[purchasing] Rework purchasing as a state machine

codex/purchasing/purchase.nim

@@ -1,46 +1,31 @@
-import ../market
-import ../clock
+import ./statemachine
+import ./states/pending
 import ./purchaseid
 
-export purchaseid
+# Purchase is implemented as a state machine:
+#
+#     pending ----> submitted ----------> started
+#        \             \    \
+#         \             \    -----------> cancelled
+#          \             \                   \
+#           --------------------------------------> error
+#
 
-type
-  Purchase* = ref object
-    future: Future[void]
-    market: Market
-    clock: Clock
-    request*: StorageRequest
+export Purchase
+export purchaseid
 
 func newPurchase*(request: StorageRequest,
                   market: Market,
                   clock: Clock): Purchase =
-  Purchase(request: request, market: market, clock: clock)
-
-proc run(purchase: Purchase) {.async.} =
-  let market = purchase.market
-  let clock = purchase.clock
-
-  proc requestStorage {.async.} =
-    purchase.request = await market.requestStorage(purchase.request)
-
-  proc waitUntilFulfilled {.async.} =
-    let done = newFuture[void]()
-    proc callback(_: RequestId) =
-      done.complete()
-    let request = purchase.request
-    let subscription = await market.subscribeFulfillment(request.id, callback)
-    await done
-    await subscription.unsubscribe()
-
-  proc withTimeout(future: Future[void]) {.async.} =
-    let expiry = purchase.request.expiry.truncate(int64)
-    await future.withTimeout(clock, expiry)
-
-  await requestStorage()
-  await waitUntilFulfilled().withTimeout()
+  Purchase(
+    future: Future[void].new(),
+    request: request,
+    market: market,
+    clock: clock
+  )
 
 proc start*(purchase: Purchase) =
-  purchase.future = purchase.run()
+  purchase.switch(PurchasePending())
 
 proc wait*(purchase: Purchase) {.async.} =
   await purchase.future

codex/purchasing/statemachine.nim

@@ -0,0 +1,15 @@
+import ../utils/statemachine
+import ../market
+import ../clock
+
+export market
+export clock
+export statemachine
+
+type
+  Purchase* = ref object of StateMachine
+    future*: Future[void]
+    market*: Market
+    clock*: Clock
+    request*: StorageRequest
+  PurchaseState* = ref object of AsyncState

codex/purchasing/states/cancelled.nim

@@ -0,0 +1,17 @@
+import ../statemachine
+import ./error
+
+type PurchaseCancelled* = ref object of PurchaseState
+
+method enterAsync*(state: PurchaseCancelled) {.async.} =
+  without purchase =? (state.context as Purchase):
+    raiseAssert "invalid state"
+
+  try:
+    await purchase.market.withdrawFunds(purchase.request.id)
+  except CatchableError as error:
+    state.switch(PurchaseError(error: error))
+    return
+
+  let error = newException(Timeout, "Purchase cancelled due to timeout")
+  state.switch(PurchaseError(error: error))

codex/purchasing/states/error.nim

@@ -0,0 +1,10 @@
+import ../statemachine
+
+type PurchaseError* = ref object of PurchaseState
+  error*: ref CatchableError
+
+method enter*(state: PurchaseError) =
+  without purchase =? (state.context as Purchase):
+    raiseAssert "invalid state"
+
+  purchase.future.fail(state.error)

codex/purchasing/states/pending.nim

@@ -0,0 +1,17 @@
+import ../statemachine
+import ./submitted
+import ./error
+
+type PurchasePending* = ref object of PurchaseState
+
+method enterAsync(state: PurchasePending) {.async.} =
+  without purchase =? (state.context as Purchase):
+    raiseAssert "invalid state"
+
+  try:
+    purchase.request = await purchase.market.requestStorage(purchase.request)
+  except CatchableError as error:
+    state.switch(PurchaseError(error: error))
+    return
+
+  state.switch(PurchaseSubmitted())

codex/purchasing/states/started.nim

@@ -0,0 +1,9 @@
+import ../statemachine
+
+type PurchaseStarted* = ref object of PurchaseState
+
+method enter*(state: PurchaseStarted) =
+  without purchase =? (state.context as Purchase):
+    raiseAssert "invalid state"
+
+  purchase.future.complete()

codex/purchasing/states/submitted.nim

@@ -0,0 +1,37 @@
+import ../statemachine
+import ./error
+import ./started
+import ./cancelled
+
+type PurchaseSubmitted* = ref object of PurchaseState
+
+method enterAsync(state: PurchaseSubmitted) {.async.} =
+  without purchase =? (state.context as Purchase):
+    raiseAssert "invalid state"
+
+  let market = purchase.market
+  let clock = purchase.clock
+
+  proc wait {.async.} =
+    let done = newFuture[void]()
+    proc callback(_: RequestId) =
+      done.complete()
+    let request = purchase.request
+    let subscription = await market.subscribeFulfillment(request.id, callback)
+    await done
+    await subscription.unsubscribe()
+
+  proc withTimeout(future: Future[void]) {.async.} =
+    let expiry = purchase.request.expiry.truncate(int64)
+    await future.withTimeout(clock, expiry)
+
+  try:
+    await wait().withTimeout()
+  except Timeout:
+    state.switch(PurchaseCancelled())
+    return
+  except CatchableError as error:
+    state.switch(PurchaseError(error: error))
+    return
+
+  state.switch(PurchaseStarted())