improve garbage collection

by shortening `.handle` reference chains

Extending test suite to get coverage for such cases

src/Promise.js

@@ -168,6 +168,7 @@ export class Future extends Core {
 	}
 
 	_become (p) {
+		/* eslint complexity:[2,6] */
 		if (p === this) {
 			p = cycle()
 		}
@@ -185,8 +186,11 @@ export class Future extends Core {
 				// for not unnecessarily creating handles that never see any actions
 				// works well because it has a near() method
 				this.handle = p
+			} else if (p.handle) {
+				this.handle = p.handle
 			} else {
-				this.handle = new Handle(p)
+				// explicit handle to avoid reference chain between multiple futures
+				this.handle = p.handle = new Handle(p)
 			}
 		}
 	}

test/future-test.js

@@ -44,6 +44,29 @@ describe('future', () => {
 			resolve(reject(expected))
 			return promise.then(assert.ifError, x => assert.strictEqual(expected, x))
 		})
+
+		it('should resolve with pending promise', () => {
+			const { resolve: resolve1, promise: promise1 } = future()
+			const { resolve: resolve2, promise: promise2 } = future()
+			const { resolve: resolve3, promise: promise3 } = future()
+			const expected = {}
+			setTimeout(resolve2, 1, promise1)
+			setTimeout(resolve1, 2, expected)
+			setTimeout(resolve3, 2, promise2)
+			return promise3.then(x => assert.strictEqual(expected, x))
+		})
+
+		it('should resolve with pending promise that already has handle', () => {
+			const { resolve: resolve1, promise: promise1 } = future()
+			const { resolve: resolve2, promise: promise2 } = future()
+			const { resolve: resolve3, promise: promise3 } = future()
+			const expected = {}
+			promise1.map(x => x === expected)
+			setTimeout(resolve2, 1, promise1)
+			setTimeout(resolve1, 2, expected)
+			setTimeout(resolve3, 2, promise2)
+			return promise3.then(x => assert.strictEqual(expected, x))
+		})
 	})
 
 	describe('when resolved to another promise', () => {