Add MaxDepth property to CborReader

[bartonjs]

Background and motivation

CBOR is structured like JSON, meaning that skipping over an array or map requires processing all of the elements within that array or map to find where the container ends. This means that a deeply nested array-or-map can use a surprising amount of memory on a skip operation.

Also, deeply nested data is always risky for recursive algorithms.

API Proposal

• A new default maxDepth parameter to the ctor.
• A new ctor that has no defaults (per the more-than-2-defaults guideline)
• A new MaxDepth property

namespace System.Formats.Cbor
{
    public partial class CborReader
    {
        public CborReader(System.ReadOnlyMemory<byte> data);
        public CborReader(System.ReadOnlyMemory<byte> data, System.Formats.Cbor.CborConformanceMode conformanceMode = System.Formats.Cbor.CborConformanceMode.Strict, bool allowMultipleRootLevelValues = false, int maxDepth = 0);
    }
}

And hide the existing ctor with defaults:

namespace System.Formats.Cbor
{
    public partial class CborReader
    {
+       [System.ComponentModel.EditorBrowsableAttribute(System.ComponentModel.EditorBrowsableState.Never)]
        public CborReader(System.ReadOnlyMemory<byte> data, System.Formats.Cbor.CborConformanceMode conformanceMode, bool allowMultipleRootLevelValues) { }
    }
}

API Usage

var reader = new CborReader({ 0x81, 0x81, 0x00 }, maxDepth: 1);

reader.ReadStartArray();

// throws, too deep
reader.ReadStartArray();

Alternative Designs

Decide that 3 options is enough to warrant an options type:

namespace System.Formats.Cbor
{
    public struct CborReaderOptions
    {
        public bool AllowMultipleRootLevelValues { get; set; }
        public CborConformanceMode ConformanceMode { get; set; }
        public int MaxDepth { get; set; }
    }

    public partial class CborReader
    {
        public CborReader(System.ReadOnlyMemory<byte> data, CborReaderOptions options);
    }
}

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[vcsjones]

bool allowMultipleRootLevelValues = false, int maxDepth = 0

Is it time to think about CborReaderOptions?

[bartonjs]

Is it time to think about CborReaderOptions?

Yeah, probably. I'll doodle that one up.

[eiriktsarpalis]

We might want a max depth on the write side too, to guard against potential serialization of cyclic structures.