fix race condition when $millis() / $now() used concurrently

src/jsonata.js

@@ -2137,12 +2137,11 @@ var jsonata = (function() {
         }
         var environment = createFrame(staticFrame);
 
-        var timestamp = new Date(); // will be overridden on each call to evalute()
         environment.bind('now', defineFunction(function(picture, timezone) {
-            return datetime.fromMillis(timestamp.getTime(), picture, timezone);
+            return datetime.fromMillis(this.environment.timestamp.getTime(), picture, timezone);
         }, '<s?s?:s>'));
         environment.bind('millis', defineFunction(function() {
-            return timestamp.getTime();
+            return this.environment.timestamp.getTime();
         }, '<:n>'));
 
         return {
@@ -2169,8 +2168,7 @@ var jsonata = (function() {
 
                 // capture the timestamp and put it in the execution environment
                 // the $now() and $millis() functions will return this value - whenever it is called
-                timestamp = new Date();
-                exec_env.timestamp = timestamp;
+                exec_env.timestamp = new Date();
                 exec_env.options = options;
 
                 exec_env.createSequence = function() {

test/implementation-tests.js

@@ -176,6 +176,37 @@ describe("Functions with side-effects", () => {
         });
     });
 
+    describe("$millis() and $now() are not affected by an overlapping evaluate() call", function() {
+        it("should return the same value within each evaluation", async function() {
+            var expr = jsonata("($m := $millis(); $n := $now(); $pause($m); [$m = $millis(), $n = $now()])");
+            var paused, release;
+            var reached = new Promise((resolve) => { paused = resolve; });
+            var resume = new Promise((resolve) => { release = resolve; });
+            // the first evaluation suspends part way through, until released
+            var first = expr.evaluate(testdata2, {
+                pause: function(millis) {
+                    paused(millis);
+                    return resume;
+                }
+            });
+            var firstMillis = await reached;
+            // make sure the overlapping evaluation captures a later timestamp
+            while (Date.now() <= firstMillis) {
+                await new Promise((resolve) => setTimeout(resolve, 1));
+            }
+            var secondMillis;
+            var second = await expr.evaluate(testdata2, {
+                pause: function(millis) {
+                    secondMillis = millis;
+                }
+            });
+            release();
+            expect(secondMillis).to.be.above(firstMillis);
+            expect(second).to.deep.equal([true, true]);
+            expect(await first).to.deep.equal([true, true]);
+        });
+    });
+
     describe("$millis() returns milliseconds since the epoch", function() {
         it("should return result object", function() {
             var expr = jsonata("$millis()");