Quote strings that start or end with whitespace when writing YAML

Frontmatter.Test/EdgeWhitespaceRoundTripTests.cs

@@ -0,0 +1,56 @@
+// Copyright (c) 2023-2026 ktsu-dev contributors
+
+namespace ktsu.Frontmatter.Test;
+
+using System.Collections.Generic;
+
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+/// <summary>
+/// Regression tests for #183: a string that starts or ends with whitespace must be written so that the
+/// next read gets the same string back. YAML strips that whitespace from a plain scalar.
+/// </summary>
+[TestClass]
+public class EdgeWhitespaceRoundTripTests
+{
+	[TestMethod]
+	[DataRow("tab\t", DisplayName = "Trailing tab")]
+	[DataRow("x \t", DisplayName = "Trailing space and tab")]
+	[DataRow("x\t ", DisplayName = "Trailing tab and space")]
+	[DataRow("sp ", DisplayName = "Trailing space")]
+	[DataRow("\ttab", DisplayName = "Leading tab")]
+	[DataRow(" sp", DisplayName = "Leading space")]
+	public void AddFrontmatter_ValueWithEdgeWhitespace_RoundTrips(string value)
+	{
+		string document = Frontmatter.AddFrontmatter("Body", new Dictionary<string, object> { ["k"] = value, ["z"] = "1" });
+
+		Assert.AreEqual(value, Frontmatter.ExtractFrontmatter(document)!["k"]);
+	}
+
+	[TestMethod]
+	public void CombineFrontmatter_QuotedValueEndingInTab_StaysQuoted()
+	{
+		string input = "---\nk: \"tab\\t\"\nz: 1\n---\nBody\n";
+
+		string combined = Frontmatter.CombineFrontmatter(input, FrontmatterNaming.AsIs, FrontmatterOrder.AsIs, FrontmatterMergeStrategy.None);
+
+		Assert.AreEqual("tab\t", Frontmatter.ExtractFrontmatter(combined)!["k"]);
+	}
+
+	[TestMethod]
+	public void SerializeYamlObject_ValueEndingInTab_ParsesBackUnchanged()
+	{
+		string yaml = YamlSerializer.SerializeYamlObject(new Dictionary<string, object> { ["k"] = "tab\t", ["z"] = "1" });
+
+		Assert.IsTrue(YamlSerializer.TryParseYamlObject(yaml, out Dictionary<string, object>? parsed));
+		Assert.AreEqual("tab\t", parsed["k"]);
+	}
+
+	[TestMethod]
+	public void SerializeYamlObject_PlainValue_StaysPlain()
+	{
+		string yaml = YamlSerializer.SerializeYamlObject(new Dictionary<string, object> { ["k"] = "plain value" });
+
+		Assert.AreEqual("k: plain value", yaml.Trim());
+	}
+}

Frontmatter/YamlSerializer.cs

@@ -6,6 +6,7 @@
 
 using YamlDotNet.Core;
 using YamlDotNet.Serialization;
+using YamlDotNet.Serialization.EventEmitters;
 using YamlDotNet.Serialization.NamingConventions;
 
 /// <summary>
@@ -43,6 +44,7 @@
 		.WithNamingConvention(NullNamingConvention.Instance)
 		.ConfigureDefaultValuesHandling(DefaultValuesHandling.Preserve)
 		.WithQuotingNecessaryStrings()
+		.WithEventEmitter(next => new EdgeWhitespaceQuotingEventEmitter(next))
 		.Build();
 
 	/// <summary>
@@ -51,7 +53,7 @@
 	/// <param name="input">The YAML string to parse.</param>
 	/// <param name="result">When this method returns, contains the deserialized dictionary if parsing succeeded, or null if parsing failed.</param>
 	/// <returns>true if the YAML was successfully parsed; otherwise, false.</returns>
 	public static bool TryParseYamlObject(string input, [NotNullWhen(true)] out Dictionary<string, object>? result)
 	{
 		result = null;
 
@@ -214,6 +216,25 @@
 		public int ValueCount { get; set; }
 	}
 
+	/// <summary>
+	/// Double-quotes any string that starts or ends with whitespace. YAML strips that whitespace from a
+	/// plain scalar, and <c>WithQuotingNecessaryStrings</c> does not quote a string that ends in a tab, so
+	/// without this the value would come back changed on the next read.
+	/// </summary>
+	private sealed class EdgeWhitespaceQuotingEventEmitter(IEventEmitter nextEmitter) : ChainedEventEmitter(nextEmitter)
+	{
+		public override void Emit(ScalarEventInfo eventInfo, IEmitter emitter)
+		{
+			if (eventInfo.Source.Value is string { Length: > 0 } text
+				&& (char.IsWhiteSpace(text[0]) || char.IsWhiteSpace(text[^1])))
+			{
+				eventInfo.Style = ScalarStyle.DoubleQuoted;
+			}
+
+			base.Emit(eventInfo, emitter);
+		}
+	}
+
 	/// <summary>
 	/// Compares objects by reference. ReferenceEqualityComparer is not available on .NET Standard.
 	/// </summary>