Add support for multiple level string variable interpolation (#342)

## Changes
Traverses the variables referred in a depth first manner to resolve
string fields.
Errors out if a cycle is detected

## Tests
Manually and unit/blackbox tests

bundle/config/interpolation/interpolation.go

@@ -6,9 +6,12 @@ import (
 	"fmt"
 	"reflect"
 	"regexp"
+	"sort"
 	"strings"
 
 	"github.com/databricks/bricks/bundle"
+	"golang.org/x/exp/maps"
+	"golang.org/x/exp/slices"
 )
 
 const Delimiter = "."
@@ -63,7 +66,14 @@ func (s *stringField) interpolate(fns []LookupFunction, lookup map[string]string
 }
 
 type accumulator struct {
+	// all string fields in the bundle config
 	strings map[string]*stringField
+
+	// contains path -> resolved_string mapping for string fields in the config
+	// The resolved strings will NOT contain any variable references that could
+	// have been resolved, however there might still be references that cannot
+	// be resolved
+	memo map[string]string
 }
 
 // jsonFieldName returns the name in a field's `json` tag.
@@ -138,25 +148,7 @@ func (a *accumulator) walk(scope []string, rv reflect.Value, s setter) {
 	}
 }
 
-// Gathers the strings for a list of paths.
-// The fields in these paths may not depend on other fields,
-// as we don't support full DAG lookup yet (only single level).
-func (a *accumulator) gather(paths []string) (map[string]string, error) {
-	var out = make(map[string]string)
-	for _, path := range paths {
-		f, ok := a.strings[path]
-		if !ok {
-			return nil, fmt.Errorf("%s is not defined", path)
-		}
-		deps := f.dependsOn()
-		if len(deps) > 0 {
-			return nil, fmt.Errorf("%s depends on %s", path, strings.Join(deps, ", "))
-		}
-		out[path] = f.Get()
-	}
-	return out, nil
-}
-
+// walk and gather all string fields in the config
 func (a *accumulator) start(v any) {
 	rv := reflect.ValueOf(v)
 	if rv.Type().Kind() != reflect.Pointer {
@@ -168,25 +160,64 @@ func (a *accumulator) start(v any) {
 	}
 
 	a.strings = make(map[string]*stringField)
+	a.memo = make(map[string]string)
 	a.walk([]string{}, rv, nilSetter{})
 }
 
-func (a *accumulator) expand(fns ...LookupFunction) error {
-	for path, v := range a.strings {
-		ds := v.dependsOn()
-		if len(ds) == 0 {
-			continue
-		}
-
-		// Create map to be used for interpolation
-		m, err := a.gather(ds)
-		if err != nil {
-			return fmt.Errorf("cannot interpolate %s: %w", path, err)
-		}
-
-		v.interpolate(fns, m)
+// recursively interpolate variables in a depth first manner
+func (a *accumulator) Resolve(path string, seenPaths []string, fns ...LookupFunction) error {
+	// return early if the path is already resolved
+	if _, ok := a.memo[path]; ok {
+		return nil
 	}
 
+	// fetch the string node to resolve
+	field, ok := a.strings[path]
+	if !ok {
+		return fmt.Errorf("could not find string field with path %s", path)
+	}
+
+	// return early if the string field has no variables to interpolate
+	if len(field.dependsOn()) == 0 {
+		a.memo[path] = field.Get()
+		return nil
+	}
+
+	// resolve all variables refered in the root string field
+	for _, childFieldPath := range field.dependsOn() {
+		// error if there is a loop in variable interpolation
+		if slices.Contains(seenPaths, childFieldPath) {
+			return fmt.Errorf("cycle detected in field resolution: %s", strings.Join(append(seenPaths, childFieldPath), " -> "))
+		}
+
+		// recursive resolve variables in the child fields
+		err := a.Resolve(childFieldPath, append(seenPaths, childFieldPath), fns...)
+		if err != nil {
+			return err
+		}
+	}
+
+	// interpolate root string once all variable references in it have been resolved
+	field.interpolate(fns, a.memo)
+
+	// record interpolated string in memo
+	a.memo[path] = field.Get()
+	return nil
+}
+
+// Interpolate all string fields in the config
+func (a *accumulator) expand(fns ...LookupFunction) error {
+	// sorting paths for stable order of iteration
+	paths := maps.Keys(a.strings)
+	sort.Strings(paths)
+
+	// iterate over paths for all strings fields in the config
+	for _, path := range paths {
+		err := a.Resolve(path, []string{path}, fns...)
+		if err != nil {
+			return err
+		}
+	}
 	return nil
 }
 

bundle/config/interpolation/interpolation_test.go

@@ -97,3 +97,31 @@ func TestInterpolationWithMap(t *testing.T) {
 	assert.Equal(t, "a", f.F["a"])
 	assert.Equal(t, "a", f.F["b"])
 }
+
+func TestInterpolationWithResursiveVariableReferences(t *testing.T) {
+	f := foo{
+		A: "a",
+		B: "(${a})",
+		C: "${a} ${b}",
+	}
+
+	err := expand(&f)
+	require.NoError(t, err)
+
+	assert.Equal(t, "a", f.A)
+	assert.Equal(t, "(a)", f.B)
+	assert.Equal(t, "a (a)", f.C)
+}
+
+func TestInterpolationVariableLoopError(t *testing.T) {
+	d := "${b}"
+	f := foo{
+		A: "a",
+		B: "${c}",
+		C: "${d}",
+		D: &d,
+	}
+
+	err := expand(&f)
+	assert.ErrorContains(t, err, "cycle detected in field resolution: b -> c -> d -> b")
+}

bundle/tests/interpolation/bundle.yml

@@ -0,0 +1,10 @@
+bundle:
+  name: foo ${workspace.profile}
+
+workspace:
+  profile: bar
+
+resources:
+  jobs:
+    my_job:
+      name: "${bundle.name} | ${workspace.profile}"

bundle/tests/interpolation_test.go

@@ -0,0 +1,23 @@
+package config_tests
+
+import (
+	"context"
+	"testing"
+
+	"github.com/databricks/bricks/bundle"
+	"github.com/databricks/bricks/bundle/config/interpolation"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestInterpolation(t *testing.T) {
+	b := load(t, "./interpolation")
+	err := bundle.Apply(context.Background(), b, []bundle.Mutator{
+		interpolation.Interpolate(
+			interpolation.IncludeLookupsInPath("bundle"),
+			interpolation.IncludeLookupsInPath("workspace"),
+		)})
+	require.NoError(t, err)
+	assert.Equal(t, "foo bar", b.Config.Bundle.Name)
+	assert.Equal(t, "foo bar | bar", b.Config.Resources.Jobs["my_job"].Name)
+}