{
+		Object: nil,
+		Fs:     fs,
+	})
+	require.NoError(t, err, "could not evaluate")
+}
+
+func TestListYamlsUsingLSGlob(t *testing.T) {
+	t.Parallel()
+
+	fs := memfs.New()
+
+	require.NoError(t, fs.MkdirAll(".github", 0755))
+	require.NoError(t, fs.MkdirAll(".github/workflows", 0755))
+
+	_, err := fs.Create(".github/workflows/security.yaml")
+	require.NoError(t, err, "could not create sec workflow file")
+
+	_, err = fs.Create(".github/workflows/build.yml")
+	require.NoError(t, err, "could not create build workflow file")
+
+	_, err = fs.Create(".github/workflows/release.yaml")
+	require.NoError(t, err, "could not create release workflow file")
+
+	e, err := rego.NewRegoEvaluator(
+		&minderv1.RuleType_Definition_Eval_Rego{
+			Type: rego.DenyByDefaultEvaluationType.String(),
+			Def: `
+package minder
+
+default allow = false
+
+allow {
+	files := file.ls_glob(".github/workflows/*.y*ml")
+	count(files) == 3
+}`,
+		},
+	)
+	require.NoError(t, err, "could not create evaluator")
+
+	emptyPol := map[string]any{}
+
+	err = e.Eval(context.Background(), emptyPol, &engif.Result{
+		Object: nil,
+		Fs:     fs,
+	})
+	require.NoError(t, err, "could not evaluate")
+}