Merge pull request #8 from rameel/fix/repeat-parser

Fix for infinite loop prevention

Author: rameel

src/Ramstack.Parsing/Parser.Repeat.cs

@@ -318,14 +318,27 @@ public override bool TryParse(ref ParseContext context, [NotNullWhen(true)] out
 
             do
             {
-                if (!parser.TryParse(ref context, out var result))
-                    break;
+                var last = context.Position;
 
-                // Prevent pointless loop with zero-width parser
-                if (context.MatchedSegment.Length == 0 && list.Count >= _min)
+                if (!parser.TryParse(ref context, out var result))
                     break;
 
                 list.Add(result);
+
+                //
+                // Prevent infinite loop
+                //
+                if (list.Count >= _min)
+                {
+                    //
+                    // Parsing failed in this case because:
+                    // 1. The parser matched, but the position remained unchanged.
+                    // 2. Rechecking would yield the same result, making it redundant.
+                    // 3. If a parser matches but the position remains unchanged, it results in an infinite loop.
+                    //
+                    if (context.Position == last)
+                        break;
+                }
             }
             while (list.Count < _max);
 
@@ -380,11 +393,14 @@ public override bool TryParse(ref ParseContext context, out Unit value)
 
             do
             {
+                var last = context.Position;
                 if (!parser.TryParse(ref context, out value))
                     break;
 
-                // Prevent pointless loop with zero-width parser
-                if (context.MatchedSegment.Length != 0)
+                //
+                // Prevent infinite loop
+                //
+                if (context.Position != last)
                     continue;
 
                 count = _min;

src/Ramstack.Parsing/Parser.Until.cs

@@ -58,24 +58,28 @@ public override bool TryParse(ref ParseContext context, [NotNullWhen(true)] out
                     return true;
                 }
 
+                var position = context.Position;
+
                 if (!parser.TryParse(ref context, out var result))
                     break;
 
-                // Prevent infinite loop with zero-width match
-                if (context.MatchedSegment.Length == 0)
+                list.Add(result);
+
+                //
+                // Prevent infinite loop
+                //
+                if (context.Position == position)
                 {
                     //
                     // Parsing failed in this case because:
-                    // 1. The main parser matched zero length, so the position remains unchanged.
+                    // 1. The main parser matched, but the position remained unchanged.
                     // 2. The terminator didn't match before, and it won't match now.
                     // 3. Rechecking would yield the same result, making it redundant.
-                    // 4. Without a terminator, parsing is considered unsuccessful, and it will never be matched now.
-                    // 5. With a zero-width match, it results in an infinite loop.
+                    // 4. Without a terminator, parsing is considered unsuccessful, and it will never match now.
+                    // 5. If a parser matches but the position remains unchanged, it results in an infinite loop.
                     //
                     break;
                 }
-
-                list.Add(result);
             }
 
             value = null;
@@ -115,19 +119,23 @@ public override bool TryParse(ref ParseContext context, out Unit value)
                     return true;
                 }
 
+                var position = context.Position;
+
                 if (!parser.TryParse(ref context, out value))
                     break;
 
-                // Prevent infinite loop with zero-width match
-                if (context.MatchedSegment.Length == 0)
+                //
+                // Prevent infinite loop
+                //
+                if (context.Position == position)
                 {
                     //
                     // Parsing failed in this case because:
-                    // 1. The main parser matched zero length, so the position remains unchanged.
+                    // 1. The main parser matched, but the position remained unchanged.
                     // 2. The terminator didn't match before, and it won't match now.
                     // 3. Rechecking would yield the same result, making it redundant.
-                    // 4. Without a terminator, parsing is considered unsuccessful, and it will never be matched now.
-                    // 5. With a zero-width match, it results in an infinite loop.
+                    // 4. Without a terminator, parsing is considered unsuccessful, and it will never match now.
+                    // 5. If a parser matches but the position remains unchanged, it results in an infinite loop.
                     //
                     break;
                 }

tests/Ramstack.Parsing.Tests/ParsersTests.Repeat.cs

@@ -51,4 +51,30 @@ public void Repeat_5_10_Test()
 
         Assert.That(parser.Parse("aaaa").Success, Is.False);
     }
+
+    [Test]
+    [SuppressMessage("ReSharper", "InconsistentNaming")]
+    public void Repeat_InfiniteLoopPrevention_ZeroLength()
+    {
+        var digit = Character.Digit;
+        var digit_list = digit.Separated(L(','));
+        var index_list = digit_list.Between(L('['), L(']')).Many();
+
+        Assert.That(index_list.Parse("[]").Length, Is.EqualTo(2));
+        Assert.That(index_list.Parse("[][]").Length, Is.EqualTo(4));
+        Assert.That(index_list.Parse("[][][]").Length, Is.EqualTo(6));
+
+        Assert.That(index_list.Parse("[1,2][1,2][2,6]").Length, Is.EqualTo(15));
+        Assert.That(index_list.Parse("[][][1,2][][][1,2][][][2,6][][]").Length, Is.EqualTo(31));
+    }
+
+    [Test]
+    public void Repeat_InfiniteLoopPrevention_ZeroConsuming()
+    {
+        var parser = And(Character.Digit).AtLeast(2);
+
+        Assert.That(parser.Parse("1234567890").Success, Is.True);
+        Assert.That(parser.Parse("1234567890").Length, Is.Zero);
+        Assert.That(parser.Text().Parse("1234567890").Value, Is.Empty);
+    }
 }