Fix off-by-one in random error plugins' failure rate check

DevProxy.Integration.Tests/BehaviorPluginsIntegrationTests.cs

@@ -66,6 +66,46 @@ public async Task GenericRandomError_Rate100_OverridesOriginWithConfiguredError(
         Assert.Equal(HttpStatusCode.ServiceUnavailable, response.StatusCode);
     }
 
+    [Fact]
+    public async Task GenericRandomError_Rate99_SometimesPassesThrough()
+    {
+        await using var origin = await FakeOrigin.StartAsync();
+        var urls = KestrelProxyHarness.BuildUrlsToWatch(origin.Host);
+
+        // rate:99 ⇒ ~1% of requests pass through. Previously rate:99 behaved like rate:100.
+        var config = PluginConfig.FromJson($$"""
+            {
+              "rate": 99,
+              "errors": [
+                {
+                  "request": { "url": "http://{{origin.Host}}/*" },
+                  "responses": [ { "statusCode": 503 } ]
+                }
+              ]
+            }
+            """);
+        var plugin = new GenericRandomErrorPlugin(
+            SharedHttpClient,
+            NullLogger<GenericRandomErrorPlugin>.Instance,
+            urls,
+            ProxyConfig,
+            config);
+
+        await using var proxy = await KestrelProxyHarness.StartAsync(
+            origin.Host, [plugin]);
+        using var client = proxy.CreateHttpClient();
+
+        // Probability of 2000 consecutive failures at 99% is ~2e-9.
+        var passedThrough = false;
+        for (var i = 0; i < 2000 && !passedThrough; i++)
+        {
+            using var response = await client.GetAsync(new Uri($"http://{origin.Host}/get"));
+            passedThrough = response.StatusCode == HttpStatusCode.OK;
+        }
+
+        Assert.True(passedThrough);
+    }
+
     [Fact]
     public async Task GenericRandomError_DynamicRetryAfterOnNon429_IsResolved()
     {

DevProxy.Plugins/Behavior/GenericRandomErrorPlugin.cs

@@ -138,7 +138,7 @@ public override Task BeforeRequestAsync(ProxyRequestArgs e, CancellationToken ca
     }
 
     // uses config to determine if a request should be failed
-    private GenericRandomErrorFailMode ShouldFail() => _random.Next(1, 100) <= Configuration.Rate ? GenericRandomErrorFailMode.Random : GenericRandomErrorFailMode.PassThru;
+    private GenericRandomErrorFailMode ShouldFail() => _random.Next(0, 100) < Configuration.Rate ? GenericRandomErrorFailMode.Random : GenericRandomErrorFailMode.PassThru;
 
     private void FailResponse(ProxyRequestArgs e)
     {

DevProxy.Plugins/Behavior/GraphRandomErrorPlugin.cs

@@ -206,7 +206,7 @@ public override Task BeforeRequestAsync(ProxyRequestArgs e, CancellationToken ca
     }
 
     // uses config to determine if a request should be failed
-    private GraphRandomErrorFailMode ShouldFail() => _random.Next(1, 100) <= Configuration.Rate ? GraphRandomErrorFailMode.Random : GraphRandomErrorFailMode.PassThru;
+    private GraphRandomErrorFailMode ShouldFail() => _random.Next(0, 100) < Configuration.Rate ? GraphRandomErrorFailMode.Random : GraphRandomErrorFailMode.PassThru;
 
     private void FailResponse(ProxyRequestArgs e)
     {