Resolve variable references inside variable lookup fields (#1368)

## Changes
Allows for the syntax below

```
variables:
  service_principal_app_id:
    description: 'The app id of the service principal for running workflows as.'
    lookup:
      service_principal: "sp-${bundle.environment}"
```

Fixes #1259

## Tests
Added regression test

bundle/config/mutator/resolve_resource_references_test.go

@@ -133,3 +133,64 @@ func TestResolveServicePrincipal(t *testing.T) {
 	require.NoError(t, diags.Error())
 	require.Equal(t, "app-1234", *b.Config.Variables["my-sp"].Value)
 }
+
+func TestResolveVariableReferencesInVariableLookups(t *testing.T) {
+	s := func(s string) *string {
+		return &s
+	}
+
+	b := &bundle.Bundle{
+		Config: config.Root{
+			Bundle: config.Bundle{
+				Target: "dev",
+			},
+			Variables: map[string]*variable.Variable{
+				"foo": {
+					Value: s("bar"),
+				},
+				"lookup": {
+					Lookup: &variable.Lookup{
+						Cluster: "cluster-${var.foo}-${bundle.target}",
+					},
+				},
+			},
+		},
+	}
+
+	m := mocks.NewMockWorkspaceClient(t)
+	b.SetWorkpaceClient(m.WorkspaceClient)
+	clusterApi := m.GetMockClustersAPI()
+	clusterApi.EXPECT().GetByClusterName(mock.Anything, "cluster-bar-dev").Return(&compute.ClusterDetails{
+		ClusterId: "1234-5678-abcd",
+	}, nil)
+
+	diags := bundle.Apply(context.Background(), b, bundle.Seq(ResolveVariableReferencesInLookup(), ResolveResourceReferences()))
+	require.NoError(t, diags.Error())
+	require.Equal(t, "cluster-bar-dev", b.Config.Variables["lookup"].Lookup.Cluster)
+	require.Equal(t, "1234-5678-abcd", *b.Config.Variables["lookup"].Value)
+}
+
+func TestResolveLookupVariableReferencesInVariableLookups(t *testing.T) {
+	b := &bundle.Bundle{
+		Config: config.Root{
+			Variables: map[string]*variable.Variable{
+				"another_lookup": {
+					Lookup: &variable.Lookup{
+						Cluster: "cluster",
+					},
+				},
+				"lookup": {
+					Lookup: &variable.Lookup{
+						Cluster: "cluster-${var.another_lookup}",
+					},
+				},
+			},
+		},
+	}
+
+	m := mocks.NewMockWorkspaceClient(t)
+	b.SetWorkpaceClient(m.WorkspaceClient)
+
+	diags := bundle.Apply(context.Background(), b, bundle.Seq(ResolveVariableReferencesInLookup(), ResolveResourceReferences()))
+	require.ErrorContains(t, diags.Error(), "lookup variables cannot contain references to another lookup variables")
+}

bundle/config/mutator/resolve_variable_references.go

@@ -2,8 +2,10 @@ package mutator
 
 import (
 	"context"
+	"fmt"
 
 	"github.com/databricks/cli/bundle"
+	"github.com/databricks/cli/bundle/config/variable"
 	"github.com/databricks/cli/libs/diag"
 	"github.com/databricks/cli/libs/dyn"
 	"github.com/databricks/cli/libs/dyn/convert"
@@ -13,10 +15,50 @@ import (
 
 type resolveVariableReferences struct {
 	prefixes []string
+	pattern  dyn.Pattern
+	lookupFn func(dyn.Value, dyn.Path) (dyn.Value, error)
 }
 
 func ResolveVariableReferences(prefixes ...string) bundle.Mutator {
-	return &resolveVariableReferences{prefixes: prefixes}
+	return &resolveVariableReferences{prefixes: prefixes, lookupFn: lookup}
+}
+
+func ResolveVariableReferencesInLookup() bundle.Mutator {
+	return &resolveVariableReferences{prefixes: []string{
+		"bundle",
+		"workspace",
+		"variables",
+	}, pattern: dyn.NewPattern(dyn.Key("variables"), dyn.AnyKey(), dyn.Key("lookup")), lookupFn: lookupForVariables}
+}
+
+func lookup(v dyn.Value, path dyn.Path) (dyn.Value, error) {
+	// Future opportunity: if we lookup this path in both the given root
+	// and the synthesized root, we know if it was explicitly set or implied to be empty.
+	// Then we can emit a warning if it was not explicitly set.
+	return dyn.GetByPath(v, path)
+}
+
+func lookupForVariables(v dyn.Value, path dyn.Path) (dyn.Value, error) {
+	if path[0].Key() != "variables" {
+		return lookup(v, path)
+	}
+
+	varV, err := dyn.GetByPath(v, path[:len(path)-1])
+	if err != nil {
+		return dyn.InvalidValue, err
+	}
+
+	var vv variable.Variable
+	err = convert.ToTyped(&vv, varV)
+	if err != nil {
+		return dyn.InvalidValue, err
+	}
+
+	if vv.Lookup != nil && vv.Lookup.String() != "" {
+		return dyn.InvalidValue, fmt.Errorf("lookup variables cannot contain references to another lookup variables")
+	}
+
+	return lookup(v, path)
 }
 
 func (*resolveVariableReferences) Name() string {
@@ -52,15 +94,11 @@ func (m *resolveVariableReferences) Apply(ctx context.Context, b *bundle.Bundle)
 		// has already been normalized when it was first loaded from the configuration file.
 		//
 		normalized, _ := convert.Normalize(b.Config, root, convert.IncludeMissingFields)
-		lookup := func(path dyn.Path) (dyn.Value, error) {
-			// Future opportunity: if we lookup this path in both the given root
-			// and the synthesized root, we know if it was explicitly set or implied to be empty.
-			// Then we can emit a warning if it was not explicitly set.
-			return dyn.GetByPath(normalized, path)
-		}
 
+		// If the pattern is nil, we resolve references in the entire configuration.
+		root, err := dyn.MapByPattern(root, m.pattern, func(p dyn.Path, v dyn.Value) (dyn.Value, error) {
 			// Resolve variable references in all values.
-		root, err := dynvar.Resolve(root, func(path dyn.Path) (dyn.Value, error) {
+			return dynvar.Resolve(v, func(path dyn.Path) (dyn.Value, error) {
 				// Rewrite the shorthand path ${var.foo} into ${variables.foo.value}.
 				if path.HasPrefix(varPath) && len(path) == 2 {
 					path = dyn.NewPath(
@@ -73,12 +111,14 @@ func (m *resolveVariableReferences) Apply(ctx context.Context, b *bundle.Bundle)
 				// Perform resolution only if the path starts with one of the specified prefixes.
 				for _, prefix := range prefixes {
 					if path.HasPrefix(prefix) {
-					return lookup(path)
+						return m.lookupFn(normalized, path)
 					}
 				}
 
 				return dyn.InvalidValue, dynvar.ErrSkipResolution
 			})
+		})
+
 		if err != nil {
 			return dyn.InvalidValue, err
 		}

bundle/phases/initialize.go

@@ -28,6 +28,7 @@ func Initialize() bundle.Mutator {
 			mutator.ExpandWorkspaceRoot(),
 			mutator.DefineDefaultWorkspacePaths(),
 			mutator.SetVariables(),
+			mutator.ResolveVariableReferencesInLookup(),
 			mutator.ResolveResourceReferences(),
 			mutator.ResolveVariableReferences(
 				"bundle",