fix(logs): fix infinite retention for jsii users (aws#3250)

The retention period was still accepting the value `Infinity` for
"infinite retention", even though the property has been changed to an
enum instead of `number`.

Even though this apparently works for TypeScript customers, jsii
customers will not be able to pass in a number where an enum value
is expected, and so won't be able to configure infinite retention.

Add an `INFINITE` value to the enum.

packages/@aws-cdk/aws-logs/lib/log-group.ts

@@ -259,7 +259,12 @@ export enum RetentionDays {
   /**
    * 10 years
    */
-  TEN_YEARS = 3653
+  TEN_YEARS = 3653,
+
+  /**
+   * Retain logs forever
+   */
+  INFINITE = 9999,
 }
 
 /**
@@ -276,9 +281,9 @@ export interface LogGroupProps {
   /**
    * How long, in days, the log contents will be retained.
    *
-   * To retain all logs, set this value to Infinity.
+   * To retain all logs, set this value to RetentionDays.INFINITE.
    *
-   * @default RetentionDays.TwoYears
+   * @default RetentionDays.TWO_YEARS
    */
   readonly retention?: RetentionDays;
 
@@ -328,7 +333,7 @@ export class LogGroup extends LogGroupBase {
 
     let retentionInDays = props.retention;
     if (retentionInDays === undefined) { retentionInDays = RetentionDays.TWO_YEARS; }
-    if (retentionInDays === Infinity) { retentionInDays = undefined; }
+    if (retentionInDays === Infinity || retentionInDays === RetentionDays.INFINITE) { retentionInDays = undefined; }
 
     if (retentionInDays !== undefined && retentionInDays <= 0) {
       throw new Error(`retentionInDays must be positive, got ${retentionInDays}`);

packages/@aws-cdk/aws-logs/test/test.loggroup.ts

@@ -43,6 +43,32 @@ export = {
 
     // WHEN
     new LogGroup(stack, 'LogGroup', {
+      retention: RetentionDays.INFINITE,
+    });
+
+    // THEN
+    expect(stack).to(matchTemplate({
+      Resources: {
+        LogGroupF5B46931: {
+          Type: "AWS::Logs::LogGroup",
+          DeletionPolicy: "Retain",
+          UpdateReplacePolicy: "Retain"
+        }
+      }
+    }));
+
+    test.done();
+  },
+
+  'infinite retention via legacy method'(test: Test) {
+    // GIVEN
+    const stack = new Stack();
+
+    // WHEN
+    new LogGroup(stack, 'LogGroup', {
+      // Don't know why TypeScript doesn't complain about passing Infinity to
+      // something where an enum is expected, but better keep this behavior for
+      // existing clients.
       retention: Infinity
     });