balancer: provides resolver.State Attributes in PickerBuildInfo

[nexcode]

In gRPC you can create manual Resolver for manual name resolution management. This resolver has an UpdateState method, which receives a State object. This object has an Attributes field, which is used to pass various parameters to load balancers.

The PickerBuilder interface contains a build method that receives a PickerBuildInfo object containing connection information, but lacking the Attributes that are passed when updating the resolver state.

PickerBuildInfo now contains a new Attributes field:

// PickerBuildInfo contains information needed by the picker builder to
// construct a picker.
type PickerBuildInfo struct {
	// ReadySCs is a map from all ready SubConns to the Addresses used to
	// create them.
	ReadySCs map[balancer.SubConn]SubConnInfo

	// Attributes contains arbitrary data about the resolver intended for
	// consumption by the load balancing policy. This is a pointer to resolver.State Attributes.
	Attributes *attributes.Attributes
}

Now when implementing the PickerBuilder interface you can get these Attributes (and based on their values, configure the load balancer's operating logic):

type pickerBuilder struct{}

func (*pickerBuilder) Build(pickerInfo base.PickerBuildInfo) balancer.Picker {
	myKey := pickerInfo.Attributes.Value("myKey")
	...
}

This change does not change any external APIs and is fully backwards compatible.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: nexcode / name: Oleg Trifonov (c3e965c)

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (master@7472d57). Learn more about missing BASE report.
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
balancer/base/balancer.go	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #8696   +/-   ##
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  Coverage          ?   83.24%           
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  Files             ?      416           
  Lines             ?    32304           
  Branches          ?        0           
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  Hits              ?    26892           
  Misses            ?     4027           
  Partials          ?     1385           

Files with missing lines	Coverage Δ
balancer/base/base.go	0.00% <ø> (ø)
balancer/base/balancer.go	51.37% <90.00%> (ø)

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[Pranjali-2501]

Hi @nexcode,

Thank you for raising this PR and for your contribution!

We generally prefer to discuss new features or standard API changes in a GitHub issue—or via the gRFC for larger, cross-language proposals—before they are implemented in a PR. This helps us discuss the design, necessity, and potential advantages of proposed changes first.

Could you please raise an issue/gRFC detailing the need for this change and the use cases it enables?

Thanks.