evaluate replace_triggered_by expressions

Evaluate the expressions stored in replace_triggered_by into the
*addrs.Reference needed to lookup changes in the plan.

internal/terraform/evaluate_triggers.go

@@ -0,0 +1,143 @@
+package terraform
+
+import (
+	"strings"
+
+	"github.com/hashicorp/hcl/v2"
+	"github.com/hashicorp/hcl/v2/hclsyntax"
+	"github.com/hashicorp/terraform/internal/addrs"
+	"github.com/hashicorp/terraform/internal/instances"
+	"github.com/hashicorp/terraform/internal/tfdiags"
+	"github.com/zclconf/go-cty/cty"
+)
+
+func evalReplaceTriggeredByExpr(expr hcl.Expression, keyData instances.RepetitionData) (*addrs.Reference, tfdiags.Diagnostics) {
+	var ref *addrs.Reference
+	var diags tfdiags.Diagnostics
+
+	traversal, diags := triggersExprToTraversal(expr, keyData)
+	if diags.HasErrors() {
+		return nil, diags
+	}
+
+	// We now have a static traversal, so we can just turn it into an addrs.Reference.
+	ref, ds := addrs.ParseRef(traversal)
+	diags = diags.Append(ds)
+
+	return ref, diags
+}
+
+// trggersExprToTraversal takes an hcl expression limited to the syntax allowed
+// in replace_triggered_by, and converts it to a static traversal. The
+// RepetitionData contains the data necessary to evaluate the only allowed
+// variables in the expression, count.index and each.key.
+func triggersExprToTraversal(expr hcl.Expression, keyData instances.RepetitionData) (hcl.Traversal, tfdiags.Diagnostics) {
+	var trav hcl.Traversal
+	var diags tfdiags.Diagnostics
+
+	switch e := expr.(type) {
+	case *hclsyntax.RelativeTraversalExpr:
+		t, d := triggersExprToTraversal(e.Source, keyData)
+		diags = diags.Append(d)
+		trav = append(trav, t...)
+		trav = append(trav, e.Traversal...)
+
+	case *hclsyntax.ScopeTraversalExpr:
+		// a static reference, we can just append the traversal
+		trav = append(trav, e.Traversal...)
+
+	case *hclsyntax.IndexExpr:
+		// Get the collection from the index expression
+		t, d := triggersExprToTraversal(e.Collection, keyData)
+		diags = diags.Append(d)
+		if diags.HasErrors() {
+			return nil, diags
+		}
+		trav = append(trav, t...)
+
+		// The index key is the only place where we could have variables that
+		// reference count and each, so we need to parse those independently.
+		idx, hclDiags := parseIndexKeyExpr(e.Key, keyData)
+		diags = diags.Append(hclDiags)
+
+		trav = append(trav, idx)
+
+	default:
+		// Something unexpected got through config validation. We're not sure
+		// what it is, but we'll point it out in the diagnostics for the user
+		// to fix.
+		diags = diags.Append(&hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Invalid replace_triggered_by expression",
+			Detail:   "Unexpected expression found in replace_triggered_by.",
+			Subject:  e.Range().Ptr(),
+		})
+	}
+
+	return trav, diags
+}
+
+// parseIndexKeyExpr takes an hcl.Expression and parses it as an index key, while
+// evaluating any references to count.index or each.key.
+func parseIndexKeyExpr(expr hcl.Expression, keyData instances.RepetitionData) (hcl.TraverseIndex, hcl.Diagnostics) {
+	idx := hcl.TraverseIndex{
+		SrcRange: expr.Range(),
+	}
+
+	trav, diags := hcl.RelTraversalForExpr(expr)
+	if diags.HasErrors() {
+		return idx, diags
+	}
+
+	keyParts := []string{}
+
+	for _, t := range trav {
+		attr, ok := t.(hcl.TraverseAttr)
+		if !ok {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Invalid index expression",
+				Detail:   "Only constant values, count.index or each.key are allowed in index expressions.",
+				Subject:  expr.Range().Ptr(),
+			})
+			return idx, diags
+		}
+		keyParts = append(keyParts, attr.Name)
+	}
+
+	switch strings.Join(keyParts, ".") {
+	case "count.index":
+		if keyData.CountIndex == cty.NilVal {
+			diags = diags.Append(&hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  `Reference to "count" in non-counted context`,
+				Detail:   `The "count" object can only be used in "resource" blocks when the "count" argument is set.`,
+				Subject:  expr.Range().Ptr(),
+			})
+		}
+		idx.Key = keyData.CountIndex
+
+	case "each.key":
+		if keyData.EachKey == cty.NilVal {
+			diags = diags.Append(&hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  `Reference to "each" in context without for_each`,
+				Detail:   `The "each" object can be used only in "resource" blocks when the "for_each" argument is set.`,
+				Subject:  expr.Range().Ptr(),
+			})
+		}
+		idx.Key = keyData.EachKey
+	default:
+		// Something may have slipped through validation, probably from a json
+		// configuration.
+		diags = append(diags, &hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Invalid index expression",
+			Detail:   "Only constant values, count.index or each.key are allowed in index expressions.",
+			Subject:  expr.Range().Ptr(),
+		})
+	}
+
+	return idx, diags
+
+}

internal/terraform/evaluate_triggers_test.go

@@ -0,0 +1,94 @@
+package terraform
+
+import (
+	"testing"
+
+	"github.com/hashicorp/hcl/v2"
+	"github.com/hashicorp/hcl/v2/hclsyntax"
+	"github.com/hashicorp/terraform/internal/addrs"
+	"github.com/hashicorp/terraform/internal/instances"
+	"github.com/zclconf/go-cty/cty"
+)
+
+func TestEvalReplaceTriggeredBy(t *testing.T) {
+	tests := map[string]struct {
+		// Raw config expression from within replace_triggered_by list.
+		// If this does not contains any count or each references, it should
+		// directly parse into the same *addrs.Reference.
+		expr string
+
+		// If the expression contains count or each, then we need to add
+		// repetition data, and the static string to parse into the desired
+		// *addrs.Reference
+		repData   instances.RepetitionData
+		reference string
+	}{
+		"single resource": {
+			expr: "test_resource.a",
+		},
+
+		"resource instance attr": {
+			expr: "test_resource.a.attr",
+		},
+
+		"resource instance index attr": {
+			expr: "test_resource.a[0].attr",
+		},
+
+		"resource instance count": {
+			expr: "test_resource.a[count.index]",
+			repData: instances.RepetitionData{
+				CountIndex: cty.NumberIntVal(0),
+			},
+			reference: "test_resource.a[0]",
+		},
+		"resource instance for_each": {
+			expr: "test_resource.a[each.key].attr",
+			repData: instances.RepetitionData{
+				EachKey: cty.StringVal("k"),
+			},
+			reference: `test_resource.a["k"].attr`,
+		},
+		"resource instance for_each map attr": {
+			expr: "test_resource.a[each.key].attr[each.key]",
+			repData: instances.RepetitionData{
+				EachKey: cty.StringVal("k"),
+			},
+			reference: `test_resource.a["k"].attr["k"]`,
+		},
+	}
+
+	for name, tc := range tests {
+		pos := hcl.Pos{Line: 1, Column: 1}
+		t.Run(name, func(t *testing.T) {
+			expr, hclDiags := hclsyntax.ParseExpression([]byte(tc.expr), "", pos)
+			if hclDiags.HasErrors() {
+				t.Fatal(hclDiags)
+			}
+
+			got, diags := evalReplaceTriggeredByExpr(expr, tc.repData)
+			if diags.HasErrors() {
+				t.Fatal(diags.Err())
+			}
+
+			want := tc.reference
+			if want == "" {
+				want = tc.expr
+			}
+
+			// create the desired reference
+			traversal, travDiags := hclsyntax.ParseTraversalAbs([]byte(want), "", pos)
+			if travDiags.HasErrors() {
+				t.Fatal(travDiags)
+			}
+			ref, diags := addrs.ParseRef(traversal)
+			if diags.HasErrors() {
+				t.Fatal(diags.Err())
+			}
+
+			if got.DisplayString() != ref.DisplayString() {
+				t.Fatalf("expected %q: got %q", ref.DisplayString(), got.DisplayString())
+			}
+		})
+	}
+}