Merge pull request #6117 from bdach/beatmap-parsing-fallback-v2

Add fallback decoder option for badly-headered files

osu.Game.Rulesets.Osu.Tests/StackingTest.cs

@@ -7,6 +7,7 @@ using System.Linq;
 using System.Text;
 using NUnit.Framework;
 using osu.Game.Beatmaps;
+using osu.Game.IO;
 using osu.Game.Rulesets.Mods;
 using osu.Game.Rulesets.Osu.Objects;
 using osu.Game.Tests.Beatmaps;
@@ -21,7 +22,7 @@ namespace osu.Game.Rulesets.Osu.Tests
         public void TestStacking()
         {
             using (var stream = new MemoryStream(Encoding.UTF8.GetBytes(beatmap_data)))
-            using (var reader = new StreamReader(stream))
+            using (var reader = new LineBufferedReader(stream))
             {
                 var beatmap = Decoder.GetDecoder<Beatmap>(reader).Decode(reader);
                 var converted = new TestWorkingBeatmap(beatmap).GetPlayableBeatmap(new OsuRuleset().RulesetInfo, Array.Empty<Mod>());

osu.Game.Tests/Beatmaps/Formats/LegacyBeatmapDecoderTest.cs

@@ -13,6 +13,7 @@ using osu.Game.Beatmaps;
 using osu.Game.Rulesets.Objects.Types;
 using osu.Game.Beatmaps.Formats;
 using osu.Game.Beatmaps.Timing;
+using osu.Game.IO;
 using osu.Game.Rulesets.Catch.Beatmaps;
 using osu.Game.Rulesets.Mods;
 using osu.Game.Rulesets.Objects;
@@ -30,13 +31,9 @@ namespace osu.Game.Tests.Beatmaps.Formats
         public void TestDecodeBeatmapVersion()
         {
             using (var resStream = TestResources.OpenResource("beatmap-version.osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var decoder = Decoder.GetDecoder<Beatmap>(stream);
-
-                stream.BaseStream.Position = 0;
-                stream.DiscardBufferedData();
-
                 var working = new TestWorkingBeatmap(decoder.Decode(stream));
 
                 Assert.AreEqual(6, working.BeatmapInfo.BeatmapVersion);
@@ -51,7 +48,7 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LegacyBeatmapDecoder { ApplyOffsets = false };
 
             using (var resStream = TestResources.OpenResource("Soleily - Renatus (Gamu) [Insane].osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var beatmap = decoder.Decode(stream);
                 var beatmapInfo = beatmap.BeatmapInfo;
@@ -75,7 +72,7 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LegacyBeatmapDecoder();
 
             using (var resStream = TestResources.OpenResource("Soleily - Renatus (Gamu) [Insane].osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var beatmapInfo = decoder.Decode(stream).BeatmapInfo;
 
@@ -101,7 +98,7 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LegacyBeatmapDecoder();
 
             using (var resStream = TestResources.OpenResource("Soleily - Renatus (Gamu) [Insane].osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var beatmap = decoder.Decode(stream);
                 var beatmapInfo = beatmap.BeatmapInfo;
@@ -126,7 +123,7 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LegacyBeatmapDecoder();
 
             using (var resStream = TestResources.OpenResource("Soleily - Renatus (Gamu) [Insane].osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var difficulty = decoder.Decode(stream).BeatmapInfo.BaseDifficulty;
 
@@ -145,7 +142,7 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LegacyBeatmapDecoder { ApplyOffsets = false };
 
             using (var resStream = TestResources.OpenResource("Soleily - Renatus (Gamu) [Insane].osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var beatmap = decoder.Decode(stream);
                 var metadata = beatmap.Metadata;
@@ -164,7 +161,7 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LegacyBeatmapDecoder { ApplyOffsets = false };
 
             using (var resStream = TestResources.OpenResource("Soleily - Renatus (Gamu) [Insane].osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var beatmap = decoder.Decode(stream);
                 var controlPoints = beatmap.ControlPointInfo;
@@ -239,7 +236,7 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LegacyBeatmapDecoder { ApplyOffsets = false };
 
             using (var resStream = TestResources.OpenResource("overlapping-control-points.osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var controlPoints = decoder.Decode(stream).ControlPointInfo;
 
@@ -271,7 +268,7 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LegacySkinDecoder();
 
             using (var resStream = TestResources.OpenResource("Soleily - Renatus (Gamu) [Insane].osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var comboColors = decoder.Decode(stream).ComboColours;
 
@@ -297,7 +294,7 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LegacyBeatmapDecoder();
 
             using (var resStream = TestResources.OpenResource("hitobject-combo-offset.osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var beatmap = decoder.Decode(stream);
 
@@ -320,7 +317,7 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LegacyBeatmapDecoder();
 
             using (var resStream = TestResources.OpenResource("hitobject-combo-offset.osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var beatmap = decoder.Decode(stream);
 
@@ -343,7 +340,7 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LegacyBeatmapDecoder { ApplyOffsets = false };
 
             using (var resStream = TestResources.OpenResource("Soleily - Renatus (Gamu) [Insane].osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var hitObjects = decoder.Decode(stream).HitObjects;
 
@@ -371,7 +368,7 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LegacyBeatmapDecoder { ApplyOffsets = false };
 
             using (var resStream = TestResources.OpenResource("controlpoint-custom-samplebank.osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var hitObjects = decoder.Decode(stream).HitObjects;
 
@@ -393,7 +390,7 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LegacyBeatmapDecoder { ApplyOffsets = false };
 
             using (var resStream = TestResources.OpenResource("hitobject-custom-samplebank.osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var hitObjects = decoder.Decode(stream).HitObjects;
 
@@ -411,7 +408,7 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LegacyBeatmapDecoder { ApplyOffsets = false };
 
             using (var resStream = TestResources.OpenResource("hitobject-file-samples.osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var hitObjects = decoder.Decode(stream).HitObjects;
 
@@ -431,7 +428,7 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LegacyBeatmapDecoder { ApplyOffsets = false };
 
             using (var resStream = TestResources.OpenResource("slider-samples.osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var hitObjects = decoder.Decode(stream).HitObjects;
 
@@ -475,7 +472,7 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LegacyBeatmapDecoder { ApplyOffsets = false };
 
             using (var resStream = TestResources.OpenResource("hitobject-no-addition-bank.osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var hitObjects = decoder.Decode(stream).HitObjects;
 
@@ -489,10 +486,110 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LegacyBeatmapDecoder { ApplyOffsets = false };
 
             using (var badResStream = TestResources.OpenResource("invalid-events.osu"))
-            using (var badStream = new StreamReader(badResStream))
+            using (var badStream = new LineBufferedReader(badResStream))
             {
                 Assert.DoesNotThrow(() => decoder.Decode(badStream));
             }
         }
+
+        [Test]
+        public void TestFallbackDecoderForCorruptedHeader()
+        {
+            Decoder<Beatmap> decoder = null;
+            Beatmap beatmap = null;
+
+            using (var resStream = TestResources.OpenResource("corrupted-header.osu"))
+            using (var stream = new LineBufferedReader(resStream))
+            {
+                Assert.DoesNotThrow(() => decoder = Decoder.GetDecoder<Beatmap>(stream));
+                Assert.IsInstanceOf<LegacyBeatmapDecoder>(decoder);
+                Assert.DoesNotThrow(() => beatmap = decoder.Decode(stream));
+                Assert.IsNotNull(beatmap);
+                Assert.AreEqual("Beatmap with corrupted header", beatmap.Metadata.Title);
+                Assert.AreEqual("Evil Hacker", beatmap.Metadata.AuthorString);
+            }
+        }
+
+        [Test]
+        public void TestFallbackDecoderForMissingHeader()
+        {
+            Decoder<Beatmap> decoder = null;
+            Beatmap beatmap = null;
+
+            using (var resStream = TestResources.OpenResource("missing-header.osu"))
+            using (var stream = new LineBufferedReader(resStream))
+            {
+                Assert.DoesNotThrow(() => decoder = Decoder.GetDecoder<Beatmap>(stream));
+                Assert.IsInstanceOf<LegacyBeatmapDecoder>(decoder);
+                Assert.DoesNotThrow(() => beatmap = decoder.Decode(stream));
+                Assert.IsNotNull(beatmap);
+                Assert.AreEqual("Beatmap with no header", beatmap.Metadata.Title);
+                Assert.AreEqual("Incredibly Evil Hacker", beatmap.Metadata.AuthorString);
+            }
+        }
+
+        [Test]
+        public void TestDecodeFileWithEmptyLinesAtStart()
+        {
+            Decoder<Beatmap> decoder = null;
+            Beatmap beatmap = null;
+
+            using (var resStream = TestResources.OpenResource("empty-lines-at-start.osu"))
+            using (var stream = new LineBufferedReader(resStream))
+            {
+                Assert.DoesNotThrow(() => decoder = Decoder.GetDecoder<Beatmap>(stream));
+                Assert.IsInstanceOf<LegacyBeatmapDecoder>(decoder);
+                Assert.DoesNotThrow(() => beatmap = decoder.Decode(stream));
+                Assert.IsNotNull(beatmap);
+                Assert.AreEqual("Empty lines at start", beatmap.Metadata.Title);
+                Assert.AreEqual("Edge Case Hunter", beatmap.Metadata.AuthorString);
+            }
+        }
+
+        [Test]
+        public void TestDecodeFileWithEmptyLinesAndNoHeader()
+        {
+            Decoder<Beatmap> decoder = null;
+            Beatmap beatmap = null;
+
+            using (var resStream = TestResources.OpenResource("empty-line-instead-of-header.osu"))
+            using (var stream = new LineBufferedReader(resStream))
+            {
+                Assert.DoesNotThrow(() => decoder = Decoder.GetDecoder<Beatmap>(stream));
+                Assert.IsInstanceOf<LegacyBeatmapDecoder>(decoder);
+                Assert.DoesNotThrow(() => beatmap = decoder.Decode(stream));
+                Assert.IsNotNull(beatmap);
+                Assert.AreEqual("The dog ate the file header", beatmap.Metadata.Title);
+                Assert.AreEqual("Why does this keep happening", beatmap.Metadata.AuthorString);
+            }
+        }
+
+        [Test]
+        public void TestDecodeFileWithContentImmediatelyAfterHeader()
+        {
+            Decoder<Beatmap> decoder = null;
+            Beatmap beatmap = null;
+
+            using (var resStream = TestResources.OpenResource("no-empty-line-after-header.osu"))
+            using (var stream = new LineBufferedReader(resStream))
+            {
+                Assert.DoesNotThrow(() => decoder = Decoder.GetDecoder<Beatmap>(stream));
+                Assert.IsInstanceOf<LegacyBeatmapDecoder>(decoder);
+                Assert.DoesNotThrow(() => beatmap = decoder.Decode(stream));
+                Assert.IsNotNull(beatmap);
+                Assert.AreEqual("No empty line delimiting header from contents", beatmap.Metadata.Title);
+                Assert.AreEqual("Edge Case Hunter", beatmap.Metadata.AuthorString);
+            }
+        }
+
+        [Test]
+        public void TestDecodeEmptyFile()
+        {
+            using (var resStream = new MemoryStream())
+            using (var stream = new LineBufferedReader(resStream))
+            {
+                Assert.Throws<IOException>(() => Decoder.GetDecoder<Beatmap>(stream));
+            }
+        }
     }
 }

osu.Game.Tests/Beatmaps/Formats/LegacyDecoderTest.cs

@@ -2,9 +2,9 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System.Collections.Generic;
-using System.IO;
 using NUnit.Framework;
 using osu.Game.Beatmaps.Formats;
+using osu.Game.IO;
 using osu.Game.Tests.Resources;
 
 namespace osu.Game.Tests.Beatmaps.Formats
@@ -18,7 +18,7 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LineLoggingDecoder(14);
 
             using (var resStream = TestResources.OpenResource("comments.osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 decoder.Decode(stream);
 

osu.Game.Tests/Beatmaps/Formats/LegacyStoryboardDecoderTest.cs

@@ -1,12 +1,12 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-using System.IO;
 using System.Linq;
 using NUnit.Framework;
 using osuTK;
 using osu.Framework.Graphics;
 using osu.Game.Beatmaps.Formats;
+using osu.Game.IO;
 using osu.Game.Storyboards;
 using osu.Game.Tests.Resources;
 
@@ -21,7 +21,7 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LegacyStoryboardDecoder();
 
             using (var resStream = TestResources.OpenResource("Himeringo - Yotsuya-san ni Yoroshiku (RLC) [Winber1's Extreme].osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var storyboard = decoder.Decode(stream);
 
@@ -94,7 +94,7 @@ namespace osu.Game.Tests.Beatmaps.Formats
             var decoder = new LegacyStoryboardDecoder();
 
             using (var resStream = TestResources.OpenResource("variable-with-suffix.osb"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var storyboard = decoder.Decode(stream);
 

osu.Game.Tests/Beatmaps/Formats/OsuJsonDecoderTest.cs

@@ -8,6 +8,7 @@ using NUnit.Framework;
 using osu.Game.Audio;
 using osu.Game.Beatmaps;
 using osu.Game.Beatmaps.Formats;
+using osu.Game.IO;
 using osu.Game.IO.Serialization;
 using osu.Game.Rulesets.Objects.Types;
 using osu.Game.Rulesets.Scoring;
@@ -148,13 +149,13 @@ namespace osu.Game.Tests.Beatmaps.Formats
         private Beatmap decode(string filename, out Beatmap jsonDecoded)
         {
             using (var stream = TestResources.OpenResource(filename))
-            using (var sr = new StreamReader(stream))
+            using (var sr = new LineBufferedReader(stream))
             {
                 var legacyDecoded = new LegacyBeatmapDecoder { ApplyOffsets = false }.Decode(sr);
 
                 using (var ms = new MemoryStream())
                 using (var sw = new StreamWriter(ms))
-                using (var sr2 = new StreamReader(ms))
+                using (var sr2 = new LineBufferedReader(ms))
                 {
                     sw.Write(legacyDecoded.Serialize());
                     sw.Flush();

osu.Game.Tests/Beatmaps/IO/ImportBeatmapTest.cs

@@ -171,7 +171,7 @@ namespace osu.Game.Tests.Beatmaps.IO
 
                     var breakTemp = TestResources.GetTestBeatmapForImport();
 
-                    MemoryStream brokenOsu = new MemoryStream(new byte[] { 1, 3, 3, 7 });
+                    MemoryStream brokenOsu = new MemoryStream();
                     MemoryStream brokenOsz = new MemoryStream(File.ReadAllBytes(breakTemp));
 
                     File.Delete(breakTemp);

osu.Game.Tests/Beatmaps/IO/LineBufferedReaderTest.cs

@@ -0,0 +1,133 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System.IO;
+using System.Text;
+using NUnit.Framework;
+using osu.Game.IO;
+
+namespace osu.Game.Tests.Beatmaps.IO
+{
+    [TestFixture]
+    public class LineBufferedReaderTest
+    {
+        [Test]
+        public void TestReadLineByLine()
+        {
+            const string contents = @"line 1
+line 2
+line 3";
+
+            using (var stream = new MemoryStream(Encoding.UTF8.GetBytes(contents)))
+            using (var bufferedReader = new LineBufferedReader(stream))
+            {
+                Assert.AreEqual("line 1", bufferedReader.ReadLine());
+                Assert.AreEqual("line 2", bufferedReader.ReadLine());
+                Assert.AreEqual("line 3", bufferedReader.ReadLine());
+                Assert.IsNull(bufferedReader.ReadLine());
+            }
+        }
+
+        [Test]
+        public void TestPeekLineOnce()
+        {
+            const string contents = @"line 1
+peek this
+line 3";
+
+            using (var stream = new MemoryStream(Encoding.UTF8.GetBytes(contents)))
+            using (var bufferedReader = new LineBufferedReader(stream))
+            {
+                Assert.AreEqual("line 1", bufferedReader.ReadLine());
+                Assert.AreEqual("peek this", bufferedReader.PeekLine());
+                Assert.AreEqual("peek this", bufferedReader.ReadLine());
+                Assert.AreEqual("line 3", bufferedReader.ReadLine());
+                Assert.IsNull(bufferedReader.ReadLine());
+            }
+        }
+
+        [Test]
+        public void TestPeekLineMultipleTimes()
+        {
+            const string contents = @"peek this once
+line 2
+peek this a lot";
+
+            using (var stream = new MemoryStream(Encoding.UTF8.GetBytes(contents)))
+            using (var bufferedReader = new LineBufferedReader(stream))
+            {
+                Assert.AreEqual("peek this once", bufferedReader.PeekLine());
+                Assert.AreEqual("peek this once", bufferedReader.ReadLine());
+                Assert.AreEqual("line 2", bufferedReader.ReadLine());
+                Assert.AreEqual("peek this a lot", bufferedReader.PeekLine());
+                Assert.AreEqual("peek this a lot", bufferedReader.PeekLine());
+                Assert.AreEqual("peek this a lot", bufferedReader.PeekLine());
+                Assert.AreEqual("peek this a lot", bufferedReader.ReadLine());
+                Assert.IsNull(bufferedReader.ReadLine());
+            }
+        }
+
+        [Test]
+        public void TestPeekLineAtEndOfStream()
+        {
+            const string contents = @"first line
+second line";
+
+            using (var stream = new MemoryStream(Encoding.UTF8.GetBytes(contents)))
+            using (var bufferedReader = new LineBufferedReader(stream))
+            {
+                Assert.AreEqual("first line", bufferedReader.ReadLine());
+                Assert.AreEqual("second line", bufferedReader.ReadLine());
+                Assert.IsNull(bufferedReader.PeekLine());
+                Assert.IsNull(bufferedReader.ReadLine());
+                Assert.IsNull(bufferedReader.PeekLine());
+            }
+        }
+
+        [Test]
+        public void TestPeekReadLineOnEmptyStream()
+        {
+            using (var stream = new MemoryStream())
+            using (var bufferedReader = new LineBufferedReader(stream))
+            {
+                Assert.IsNull(bufferedReader.PeekLine());
+                Assert.IsNull(bufferedReader.ReadLine());
+                Assert.IsNull(bufferedReader.ReadLine());
+                Assert.IsNull(bufferedReader.PeekLine());
+            }
+        }
+
+        [Test]
+        public void TestReadToEndNoPeeks()
+        {
+            const string contents = @"first line
+second line";
+
+            using (var stream = new MemoryStream(Encoding.UTF8.GetBytes(contents)))
+            using (var bufferedReader = new LineBufferedReader(stream))
+            {
+                Assert.AreEqual(contents, bufferedReader.ReadToEnd());
+            }
+        }
+
+        [Test]
+        public void TestReadToEndAfterReadsAndPeeks()
+        {
+            const string contents = @"this line is gone
+this one shouldn't be
+these ones
+definitely not";
+
+            using (var stream = new MemoryStream(Encoding.UTF8.GetBytes(contents)))
+            using (var bufferedReader = new LineBufferedReader(stream))
+            {
+                Assert.AreEqual("this line is gone", bufferedReader.ReadLine());
+                Assert.AreEqual("this one shouldn't be", bufferedReader.PeekLine());
+                const string ending = @"this one shouldn't be
+these ones
+definitely not";
+                Assert.AreEqual(ending, bufferedReader.ReadToEnd());
+            }
+        }
+    }
+}

osu.Game.Tests/Beatmaps/IO/OszArchiveReaderTest.cs

@@ -7,6 +7,7 @@ using NUnit.Framework;
 using osu.Game.Beatmaps;
 using osu.Game.Tests.Resources;
 using osu.Game.Beatmaps.Formats;
+using osu.Game.IO;
 using osu.Game.IO.Archives;
 
 namespace osu.Game.Tests.Beatmaps.IO
@@ -50,7 +51,7 @@ namespace osu.Game.Tests.Beatmaps.IO
 
                 Beatmap beatmap;
 
-                using (var stream = new StreamReader(reader.GetStream("Soleily - Renatus (Deif) [Platter].osu")))
+                using (var stream = new LineBufferedReader(reader.GetStream("Soleily - Renatus (Deif) [Platter].osu")))
                     beatmap = Decoder.GetDecoder<Beatmap>(stream).Decode(stream);
 
                 var meta = beatmap.Metadata;

osu.Game.Tests/Resources/corrupted-header.osu

@@ -0,0 +1,5 @@
+ow computerosu file format v14
+
+[Metadata]
+Title: Beatmap with corrupted header
+Creator: Evil Hacker

osu.Game.Tests/Resources/empty-line-instead-of-header.osu

@@ -0,0 +1,5 @@
+
+
+[Metadata]
+Title: The dog ate the file header
+Creator: Why does this keep happening

osu.Game.Tests/Resources/empty-lines-at-start.osu

@@ -0,0 +1,8 @@
+
+
+
+osu file format v14
+
+[Metadata]
+Title: Empty lines at start
+Creator: Edge Case Hunter

osu.Game.Tests/Resources/missing-header.osu

@@ -0,0 +1,4 @@
+[Metadata]
+
+Title: Beatmap with no header
+Creator: Incredibly Evil Hacker

osu.Game.Tests/Resources/no-empty-line-after-header.osu

@@ -0,0 +1,4 @@
+osu file format v14
+[Metadata]
+Title: No empty line delimiting header from contents
+Creator: Edge Case Hunter

osu.Game.Tests/Skins/LegacySkinDecoderTest.cs

@@ -2,8 +2,8 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System.Collections.Generic;
-using System.IO;
 using NUnit.Framework;
+using osu.Game.IO;
 using osu.Game.Skinning;
 using osu.Game.Tests.Resources;
 using osuTK.Graphics;
@@ -20,7 +20,7 @@ namespace osu.Game.Tests.Skins
             var decoder = new LegacySkinDecoder();
 
             using (var resStream = TestResources.OpenResource(hasColours ? "skin.ini" : "skin-empty.ini"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var comboColors = decoder.Decode(stream).ComboColours;
 
@@ -48,7 +48,7 @@ namespace osu.Game.Tests.Skins
             var decoder = new LegacySkinDecoder();
 
             using (var resStream = TestResources.OpenResource("skin.ini"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var config = decoder.Decode(stream);
 

osu.Game.Tests/WaveformTestBeatmap.cs

@@ -9,6 +9,7 @@ using osu.Framework.Graphics.Textures;
 using osu.Framework.Graphics.Video;
 using osu.Game.Beatmaps;
 using osu.Game.Beatmaps.Formats;
+using osu.Game.IO;
 using osu.Game.IO.Archives;
 using osu.Game.Tests.Resources;
 
@@ -56,7 +57,7 @@ namespace osu.Game.Tests
         private Beatmap createTestBeatmap()
         {
             using (var beatmapStream = getBeatmapStream())
-            using (var beatmapReader = new StreamReader(beatmapStream))
+            using (var beatmapReader = new LineBufferedReader(beatmapStream))
                 return Decoder.GetDecoder<Beatmap>(beatmapReader).Decode(beatmapReader);
         }
     }

osu.Game/Beatmaps/BeatmapManager.cs

@@ -20,6 +20,7 @@ using osu.Framework.Platform;
 using osu.Framework.Threading;
 using osu.Game.Beatmaps.Formats;
 using osu.Game.Database;
+using osu.Game.IO;
 using osu.Game.IO.Archives;
 using osu.Game.Online.API;
 using osu.Game.Online.API.Requests;
@@ -264,7 +265,7 @@ namespace osu.Game.Beatmaps
             }
 
             Beatmap beatmap;
-            using (var stream = new StreamReader(reader.GetStream(mapName)))
+            using (var stream = new LineBufferedReader(reader.GetStream(mapName)))
                 beatmap = Decoder.GetDecoder<Beatmap>(stream).Decode(stream);
 
             return new BeatmapSetInfo
@@ -287,7 +288,7 @@ namespace osu.Game.Beatmaps
             {
                 using (var raw = Files.Store.GetStream(file.FileInfo.StoragePath))
                 using (var ms = new MemoryStream()) //we need a memory stream so we can seek
-                using (var sr = new StreamReader(ms))
+                using (var sr = new LineBufferedReader(ms))
                 {
                     raw.CopyTo(ms);
                     ms.Position = 0;

osu.Game/Beatmaps/BeatmapManager_WorkingBeatmap.cs

@@ -2,7 +2,6 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
-using System.IO;
 using System.Linq;
 using osu.Framework.Audio;
 using osu.Framework.Audio.Track;
@@ -11,6 +10,7 @@ using osu.Framework.Graphics.Video;
 using osu.Framework.IO.Stores;
 using osu.Framework.Logging;
 using osu.Game.Beatmaps.Formats;
+using osu.Game.IO;
 using osu.Game.Skinning;
 using osu.Game.Storyboards;
 
@@ -33,7 +33,7 @@ namespace osu.Game.Beatmaps
             {
                 try
                 {
-                    using (var stream = new StreamReader(store.GetStream(getPathForFile(BeatmapInfo.Path))))
+                    using (var stream = new LineBufferedReader(store.GetStream(getPathForFile(BeatmapInfo.Path))))
                         return Decoder.GetDecoder<Beatmap>(stream).Decode(stream);
                 }
                 catch
@@ -127,7 +127,7 @@ namespace osu.Game.Beatmaps
 
                 try
                 {
-                    using (var stream = new StreamReader(store.GetStream(getPathForFile(BeatmapInfo.Path))))
+                    using (var stream = new LineBufferedReader(store.GetStream(getPathForFile(BeatmapInfo.Path))))
                     {
                         var decoder = Decoder.GetDecoder<Storyboard>(stream);
 
@@ -136,7 +136,7 @@ namespace osu.Game.Beatmaps
                             storyboard = decoder.Decode(stream);
                         else
                         {
-                            using (var secondaryStream = new StreamReader(store.GetStream(getPathForFile(BeatmapSetInfo.StoryboardFile))))
+                            using (var secondaryStream = new LineBufferedReader(store.GetStream(getPathForFile(BeatmapSetInfo.StoryboardFile))))
                                 storyboard = decoder.Decode(stream, secondaryStream);
                         }
                     }

osu.Game/Beatmaps/Formats/Decoder.cs

@@ -5,6 +5,7 @@ using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
+using osu.Game.IO;
 
 namespace osu.Game.Beatmaps.Formats
 {
@@ -13,20 +14,21 @@ namespace osu.Game.Beatmaps.Formats
     {
         protected virtual TOutput CreateTemplateObject() => new TOutput();
 
-        public TOutput Decode(StreamReader primaryStream, params StreamReader[] otherStreams)
+        public TOutput Decode(LineBufferedReader primaryStream, params LineBufferedReader[] otherStreams)
         {
             var output = CreateTemplateObject();
-            foreach (StreamReader stream in otherStreams.Prepend(primaryStream))
+            foreach (LineBufferedReader stream in otherStreams.Prepend(primaryStream))
                 ParseStreamInto(stream, output);
             return output;
         }
 
-        protected abstract void ParseStreamInto(StreamReader stream, TOutput output);
+        protected abstract void ParseStreamInto(LineBufferedReader stream, TOutput output);
     }
 
     public abstract class Decoder
     {
         private static readonly Dictionary<Type, Dictionary<string, Func<string, Decoder>>> decoders = new Dictionary<Type, Dictionary<string, Func<string, Decoder>>>();
+        private static readonly Dictionary<Type, Func<Decoder>> fallback_decoders = new Dictionary<Type, Func<Decoder>>();
 
         static Decoder()
         {
@@ -39,7 +41,7 @@ namespace osu.Game.Beatmaps.Formats
         /// Retrieves a <see cref="Decoder"/> to parse a <see cref="Beatmap"/>.
         /// </summary>
         /// <param name="stream">A stream pointing to the <see cref="Beatmap"/>.</param>
-        public static Decoder<T> GetDecoder<T>(StreamReader stream)
+        public static Decoder<T> GetDecoder<T>(LineBufferedReader stream)
             where T : new()
         {
             if (stream == null)
@@ -48,21 +50,31 @@ namespace osu.Game.Beatmaps.Formats
             if (!decoders.TryGetValue(typeof(T), out var typedDecoders))
                 throw new IOException(@"Unknown decoder type");
 
-            string line;
+            // start off with the first line of the file
+            string line = stream.PeekLine()?.Trim();
 
-            do
+            while (line != null && line.Length == 0)
             {
-                line = stream.ReadLine()?.Trim();
-            } while (line != null && line.Length == 0);
+                // consume the previously peeked empty line and advance to the next one
+                stream.ReadLine();
+                line = stream.PeekLine()?.Trim();
+            }
 
             if (line == null)
-                throw new IOException(@"Unknown file format (null)");
+                throw new IOException("Unknown file format (null)");
 
             var decoder = typedDecoders.Select(d => line.StartsWith(d.Key, StringComparison.InvariantCulture) ? d.Value : null).FirstOrDefault();
-            if (decoder == null)
-                throw new IOException($@"Unknown file format ({line})");
 
-            return (Decoder<T>)decoder.Invoke(line);
+            // it's important the magic does NOT get consumed here, since sometimes it's part of the structure
+            // (see JsonBeatmapDecoder - the magic string is the opening brace)
+            // decoder implementations should therefore not die on receiving their own magic
+            if (decoder != null)
+                return (Decoder<T>)decoder.Invoke(line);
+
+            if (!fallback_decoders.TryGetValue(typeof(T), out var fallbackDecoder))
+                throw new IOException($"Unknown file format ({line})");
+
+            return (Decoder<T>)fallbackDecoder.Invoke();
         }
 
         /// <summary>
@@ -77,5 +89,19 @@ namespace osu.Game.Beatmaps.Formats
 
             typedDecoders[magic] = constructor;
         }
+
+        /// <summary>
+        /// Registers a fallback decoder instantiation function.
+        /// The fallback will be returned if the first non-empty line of the decoded stream does not match any known magic.
+        /// </summary>
+        /// <typeparam name="T">Type of object being decoded.</typeparam>
+        /// <param name="constructor">A function that constructs the fallback<see cref="Decoder"/>.</param>
+        protected static void SetFallbackDecoder<T>(Func<Decoder> constructor)
+        {
+            if (fallback_decoders.ContainsKey(typeof(T)))
+                throw new InvalidOperationException($"A fallback decoder was already added for type {typeof(T)}.");
+
+            fallback_decoders[typeof(T)] = constructor;
+        }
     }
 }

osu.Game/Beatmaps/Formats/JsonBeatmapDecoder.cs

@@ -1,7 +1,7 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-using System.IO;
+using osu.Game.IO;
 using osu.Game.IO.Serialization;
 
 namespace osu.Game.Beatmaps.Formats
@@ -13,11 +13,8 @@ namespace osu.Game.Beatmaps.Formats
             AddDecoder<Beatmap>("{", m => new JsonBeatmapDecoder());
         }
 
-        protected override void ParseStreamInto(StreamReader stream, Beatmap output)
+        protected override void ParseStreamInto(LineBufferedReader stream, Beatmap output)
         {
-            stream.BaseStream.Position = 0;
-            stream.DiscardBufferedData();
-
             stream.ReadToEnd().DeserializeInto(output);
 
             foreach (var hitObject in output.HitObjects)

osu.Game/Beatmaps/Formats/LegacyBeatmapDecoder.cs

@@ -8,6 +8,7 @@ using osu.Framework.IO.File;
 using osu.Game.Beatmaps.Timing;
 using osu.Game.Rulesets.Objects.Legacy;
 using osu.Game.Beatmaps.ControlPoints;
+using osu.Game.IO;
 
 namespace osu.Game.Beatmaps.Formats
 {
@@ -25,6 +26,7 @@ namespace osu.Game.Beatmaps.Formats
         public static void Register()
         {
             AddDecoder<Beatmap>(@"osu file format v", m => new LegacyBeatmapDecoder(Parsing.ParseInt(m.Split('v').Last())));
+            SetFallbackDecoder<Beatmap>(() => new LegacyBeatmapDecoder());
         }
 
         /// <summary>
@@ -41,7 +43,7 @@ namespace osu.Game.Beatmaps.Formats
             offset = FormatVersion < 5 ? 24 : 0;
         }
 
-        protected override void ParseStreamInto(StreamReader stream, Beatmap beatmap)
+        protected override void ParseStreamInto(LineBufferedReader stream, Beatmap beatmap)
         {
             this.beatmap = beatmap;
             this.beatmap.BeatmapInfo.BeatmapVersion = FormatVersion;

osu.Game/Beatmaps/Formats/LegacyDecoder.cs

@@ -3,10 +3,10 @@
 
 using System;
 using System.Collections.Generic;
-using System.IO;
 using osu.Framework.Logging;
 using osu.Game.Audio;
 using osu.Game.Beatmaps.ControlPoints;
+using osu.Game.IO;
 using osuTK.Graphics;
 
 namespace osu.Game.Beatmaps.Formats
@@ -21,7 +21,7 @@ namespace osu.Game.Beatmaps.Formats
             FormatVersion = version;
         }
 
-        protected override void ParseStreamInto(StreamReader stream, T output)
+        protected override void ParseStreamInto(LineBufferedReader stream, T output)
         {
             Section section = Section.None;
 

osu.Game/Beatmaps/Formats/LegacyDifficultyCalculatorBeatmapDecoder.cs

@@ -24,6 +24,7 @@ namespace osu.Game.Beatmaps.Formats
         public new static void Register()
         {
             AddDecoder<Beatmap>(@"osu file format v", m => new LegacyDifficultyCalculatorBeatmapDecoder(int.Parse(m.Split('v').Last())));
+            SetFallbackDecoder<Beatmap>(() => new LegacyDifficultyCalculatorBeatmapDecoder());
         }
 
         protected override TimingControlPoint CreateTimingControlPoint()

osu.Game/Beatmaps/Formats/LegacyStoryboardDecoder.cs

@@ -10,6 +10,7 @@ using osuTK;
 using osuTK.Graphics;
 using osu.Framework.Graphics;
 using osu.Framework.IO.File;
+using osu.Game.IO;
 using osu.Game.Storyboards;
 
 namespace osu.Game.Beatmaps.Formats
@@ -33,9 +34,10 @@ namespace osu.Game.Beatmaps.Formats
             // note that this isn't completely correct
             AddDecoder<Storyboard>(@"osu file format v", m => new LegacyStoryboardDecoder());
             AddDecoder<Storyboard>(@"[Events]", m => new LegacyStoryboardDecoder());
+            SetFallbackDecoder<Storyboard>(() => new LegacyStoryboardDecoder());
         }
 
-        protected override void ParseStreamInto(StreamReader stream, Storyboard storyboard)
+        protected override void ParseStreamInto(LineBufferedReader stream, Storyboard storyboard)
         {
             this.storyboard = storyboard;
             base.ParseStreamInto(stream, storyboard);

osu.Game/IO/LineBufferedReader.cs

@@ -0,0 +1,72 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Text;
+
+namespace osu.Game.IO
+{
+    /// <summary>
+    /// A <see cref="StreamReader"/>-like decorator (with more limited API) for <see cref="Stream"/>s
+    /// that allows lines to be peeked without consuming.
+    /// </summary>
+    public class LineBufferedReader : IDisposable
+    {
+        private readonly StreamReader streamReader;
+        private readonly Queue<string> lineBuffer;
+
+        public LineBufferedReader(Stream stream)
+        {
+            streamReader = new StreamReader(stream);
+            lineBuffer = new Queue<string>();
+        }
+
+        /// <summary>
+        /// Reads the next line from the stream without consuming it.
+        /// Subsequent calls to <see cref="PeekLine"/> without a <see cref="ReadLine"/> will return the same string.
+        /// </summary>
+        public string PeekLine()
+        {
+            if (lineBuffer.Count > 0)
+                return lineBuffer.Peek();
+
+            var line = streamReader.ReadLine();
+            if (line != null)
+                lineBuffer.Enqueue(line);
+            return line;
+        }
+
+        /// <summary>
+        /// Reads the next line from the stream and consumes it.
+        /// If a line was peeked, that same line will then be consumed and returned.
+        /// </summary>
+        public string ReadLine() => lineBuffer.Count > 0 ? lineBuffer.Dequeue() : streamReader.ReadLine();
+
+        /// <summary>
+        /// Reads the stream to its end and returns the text read.
+        /// This includes any peeked but unconsumed lines.
+        /// </summary>
+        public string ReadToEnd()
+        {
+            var remainingText = streamReader.ReadToEnd();
+            if (lineBuffer.Count == 0)
+                return remainingText;
+
+            var builder = new StringBuilder();
+
+            // this might not be completely correct due to varying platform line endings
+            while (lineBuffer.Count > 0)
+                builder.AppendLine(lineBuffer.Dequeue());
+            builder.Append(remainingText);
+
+            return builder.ToString();
+        }
+
+        public void Dispose()
+        {
+            streamReader?.Dispose();
+        }
+    }
+}

osu.Game/Skinning/LegacySkin.cs

@@ -12,6 +12,7 @@ using osu.Framework.Graphics;
 using osu.Framework.Graphics.Textures;
 using osu.Framework.IO.Stores;
 using osu.Game.Audio;
+using osu.Game.IO;
 using osu.Game.Rulesets.Scoring;
 using osuTK.Graphics;
 
@@ -35,7 +36,7 @@ namespace osu.Game.Skinning
         {
             Stream stream = storage?.GetStream(filename);
             if (stream != null)
-                using (StreamReader reader = new StreamReader(stream))
+                using (LineBufferedReader reader = new LineBufferedReader(stream))
                     Configuration = new LegacySkinDecoder().Decode(reader);
             else
                 Configuration = new DefaultSkinConfiguration();

osu.Game/Tests/Beatmaps/BeatmapConversionTest.cs

@@ -13,6 +13,7 @@ using osu.Framework.Graphics.Textures;
 using osu.Framework.Graphics.Video;
 using osu.Game.Beatmaps;
 using osu.Game.Beatmaps.Formats;
+using osu.Game.IO;
 using osu.Game.Rulesets;
 using osu.Game.Rulesets.Mods;
 using osu.Game.Rulesets.Objects;
@@ -142,7 +143,7 @@ namespace osu.Game.Tests.Beatmaps
         private IBeatmap getBeatmap(string name)
         {
             using (var resStream = openResource($"{resource_namespace}.{name}.osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var decoder = Decoder.GetDecoder<Beatmap>(stream);
                 ((LegacyBeatmapDecoder)decoder).ApplyOffsets = false;

osu.Game/Tests/Beatmaps/DifficultyCalculatorTest.cs

@@ -7,6 +7,7 @@ using System.Reflection;
 using NUnit.Framework;
 using osu.Game.Beatmaps;
 using osu.Game.Beatmaps.Formats;
+using osu.Game.IO;
 using osu.Game.Rulesets;
 using osu.Game.Rulesets.Difficulty;
 using osu.Game.Rulesets.Mods;
@@ -26,7 +27,7 @@ namespace osu.Game.Tests.Beatmaps
         private WorkingBeatmap getBeatmap(string name)
         {
             using (var resStream = openResource($"{resource_namespace}.{name}.osu"))
-            using (var stream = new StreamReader(resStream))
+            using (var stream = new LineBufferedReader(resStream))
             {
                 var decoder = Decoder.GetDecoder<Beatmap>(stream);
                 ((LegacyBeatmapDecoder)decoder).ApplyOffsets = false;

osu.Game/Tests/Beatmaps/TestBeatmap.cs

@@ -5,6 +5,7 @@ using System.Collections.Generic;
 using System.IO;
 using System.Text;
 using osu.Game.Beatmaps;
+using osu.Game.IO;
 using osu.Game.Rulesets;
 using Decoder = osu.Game.Beatmaps.Formats.Decoder;
 
@@ -39,7 +40,7 @@ namespace osu.Game.Tests.Beatmaps
         private static Beatmap createTestBeatmap()
         {
             using (var stream = new MemoryStream(Encoding.UTF8.GetBytes(test_beatmap_data)))
-            using (var reader = new StreamReader(stream))
+            using (var reader = new LineBufferedReader(stream))
                 return Decoder.GetDecoder<Beatmap>(reader).Decode(reader);
         }