Update notExists to true when value is empty (#2412)

* Update notExists to true when value is empty

* Simplify the condition further

---------

Co-authored-by: Casey Sun <caseysun@microsoft.com>
Co-authored-by: Miha Zupan <mihazupan.zupan1@gmail.com>

src/ReverseProxy/Routing/HeaderMatcherPolicy.cs

@@ -70,13 +70,13 @@ public Task ApplyAsync(HttpContext httpContext, CandidateSet candidates)
 
             foreach (var matcher in matchers)
             {
-                var headerExists = headers.TryGetValue(matcher.Name, out var requestHeaderValues);
+                headers.TryGetValue(matcher.Name, out var requestHeaderValues);
                 var valueIsEmpty = StringValues.IsNullOrEmpty(requestHeaderValues);
 
                 var matched = matcher.Mode switch
                 {
                     HeaderMatchMode.Exists => !valueIsEmpty,
-                    HeaderMatchMode.NotExists => !headerExists,
+                    HeaderMatchMode.NotExists => valueIsEmpty,
                     HeaderMatchMode.ExactHeader => !valueIsEmpty && TryMatchExactOrPrefix(matcher, requestHeaderValues),
                     HeaderMatchMode.HeaderPrefix => !valueIsEmpty && TryMatchExactOrPrefix(matcher, requestHeaderValues),
                     HeaderMatchMode.Contains => !valueIsEmpty && TryMatchContainsOrNotContains(matcher, requestHeaderValues),

test/ReverseProxy.Tests/Routing/HeaderMatcherPolicyTests.cs

@@ -154,7 +154,7 @@ public void AppliesToEndpoints_NoMetadata_DoesNotApply()
     [InlineData("", HeaderMatchMode.Exists, false)]
     [InlineData("abc", HeaderMatchMode.Exists, true)]
     [InlineData(null, HeaderMatchMode.NotExists, true)]
-    [InlineData("", HeaderMatchMode.NotExists, false)]
+    [InlineData("", HeaderMatchMode.NotExists, true)]
     [InlineData("abc", HeaderMatchMode.NotExists, false)]
     public async Task ApplyAsync_MatchingScenarios_AnyHeaderValue(string incomingHeaderValue, HeaderMatchMode mode, bool shouldMatch)
     {