[contracts] Add Clock that synchronizes time with EVM

dagger/contracts/clock.nim

@@ -0,0 +1,40 @@
+import std/times
+import pkg/ethers
+import pkg/chronos
+import pkg/stint
+
+type
+  Clock* = ref object
+    provider: Provider
+    subscription: Subscription
+    offset: int64
+    started: bool
+  SecondsSince1970* = int64
+
+proc new*(_: type Clock, provider: Provider): Clock =
+  Clock(provider: provider)
+
+proc start*(clock: Clock) {.async.} =
+  if clock.started:
+    return
+  clock.started = true
+
+  proc onBlock(blck: Block) {.gcsafe, upraises:[].} =
+    let blockTime = blck.timestamp.truncate(int64)
+    let computerTime = getTime().toUnix
+    clock.offset = blockTime - computerTime
+
+  onBlock(!await clock.provider.getBlock(BlockTag.latest))
+
+  clock.subscription = await clock.provider.subscribe(onBlock)
+
+proc stop*(clock: Clock) {.async.} =
+  if not clock.started:
+    return
+  clock.started = false
+
+  await clock.subscription.unsubscribe()
+
+proc now*(clock: Clock): SecondsSince1970 =
+  doAssert clock.started, "clock should be started before calling now()"
+  getTime().toUnix + clock.offset

tests/contracts/testClock.nim

@@ -0,0 +1,48 @@
+import std/times
+import pkg/chronos
+import dagger/contracts/clock
+import ../ethertest
+
+ethersuite "Clock":
+
+  var clock: Clock
+
+  setup:
+    clock = Clock.new(provider)
+    await clock.start()
+
+  teardown:
+    await clock.stop()
+
+  test "returns the current time of the EVM":
+    let latestBlock = (!await provider.getBlock(BlockTag.latest))
+    let timestamp = latestBlock.timestamp.truncate(int64)
+    check clock.now() == timestamp
+
+  test "updates time with timestamp of new blocks":
+    let future = (getTime() + 42.years).toUnix
+    discard await provider.send("evm_setNextBlockTimestamp", @[%future])
+    discard await provider.send("evm_mine")
+    check clock.now() == future
+
+  test "updates time using wall-clock in-between blocks":
+    let past = clock.now()
+    await sleepAsync(chronos.seconds(1))
+    check clock.now() == past + 1
+
+  test "raises when not started":
+    expect AssertionError:
+      discard Clock.new(provider).now()
+
+  test "raises when stopped":
+    await clock.stop()
+    expect AssertionError:
+      discard clock.now()
+
+  test "handles starting multiple times":
+    await clock.start()
+    await clock.start()
+
+  test "handles stopping multiple times":
+    await clock.stop()
+    await clock.stop()

tests/testContracts.nim

@@ -3,5 +3,6 @@ import ./contracts/testContracts
 import ./contracts/testMarket
 import ./contracts/testProofs
 import ./contracts/testInteractions
+import ./contracts/testClock
 
 {.warning[UnusedImport]:off.}