From e2841b3fd4183c2d7031c15531490a26006a5dee Mon Sep 17 00:00:00 2001
From: LTRData <ol@ltr-data.se>
Date: Tue, 12 Dec 2023 17:14:08 +0100
Subject: [PATCH] Optimized file streams for read-only file systems

---
 .../Internals/MasterFileTableEntry.cs         |  2 +-
 Library/DiscUtils.Ntfs/NtfsFileStream.cs      | 46 ++++++++++++++++---
 Library/DiscUtils.Ntfs/NtfsFileSystem.cs      |  7 ++-
 3 files changed, 47 insertions(+), 8 deletions(-)

diff --git a/Library/DiscUtils.Ntfs/Internals/MasterFileTableEntry.cs b/Library/DiscUtils.Ntfs/Internals/MasterFileTableEntry.cs
index cceff29bc..e11180672 100644
--- a/Library/DiscUtils.Ntfs/Internals/MasterFileTableEntry.cs
+++ b/Library/DiscUtils.Ntfs/Internals/MasterFileTableEntry.cs
@@ -193,7 +193,7 @@ public SparseStream Open(IAttributeLocator attr, FileAccess access)
             return null;
         }
 
-        return new NtfsFileStream(File, directoryEntry.Value, attr.AttributeType, attr.Identifier, access);
+        return NtfsFileStream.Open(File, directoryEntry.Value, attr.AttributeType, attr.Identifier, access);
     }
 
     public IEnumerable<Range<long, long>> GetClusters(IAttributeLocator attr) =>
diff --git a/Library/DiscUtils.Ntfs/NtfsFileStream.cs b/Library/DiscUtils.Ntfs/NtfsFileStream.cs
index 84a708630..3420589fb 100644
--- a/Library/DiscUtils.Ntfs/NtfsFileStream.cs
+++ b/Library/DiscUtils.Ntfs/NtfsFileStream.cs
@@ -32,26 +32,60 @@ namespace DiscUtils.Ntfs;
 internal sealed class NtfsFileStream : SparseStream
 {
     private SparseStream _baseStream;
+
+    public SparseStream BaseStream => _baseStream;
+
     private readonly DirectoryEntry _entry;
 
     private readonly File _file;
 
     private bool _isDirty;
 
-    public NtfsFileStream(File file, DirectoryEntry entry, AttributeType attrType, string attrName,
+    public static SparseStream Open(File file, DirectoryEntry entry, AttributeType attrType, string attrName,
                           FileAccess access)
+    {
+        var baseStream = file.OpenStream(attrType, attrName, access);
+
+        if (baseStream is null)
+        {
+            return null;
+        }
+
+        if (file.Context.ReadOnly)
+        {
+            return baseStream;
+        }
+        else
+        {
+            return new NtfsFileStream(entry, file, baseStream);
+        }
+    }
+
+    private NtfsFileStream(DirectoryEntry entry, File file, SparseStream baseStream)
     {
         _entry = entry;
         _file = file;
-        _baseStream = _file.OpenStream(attrType, attrName, access);
+        _baseStream = baseStream;
     }
 
-    public NtfsFileStream(File file, DirectoryEntry entry, AttributeType attrType, ushort attrId,
+    public static SparseStream Open(File file, DirectoryEntry entry, AttributeType attrType, ushort attrId,
                           FileAccess access)
     {
-        _entry = entry;
-        _file = file;
-        _baseStream = _file.OpenStream(attrId, attrType, access);
+        var baseStream = file.OpenStream(attrId, attrType, access);
+
+        if (baseStream is null)
+        {
+            return null;
+        }
+
+        if (file.Context.ReadOnly)
+        {
+            return baseStream;
+        }
+        else
+        {
+            return new NtfsFileStream(entry, file, baseStream);
+        }
     }
 
     public override bool CanRead
diff --git a/Library/DiscUtils.Ntfs/NtfsFileSystem.cs b/Library/DiscUtils.Ntfs/NtfsFileSystem.cs
index fb66615c7..e0b168327 100644
--- a/Library/DiscUtils.Ntfs/NtfsFileSystem.cs
+++ b/Library/DiscUtils.Ntfs/NtfsFileSystem.cs
@@ -2055,7 +2055,12 @@ public SparseStream OpenFile(string path, FileMode mode, FileAccess access, NewF
                 }
             }
 
-            var stream = new NtfsFileStream(file, entry.Value, attributeType, attributeName, access);
+            var stream = NtfsFileStream.Open(file, entry.Value, attributeType, attributeName, access);
+
+            if (stream is null)
+            {
+                throw new InvalidFileSystemException("File system corrupt");
+            }
 
             if (mode is FileMode.Create or FileMode.Truncate)
             {