Bound the regex and glob caches

TextFilter.Test/TextFilterTests.cs

@@ -666,4 +666,53 @@ public void AnOrdinaryRegexIsUnaffectedByTheTimeout()
 		Assert.IsTrue(TextFilter.IsMatch("hello world", "^hello", TextFilterType.Regex, TextFilterMatchOptions.ByWholeString));
 		Assert.IsFalse(TextFilter.IsMatch("hello world", "^goodbye", TextFilterType.Regex, TextFilterMatchOptions.ByWholeString));
 	}
+
+	[TestMethod]
+	public void TheRegexCacheStaysBoundedAsDistinctPatternsArrive()
+	{
+		// The keystroke case from the report: every prefix a user types is a distinct key. Feeding
+		// more distinct patterns than the cap must not leave more than the cap cached.
+		for (int i = 0; i < (TextFilter.MaxCacheEntries * 2); i++)
+		{
+			TextFilter.IsMatch("hello world", $"^p{i}x", TextFilterType.Regex, TextFilterMatchOptions.ByWholeString);
+		}
+
+		Assert.IsLessThanOrEqualTo(
+			TextFilter.MaxCacheEntries,
+			TextFilter.RegexCacheCount,
+			"The regex cache should be bounded, not grow with every pattern ever seen.");
+	}
+
+	[TestMethod]
+	public void TheGlobCacheStaysBoundedAsDistinctPatternsArrive()
+	{
+		for (int i = 0; i < (TextFilter.MaxCacheEntries * 2); i++)
+		{
+			TextFilter.IsMatch("hello world", $"*q{i}z*", TextFilterType.Glob, TextFilterMatchOptions.ByWholeString);
+		}
+
+		Assert.IsLessThanOrEqualTo(
+			TextFilter.MaxCacheEntries,
+			TextFilter.GlobCacheCount,
+			"The glob cache should be bounded, not grow with every pattern ever seen.");
+	}
+
+	[TestMethod]
+	public void ACachedPatternStillMatchesAfterTheCacheHasReset()
+	{
+		// Guards the over-correction: a bound that dropped correctness rather than entries. The same
+		// pattern must answer identically before and after enough churn to force a reset.
+		const string pattern = "^hello";
+		Assert.IsTrue(TextFilter.IsMatch("hello world", pattern, TextFilterType.Regex, TextFilterMatchOptions.ByWholeString));
+
+		for (int i = 0; i < (TextFilter.MaxCacheEntries + 1); i++)
+		{
+			TextFilter.IsMatch("hello world", $"^r{i}y", TextFilterType.Regex, TextFilterMatchOptions.ByWholeString);
+		}
+
+		Assert.IsTrue(
+			TextFilter.IsMatch("hello world", pattern, TextFilterType.Regex, TextFilterMatchOptions.ByWholeString),
+			"A pattern evicted by the bound should simply be recompiled, not answer differently.");
+		Assert.IsFalse(TextFilter.IsMatch("hello world", "^goodbye", TextFilterType.Regex, TextFilterMatchOptions.ByWholeString));
+	}
 }

TextFilter/TextFilter.cs

@@ -84,6 +84,31 @@
 	private static ConcurrentDictionary<string, Regex> RegexCache { get; } = [];
 	private static ConcurrentDictionary<string, Glob> GlobCache { get; } = [];
 
+	// Filter patterns are caller-supplied, and in the keystroke-driven filter box this library exists
+	// for, every prefix of what the user types becomes its own key. Unbounded, the caches therefore
+	// grow with everything ever typed for the lifetime of the process, rather than with the size of
+	// the data being filtered.
+	internal const int MaxCacheEntries = 4096;
+
+	internal static int RegexCacheCount => RegexCache.Count;
+
+	internal static int GlobCacheCount => GlobCache.Count;
+
+	// Clear-and-restart rather than LRU: ConcurrentDictionary keeps no eviction order, and tracking
+	// one would put a write on every cache *hit* — the path the cache exists to keep cheap. This
+	// only ever runs on a miss, where a compile is about to happen anyway, so the Count (which does
+	// take the dictionary's locks) is off the hot path. At this cap a reset is rare, and costs one
+	// recompile per pattern still in use.
+	private static void AddBounded<TValue>(ConcurrentDictionary<string, TValue> cache, string cacheKey, TValue value)
+	{
+		if (cache.Count >= MaxCacheEntries)
+		{
+			cache.Clear();
+		}
+
+		cache.TryAdd(cacheKey, value);
+	}
+
 	// Both caches are keyed by pattern text, so the same pattern compiled at two sensitivities would
 	// otherwise collide on the first one cached. The sensitivity is folded into the key rather than
 	// using a tuple key, which netstandard2.0 does not get for free.
@@ -98,7 +123,7 @@
 	private static readonly TimeSpan RegexMatchTimeout = TimeSpan.FromSeconds(1);
 
 	[System.Diagnostics.CodeAnalysis.SuppressMessage("Performance", "SYSLIB1045:Convert to 'GeneratedRegexAttribute'.", Justification = "Not available in older frameworks")]
 	private static Regex RegexMatchAnything() => new(".*", RegexOptions.Compiled);
 
 	/// <summary>
 	/// Gets a hint for the specified filter type.
@@ -250,7 +275,7 @@
 
 			return ExcludedTokenPrefixes.Contains(prefix)
 				? TextFilterTokenType.Excluded
 				: RequiredTokenPrefixes.Contains(prefix)
 				? TextFilterTokenType.Required
 				: TextFilterTokenType.Optional;
 		})
@@ -379,7 +404,7 @@
 				? Glob.Parse(filterToken, CaseInsensitiveGlobOptions)
 				: Glob.Parse(filterToken);
 
-			GlobCache.TryAdd(cacheKey, glob);
+			AddBounded(GlobCache, cacheKey, glob);
 		}
 
 		return glob;
@@ -423,7 +448,7 @@
 				regex = RegexMatchAnything();
 			}
 
-			RegexCache.TryAdd(cacheKey, regex);
+			AddBounded(RegexCache, cacheKey, regex);
 		}
 
 		Func<IEnumerable<string>, Func<string, bool>, bool> matchFunc = textFilterMatchOptions is TextFilterMatchOptions.ByWordAny