Added test for terminal characters in yaml strings

Author: darrelmiller

src/Microsoft.OpenApi/Writers/SpecialCharacterStringExtensions.cs

@@ -42,8 +42,7 @@ public static class SpecialCharacterStringExtensions
         // http://www.yaml.org/spec/1.2/spec.html#style/flow/plain
         private static readonly string[] _yamlPlainStringForbiddenCombinations =
         {
-            //": ",
-            ":",  // Even though colons can be allowed, this change prevents unquoted trailing colon which causes a problem
+            ": ",
             " #",
 
             // These are technically forbidden only inside flow collections, but
@@ -55,6 +54,13 @@ public static class SpecialCharacterStringExtensions
             ","
         };
 
+        // Plain style strings cannot end with these characters.
+        // http://www.yaml.org/spec/1.2/spec.html#style/flow/plain
+        private static readonly string[] _yamlPlainStringForbiddenTerminals =
+        {
+            ":"
+        };
+
         // Double-quoted strings are needed for these non-printable control characters.
         // http://www.yaml.org/spec/1.2/spec.html#style/flow/double-quoted
         private static readonly char[] _yamlControlCharacters =
@@ -171,6 +177,7 @@ internal static string GetYamlCompatibleString(this string input)
             // http://www.yaml.org/spec/1.2/spec.html#style/flow/plain
             if (_yamlPlainStringForbiddenCombinations.Any(fc => input.Contains(fc)) ||
                 _yamlIndicators.Any(i => input.StartsWith(i.ToString())) ||
+                _yamlPlainStringForbiddenTerminals.Any(i => input.EndsWith(i.ToString())) ||
                 input.Trim() != input)
             {
                 // Escape single quotes with two single quotes.

test/Microsoft.OpenApi.Tests/Writers/OpenApiWriterSpecialCharacterTests.cs

@@ -60,6 +60,7 @@ public void WriteStringWithSpecialCharactersAsJsonWorks(string input, string exp
         [InlineData("true", " 'true'")]
         [InlineData("trailingspace ", " 'trailingspace '")]
         [InlineData("     trailingspace", " '     trailingspace'")]
+        [InlineData("terminal:", " 'terminal:'")]
         public void WriteStringWithSpecialCharactersAsYamlWorks(string input, string expected)
         {
             // Arrange