Add zero-value guards to abaci.sol auction parameter setters

cache/test-failures

@@ -0,0 +1 @@
+testFail_linear_decrease_tau_zero_reverts|testFail_stairstep_step_zero_reverts

foundry.lock

@@ -0,0 +1,11 @@
+{
+  "lib/ds-test": {
+    "rev": "0a5da56b0d65960e6a994d2ec8245e6edd38c248"
+  },
+  "lib/ds-token": {
+    "rev": "a5e709b205383720a3d47c7b234771daf4b58766"
+  },
+  "lib/ds-value": {
+    "rev": "4049ecd2652a39cbab464bb1c2c627985f720f97"
+  }
+}

src/abaci.sol

@@ -54,7 +54,7 @@ contract LinearDecrease is Abacus {
 
     // --- Administration ---
     function file(bytes32 what, uint256 data) external auth {
-        if (what ==  "tau") tau = data;
+        if (what ==  "tau") require((tau = data) > 0, "LinearDecrease/tau-is-zero");
         else revert("LinearDecrease/file-unrecognized-param");
         emit File(what, data);
     }
@@ -120,7 +120,7 @@ contract StairstepExponentialDecrease is Abacus {
     // --- Administration ---
     function file(bytes32 what, uint256 data) external auth {
         if      (what ==  "cut") require((cut = data) <= RAY, "StairstepExponentialDecrease/cut-gt-RAY");
-        else if (what == "step") step = data;
+        else if (what == "step") require((step = data) > 0, "StairstepExponentialDecrease/step-is-zero");
         else revert("StairstepExponentialDecrease/file-unrecognized-param");
         emit File(what, data);
     }

src/test/abaci.t.sol

@@ -28,6 +28,10 @@ interface Hevm {
     function store(address,bytes32,bytes32) external;
 }
 
+interface Vm {
+    function expectRevert(bytes calldata) external;
+}
+
 contract ClipperTest is DSTest {
     Hevm hevm;
 
@@ -227,4 +231,20 @@ contract ClipperTest is DSTest {
         price = calc.price(top, now - tic);
         assertEq(price, 0);
     }
+
+    // --- Revert tests for zero-value parameter validation ---
+
+    // LinearDecrease must reject tau = 0 to prevent division-by-zero in price()
+    function test_linear_decrease_tau_zero_reverts() public {
+        LinearDecrease calc = new LinearDecrease();
+        Vm(address(CHEAT_CODE)).expectRevert(bytes("LinearDecrease/tau-is-zero"));
+        calc.file(bytes32("tau"), 0);
+    }
+
+    // StairstepExponentialDecrease must reject step = 0 to prevent division-by-zero in price()
+    function test_stairstep_step_zero_reverts() public {
+        StairstepExponentialDecrease calc = new StairstepExponentialDecrease();
+        Vm(address(CHEAT_CODE)).expectRevert(bytes("StairstepExponentialDecrease/step-is-zero"));
+        calc.file(bytes32("step"), 0);
+    }
 }