Fix: use slot start instead of request start

Proofs should be required from the moment a slot is filled,
not from the moment a request is submitted.

This reverts commit f224cb8eda.

contracts/Marketplace.sol

@@ -320,18 +320,10 @@ contract Marketplace is Collateral, Proofs {
     return _timeout();
   }
 
-  function proofStart(SlotId slotId) public view override returns (uint256) {
-    return requestStart(_slot(slotId).requestId);
-  }
-
   function proofEnd(SlotId slotId) public view override returns (uint256) {
     return requestEnd(_slot(slotId).requestId);
   }
 
-  function requestStart(RequestId requestId) public view returns (uint256) {
-    return _context(requestId).startedAt;
-  }
-
   function requestEnd(RequestId requestId) public view returns (uint256) {
     uint256 end = _context(requestId).endsAt;
     if (_requestAcceptsProofs(requestId)) {

contracts/Proofs.sol

@@ -19,6 +19,7 @@ abstract contract Proofs is Periods {
   }
 
   mapping(SlotId => bool) private slotIds;
+  mapping(SlotId => uint256) private slotStarts;
   mapping(SlotId => uint256) private probabilities;
   mapping(SlotId => uint256) private missed;
   mapping(SlotId => mapping(Period => bool)) private received;
@@ -28,10 +29,6 @@ abstract contract Proofs is Periods {
     return timeout;
   }
 
-  // Override this to let the proving system know when proofs for a
-  // slot are required.
-  function proofStart(SlotId id) public view virtual returns (uint256);
-
   // Override this to let the proving system know when proofs for a
   // slot are no longer required.
   function proofEnd(SlotId id) public view virtual returns (uint256);
@@ -46,6 +43,7 @@ abstract contract Proofs is Periods {
   function _startRequiringProofs(SlotId id, uint256 probability) internal {
     require(!slotIds[id], "Proofs already required for slot");
     slotIds[id] = true;
+    slotStarts[id] = block.timestamp;
     probabilities[id] = probability;
   }
 
@@ -90,7 +88,7 @@ abstract contract Proofs is Periods {
     SlotId id,
     Period proofPeriod
   ) internal view returns (bool isRequired, uint8 pointer) {
-    Period start = periodOf(proofStart(id));
+    Period start = periodOf(slotStarts[id]);
     Period end = periodOf(proofEnd(id));
     if (!isAfter(proofPeriod, start) || !isBefore(proofPeriod, end)) {
       return (false, 0);

contracts/TestProofs.sol

@@ -5,7 +5,6 @@ import "./Proofs.sol";
 
 // exposes internal functions of Proofs for testing
 contract TestProofs is Proofs {
-  mapping(SlotId => uint256) private starts;
   mapping(SlotId => uint256) private ends;
 
   constructor(
@@ -19,10 +18,6 @@ contract TestProofs is Proofs {
 
   }
 
-  function proofStart(SlotId slotId) public view override returns (uint256) {
-    return starts[slotId];
-  }
-
   function proofEnd(SlotId slotId) public view override returns (uint256) {
     return ends[slotId];
   }
@@ -59,10 +54,6 @@ contract TestProofs is Proofs {
     _markProofAsMissing(id, _period);
   }
 
-  function setProofStart(SlotId id, uint256 start) public {
-    starts[id] = start;
-  }
-
   function setProofEnd(SlotId id, uint256 end) public {
     ends[id] = end;
   }

test/Proofs.test.js

@@ -37,7 +37,6 @@ describe("Proofs", function () {
 
   describe("general", function () {
     beforeEach(async function () {
-      await proofs.setProofStart(slotId, await currentTime())
       await proofs.setProofEnd(slotId, (await currentTime()) + duration)
     })
 
@@ -90,7 +89,6 @@ describe("Proofs", function () {
       let id2 = hexlify(randomBytes(32))
       let id3 = hexlify(randomBytes(32))
       for (let slotId of [id1, id2, id3]) {
-        await proofs.setProofStart(slotId, await currentTime())
         await proofs.setProofEnd(slotId, (await currentTime()) + duration)
         await proofs.startRequiringProofs(slotId, probability)
       }
@@ -122,7 +120,6 @@ describe("Proofs", function () {
     }
 
     beforeEach(async function () {
-      await proofs.setProofStart(slotId, await currentTime())
       await proofs.setProofEnd(slotId, (await currentTime()) + duration)
       await proofs.startRequiringProofs(slotId, probability)
       await advanceTimeTo(periodEnd(periodOf(await currentTime())))
@@ -157,7 +154,6 @@ describe("Proofs", function () {
     const proof = hexlify(randomBytes(42))
 
     beforeEach(async function () {
-      await proofs.setProofStart(slotId, await currentTime())
       await proofs.setProofEnd(slotId, (await currentTime()) + duration)
       await proofs.startRequiringProofs(slotId, probability)
     })