Fix data corruption from length prefix mismatch in ReadBytesAt

SharpCoreDB/DataStructures/Table.cs

@@ -193,15 +193,44 @@ private List<Dictionary<string, object>> SelectInternal(string? where, string? o
                 using var reader = new BinaryReader(ms);
                 while (ms.Position < ms.Length)
                 {
-                    var row = new Dictionary<string, object>();
-                    bool valid = true;
-                    for (int i = 0; i < this.Columns.Count; i++)
+                    try
                     {
-                        try { row[this.Columns[i]] = this.ReadTypedValue(reader, this.ColumnTypes[i]); }
-                        catch { valid = false; break; }
+                        // Read the length prefix that was written by AppendBytes
+                        int recordLength = reader.ReadInt32();
+
+                        // Validate length to prevent issues with corrupted data
+                        if (recordLength < 0 || recordLength > data.Length)
+                        {
+                            break; // Corrupted record, stop scanning
+                        }
+
+                        // Read the record data
+                        var recordData = reader.ReadBytes(recordLength);
+
+                        // Deserialize the record
+                        using var recordMs = new MemoryStream(recordData);
+                        using var recordReader = new BinaryReader(recordMs);
+
+                        var row = new Dictionary<string, object>();
+                        bool valid = true;
+                        for (int i = 0; i < this.Columns.Count; i++)
+                        {
+                            try { row[this.Columns[i]] = this.ReadTypedValue(recordReader, this.ColumnTypes[i]); }
+                            catch { valid = false; break; }
+                        }
+                        if (valid && (string.IsNullOrEmpty(where) || EvaluateWhere(row, where)))
+                            results.Add(row);
+                    }
+                    catch (EndOfStreamException)
+                    {
+                        // Reached end of file
+                        break;
+                    }
+                    catch (Exception)
+                    {
+                        // Corrupted record, skip and continue
+                        break;
                     }
-                    if (valid && (string.IsNullOrEmpty(where) || EvaluateWhere(row, where)))
-                        results.Add(row);
                 }
             }
         }

SharpCoreDB/Services/Storage.cs

@@ -199,16 +199,24 @@ public long AppendBytes(string path, byte[] data)
 
         using var fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read);
         fs.Seek(position, SeekOrigin.Begin);
-        var buffer = new byte[maxLength];
-        int bytesRead = fs.Read(buffer, 0, maxLength);
-        if (bytesRead == 0)
+        using var reader = new BinaryReader(fs);
+
+        // Read the length prefix that was written by AppendBytes
+        int length = reader.ReadInt32();
+
+        // Validate length to prevent excessive memory allocation
+        if (length < 0 || length > maxLength * 10) // Allow some buffer but prevent abuse
         {
-            return null;
+            throw new InvalidDataException($"Invalid record length: {length}");
         }
-
-        if (bytesRead < maxLength)
+
+        // Read the actual data (maxLength is for the data portion, not including prefix)
+        int bytesToRead = Math.Min(length, maxLength);
+        var buffer = reader.ReadBytes(bytesToRead);
+
+        if (buffer.Length == 0)
         {
-            Array.Resize(ref buffer, bytesRead);
+            return null;
         }
 
         var effectiveNoEncrypt = noEncrypt || this.noEncryption;