Fixes environment variable prefixes with double underscores preventing environment variables from being parsed

src/libraries/Microsoft.Extensions.Configuration.EnvironmentVariables/src/EnvironmentVariablesConfigurationProvider.cs

@@ -67,9 +67,17 @@ internal void Load(IDictionary envVariables)
                 {
                     prefix = CustomPrefix;
                 }
+                else if (key.StartsWith(_prefix, StringComparison.OrdinalIgnoreCase))
+                {
+                    // This prevents the prefix from being normalized.
+                    // We can also do a fast path branch, I guess? No point in reallocating if the prefix is empty.
+                    key = NormalizeKey(key.Substring(_prefix.Length));
+                    data[key] = entry.Value as string;
+
+                    continue;
+                }
                 else
                 {
-                    AddIfPrefixed(data, NormalizeKey(key), (string)entry.Value);
                     continue;
                 }
 

src/libraries/Microsoft.Extensions.Configuration.EnvironmentVariables/tests/EnvironmentVariablesTest.cs

@@ -143,14 +143,72 @@ public void ReplaceDoubleUnderscoreInEnvironmentVariables()
             var dict = new Hashtable()
                 {
                     {"data__ConnectionString", "connection"},
-                    {"SQLCONNSTR__db1", "connStr"}
+                    {"SQLCONNSTR_db1", "connStr"}
                 };
             var envConfigSrc = new EnvironmentVariablesConfigurationProvider();
 
             envConfigSrc.Load(dict);
 
             Assert.Equal("connection", envConfigSrc.Get("data:ConnectionString"));
-            Assert.Equal("System.Data.SqlClient", envConfigSrc.Get("ConnectionStrings:_db1_ProviderName"));
+            Assert.Equal("System.Data.SqlClient", envConfigSrc.Get("ConnectionStrings:db1_ProviderName"));
+        }
+
+        [Fact]
+        public void ReplaceDoubleUnderscoreInEnvironmentVariablesButNotPrefix()
+        {
+            var dict = new Hashtable()
+                {
+                    {"test__prefix__with__double__underscores__data__ConnectionString", "connection"}
+                };
+            var envConfigSrc = new EnvironmentVariablesConfigurationProvider("test__prefix__with__double__underscores__");
+
+            envConfigSrc.Load(dict);
+
+            Assert.Equal("connection", envConfigSrc.Get("data:ConnectionString"));
+        }
+
+        [Fact]
+        public void ReplaceDoubleUnderscoreInEnvironmentVariablesButNotInAnomalousPrefix()
+        {
+            var dict = new Hashtable()
+                {
+                    {"_____EXPERIMENTAL__data__ConnectionString", "connection"}
+                };
+            var envConfigSrc = new EnvironmentVariablesConfigurationProvider("_____EXPERIMENTAL__");
+
+            envConfigSrc.Load(dict);
+
+            Assert.Equal("connection", envConfigSrc.Get("data:ConnectionString"));
+        }
+
+        [Fact]
+        public void ReplaceDoubleUnderscoreInEnvironmentVariablesWithDuplicatedPrefix()
+        {
+            var dict = new Hashtable()
+                {
+                    {"test__test__ConnectionString", "connection"}
+                };
+            var envConfigSrc = new EnvironmentVariablesConfigurationProvider("test__");
+
+            envConfigSrc.Load(dict);
+
+            Assert.Equal("connection", envConfigSrc.Get("test:ConnectionString"));
+        }
+
+        [Fact]
+        public void PrefixPreventsLoadingSqlConnectionStrings()
+        {
+            var dict = new Hashtable()
+                {
+                    {"test__test__ConnectionString", "connection"},
+                    {"SQLCONNSTR_db1", "connStr"}
+                };
+            var envConfigSrc = new EnvironmentVariablesConfigurationProvider("test__");
+
+            envConfigSrc.Load(dict);
+
+            Assert.Equal("connection", envConfigSrc.Get("test:ConnectionString"));
+            Assert.Throws<InvalidOperationException>(() => envConfigSrc.Get("ConnectionStrings:db1_ProviderName"));
         }
 
         [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsThreadingSupported))]