plan renderer: ensure JSON strings are completely JSON

internal/command/jsonformat/computed/renderers/renderer_test.go

@@ -111,6 +111,13 @@ null -> jsonencode(
     )
 `,
 		},
+		"primitive_create_fake_json": {
+			diff: computed.Diff{
+				Renderer: Primitive(nil, "[\"hello\"] and some more", cty.String),
+				Action:   plans.Create,
+			},
+			expected: `"[\"hello\"] and some more"`,
+		},
 		"primitive_create_null_string": {
 			diff: computed.Diff{
 				Renderer: Primitive(nil, nil, cty.String),

internal/command/jsonformat/computed/renderers/string.go

@@ -4,11 +4,11 @@
 package renderers
 
 import (
-	"encoding/json"
 	"fmt"
 	"strings"
 
 	"github.com/hashicorp/terraform/internal/command/jsonformat/computed"
+	"github.com/hashicorp/terraform/internal/command/jsonformat/structured"
 )
 
 type evaluatedString struct {
@@ -30,12 +30,8 @@ func evaluatePrimitiveString(value interface{}, opts computed.RenderHumanOpts) e
 	str := value.(string)
 
 	if strings.HasPrefix(str, "{") || strings.HasPrefix(str, "[") {
-
-		decoder := json.NewDecoder(strings.NewReader(str))
-		decoder.UseNumber()
-
-		var jv interface{}
-		if err := decoder.Decode(&jv); err == nil {
+		jv, err := structured.ParseJson(strings.NewReader(str))
+		if err == nil {
 			return evaluatedString{
 				String: str,
 				Json:   jv,

internal/command/jsonformat/structured/change.go

@@ -6,6 +6,8 @@ package structured
 import (
 	"bytes"
 	"encoding/json"
+	"fmt"
+	"io"
 	"reflect"
 
 	"github.com/hashicorp/terraform/internal/command/jsonformat/structured/attribute_path"
@@ -282,11 +284,8 @@ func unmarshalGeneric(raw json.RawMessage) interface{} {
 		return nil
 	}
 
-	decoder := json.NewDecoder(bytes.NewReader(raw))
-	decoder.UseNumber() // We support very big (> 2^64) numbers.
-
-	var out interface{}
-	if err := decoder.Decode(&out); err != nil {
+	out, err := ParseJson(bytes.NewReader(raw))
+	if err != nil {
 		panic("unrecognized json type: " + err.Error())
 	}
 	return out
@@ -299,3 +298,21 @@ func unwrapAttributeValues(values jsonstate.AttributeValues) map[string]interfac
 	}
 	return out
 }
+
+func ParseJson(reader io.Reader) (interface{}, error) {
+	decoder := json.NewDecoder(reader)
+	decoder.UseNumber()
+
+	var jv interface{}
+	if err := decoder.Decode(&jv); err != nil {
+		return nil, err
+	}
+
+	// The JSON decoder should have consumed the entire input stream, so
+	// we should be at EOF now.
+	if token, err := decoder.Token(); err != io.EOF {
+		return nil, fmt.Errorf("unexpected token after valid JSON: %v", token)
+	}
+
+	return jv, nil
+}