Fix IdSpan equality violation: Create("") and default now correctly unequal

[Copilot]

IdSpan.Create(string.Empty) and default(IdSpan) were equal despite having different hash codes, violating the fundamental contract that equal objects must have equal hash codes.

Root cause: Equals used Span<byte>.SequenceEqual which treats null.AsSpan() and Array.Empty<byte>().AsSpan() as equal.

Example of the bug:

IdSpan a = default;                    // _value is null, hashCode is 0
IdSpan b = IdSpan.Create("");          // _value is byte[0], hashCode is 90033154

a == b;              // Was: true, Now: false ✓
a.GetHashCode() == b.GetHashCode();    // Was: false (contract violation!)

Changes

• IdSpan.Equals: Added explicit null checks before SequenceEqual comparison. Two IdSpans are equal only if both have null _value, both have the same reference, or both have non-null arrays with identical byte sequences.

• IdSpan.CompareTo: Added null handling to maintain consistent ordering: null < empty array < non-empty arrays.

• Tests: Added IdSpanTests.cs with coverage for equality, hash code consistency, and comparison edge cases.

Note: IdSpan.IsDefault intentionally returns true for both null and empty arrays (logical emptiness), but they remain unequal (different internal representation).

Original prompt

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This section details on the original issue you should resolve

<issue_title>IdSpan.Create(string.Empty) and IdSpan = default are equal when the hashcode says they are not</issue_title>
<issue_description>Orleans 9.2.1

   IdSpan defaultGrainId = IdSpan.Create(string.Empty);
   IdSpan emptyGrainId = default;

   Assert.That(defaultGrainId, Is.EqualTo(emptyGrainId));                             // Pass
   Assert.That(defaultGrainId, Is.EqualTo(emptyGrainId));                             // Pass
   Assert.That(defaultGrainId.GetHashCode(), Is.EqualTo(emptyGrainId.GetHashCode())); // Fail

Seems to be a violation of the principle:

A hash function must have the following properties:

If two objects compare as equal, the GetHashCode() method for each object must return the same value. However, if two objects do not compare as equal, the GetHashCode() methods for the two objects do not have to return different values.

https://learn.microsoft.com/en-us/dotnet/api/system.object.gethashcode?view=net-10.0

It looks to me like the problem is actually in the IdSpan.Equals() method
public bool Equals(IdSpan obj) => _value == obj._value || _value.AsSpan().SequenceEqual(obj._value);

In the defaultGrainId case IdSpan._value is byte[0]
In the emptyGrainId case IdSpan._value is null

This I think is the fundamental issue

  Span<byte> nullByteArraySpan = ((byte[]?)null).AsSpan();
  var emptyByteArraySpan = Array.Empty<byte>();
  Assert.That(nullByteArraySpan.SequenceEqual(emptyByteArraySpan), Is.True); // Pass

It's possible IdSpan.CompareTo and other methods that rely on _value.AsSpan() might have similar behaviour.

</issue_description>

<agent_instructions>IdSpan.Create("") and default(IdSpan) should not be equal - fix the equality check</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@ReubenBond We need to make some change here: either `default(IdSpan)` and `IdSpan.Create("")` are equivalent (`a == b` implies `a.GetHashCode() == b.GetHashCode()`) or to make them not equal. I recall we block `string.Empty` at higher layers of the stack but don't block it at lower layers (so that existing serialization methods will continue working).

We should make them not equatable, it seems.</comment_new>

• Fixes IdSpan.Create(string.Empty) and IdSpan = default are equal when the hashcode says they are not #9853

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Microsoft Reviewers: Open in CodeFlow

[pentp]

Such overly verbose comments add no value.

[Copilot]

Pull request overview

This PR fixes a correctness issue in IdSpan by ensuring default(IdSpan) (null backing array) is no longer considered equal to IdSpan.Create("") (empty backing array), which previously violated the Equals/GetHashCode contract.

Changes:

• Update IdSpan.Equals(IdSpan) to treat null and empty arrays as unequal.
• Update IdSpan.CompareTo(IdSpan) to provide consistent ordering when either side has a null backing array.
• Add IdSpanTests covering equality, hash code consistency, and comparison edge cases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/Orleans.Core.Abstractions/IDs/IdSpan.cs	Fixes equality and comparison behavior for null vs empty backing arrays.
test/NonSilo.Tests/IdSpanTests.cs	Adds regression tests for the equality/hash-code contract and comparison behavior.

[Copilot]

This assertion bakes in an implementation detail that the stable hash of an empty byte array is non-zero. To make the test resilient to future hash algorithm changes while still validating the contract, consider asserting that IdSpan.Create(string.Empty) has a different hash code than default(IdSpan) (and/or that a.Equals(b) implies equal hash codes), rather than asserting != 0 specifically.

Suggested change

	Assert.True(emptyStringIdSpan.IsDefault);
	Assert.Equal("", emptyStringIdSpan.ToString());
	// Hash code should be computed from empty byte array, not 0
	Assert.NotEqual(0, emptyStringIdSpan.GetHashCode());
	IdSpan defaultIdSpan = default;
	Assert.True(emptyStringIdSpan.IsDefault);
	Assert.Equal("", emptyStringIdSpan.ToString());
	// Hash code should differ from default(IdSpan) while still representing an "empty" value
	Assert.NotEqual(defaultIdSpan.GetHashCode(), emptyStringIdSpan.GetHashCode());

[Copilot]

IdSpanCodec currently serializes based on value.AsSpan(). Since AsSpan() returns an empty span for both _value == null and _value.Length == 0, an IdSpan created from "" (empty byte[]) will be serialized the same as default and will deserialize back to default. With the new equality semantics (null != empty), that means serialization no longer round-trips the value. Consider updating IdSpanCodec to encode empty arrays distinctly from null/default (and add a regression test for that).