feat: add /r/sys/validators (gnolang#2130)

## Description

This PR introduces an initial validator set implementation in Gno (realm
based), as outlined in gnolang#1824.

It introduces a Proof of Contribution validator set management
mechanism, based on govdao proposals.
Related PRs:
- gnolang#1945 
- gnolang#2344 

I've left the door open to arbitrary protocol implementations.

~I've also added 2 example implementations:~
- ~PoS (Proof of Stake) - users can stake funds (`ugnot`) to become part
of the on-chain validator set~
- ~PoA (Proof of Authority) - new validators need to be voted in by the
majority of the existing validator set~

Update: I've moved the example PoS + PoA implementation to another
unrelated PR, since they are out of scope

Closes gnolang#1824

<details><summary>Contributors' checklist...</summary>

- [x] Added new tests, or not needed, or not feasible
- [x] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [x] Updated the official documentation or not needed
- [x] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [x] Added references to related issues and PRs
- [ ] Provided any useful hints for running manual tests
- [ ] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](https://github.com/gnolang/gno/blob/master/.benchmarks/README.md).
</details>

---------

Co-authored-by: Manfred Touron <94029+moul@users.noreply.github.com>

examples/gno.land/p/nt/poa/gno.mod

@@ -0,0 +1,10 @@
+module gno.land/p/nt/poa
+
+require (
+	gno.land/p/demo/avl v0.0.0-latest
+	gno.land/p/demo/testutils v0.0.0-latest
+	gno.land/p/demo/uassert v0.0.0-latest
+	gno.land/p/demo/ufmt v0.0.0-latest
+	gno.land/p/demo/urequire v0.0.0-latest
+	gno.land/p/sys/validators v0.0.0-latest
+)

examples/gno.land/p/nt/poa/option.gno

@@ -0,0 +1,14 @@
+package poa
+
+import "gno.land/p/sys/validators"
+
+type Option func(*PoA)
+
+// WithInitialSet sets the initial PoA validator set
+func WithInitialSet(validators []validators.Validator) Option {
+	return func(p *PoA) {
+		for _, validator := range validators {
+			p.validators.Set(validator.Address.String(), validator)
+		}
+	}
+}

examples/gno.land/p/nt/poa/poa.gno

@@ -0,0 +1,106 @@
+package poa
+
+import (
+	"errors"
+	"std"
+
+	"gno.land/p/demo/avl"
+	"gno.land/p/sys/validators"
+)
+
+var ErrInvalidVotingPower = errors.New("invalid voting power")
+
+// PoA specifies the Proof of Authority validator set, with simple add / remove constraints.
+//
+// To add:
+// - proposed validator must not be part of the set already
+// - proposed validator voting power must be > 0
+//
+// To remove:
+// - proposed validator must be part of the set already
+type PoA struct {
+	validators *avl.Tree // std.Address -> validators.Validator
+}
+
+// NewPoA creates a new empty Proof of Authority validator set
+func NewPoA(opts ...Option) *PoA {
+	// Create the empty set
+	p := &PoA{
+		validators: avl.NewTree(),
+	}
+
+	// Apply the options
+	for _, opt := range opts {
+		opt(p)
+	}
+
+	return p
+}
+
+func (p *PoA) AddValidator(address std.Address, pubKey string, power uint64) (validators.Validator, error) {
+	// Validate that the operation is a valid call.
+	// Check if the validator is already in the set
+	if p.IsValidator(address) {
+		return validators.Validator{}, validators.ErrValidatorExists
+	}
+
+	// Make sure the voting power > 0
+	if power == 0 {
+		return validators.Validator{}, ErrInvalidVotingPower
+	}
+
+	v := validators.Validator{
+		Address:     address,
+		PubKey:      pubKey, // TODO: in the future, verify the public key
+		VotingPower: power,
+	}
+
+	// Add the validator to the set
+	p.validators.Set(address.String(), v)
+
+	return v, nil
+}
+
+func (p *PoA) RemoveValidator(address std.Address) (validators.Validator, error) {
+	// Validate that the operation is a valid call
+	// Fetch the validator
+	validator, err := p.GetValidator(address)
+	if err != nil {
+		return validators.Validator{}, err
+	}
+
+	// Remove the validator from the set
+	p.validators.Remove(address.String())
+
+	return validator, nil
+}
+
+func (p *PoA) IsValidator(address std.Address) bool {
+	_, exists := p.validators.Get(address.String())
+
+	return exists
+}
+
+func (p *PoA) GetValidator(address std.Address) (validators.Validator, error) {
+	validatorRaw, exists := p.validators.Get(address.String())
+	if !exists {
+		return validators.Validator{}, validators.ErrValidatorMissing
+	}
+
+	validator := validatorRaw.(validators.Validator)
+
+	return validator, nil
+}
+
+func (p *PoA) GetValidators() []validators.Validator {
+	vals := make([]validators.Validator, 0, p.validators.Size())
+
+	p.validators.Iterate("", "", func(_ string, value interface{}) bool {
+		validator := value.(validators.Validator)
+		vals = append(vals, validator)
+
+		return false
+	})
+
+	return vals
+}

examples/gno.land/p/nt/poa/poa_test.gno

@@ -0,0 +1,237 @@
+package poa
+
+import (
+	"testing"
+
+	"gno.land/p/demo/testutils"
+	"gno.land/p/demo/uassert"
+	"gno.land/p/demo/urequire"
+	"gno.land/p/sys/validators"
+
+	"gno.land/p/demo/ufmt"
+)
+
+// generateTestValidators generates a dummy validator set
+func generateTestValidators(count int) []validators.Validator {
+	vals := make([]validators.Validator, 0, count)
+
+	for i := 0; i < count; i++ {
+		val := validators.Validator{
+			Address:     testutils.TestAddress(ufmt.Sprintf("%d", i)),
+			PubKey:      "public-key",
+			VotingPower: 1,
+		}
+
+		vals = append(vals, val)
+	}
+
+	return vals
+}
+
+func TestPoA_AddValidator_Invalid(t *testing.T) {
+	t.Parallel()
+
+	t.Run("validator already in set", func(t *testing.T) {
+		t.Parallel()
+
+		var (
+			proposalAddress = testutils.TestAddress("caller")
+			proposalKey     = "public-key"
+
+			initialSet = generateTestValidators(1)
+		)
+
+		initialSet[0].Address = proposalAddress
+		initialSet[0].PubKey = proposalKey
+
+		// Create the protocol with an initial set
+		p := NewPoA(WithInitialSet(initialSet))
+
+		// Attempt to add the validator
+		_, err := p.AddValidator(proposalAddress, proposalKey, 1)
+		uassert.ErrorIs(t, err, validators.ErrValidatorExists)
+	})
+
+	t.Run("invalid voting power", func(t *testing.T) {
+		t.Parallel()
+
+		var (
+			proposalAddress = testutils.TestAddress("caller")
+			proposalKey     = "public-key"
+		)
+
+		// Create the protocol with no initial set
+		p := NewPoA()
+
+		// Attempt to add the validator
+		_, err := p.AddValidator(proposalAddress, proposalKey, 0)
+		uassert.ErrorIs(t, err, ErrInvalidVotingPower)
+	})
+}
+
+func TestPoA_AddValidator(t *testing.T) {
+	t.Parallel()
+
+	var (
+		proposalAddress = testutils.TestAddress("caller")
+		proposalKey     = "public-key"
+	)
+
+	// Create the protocol with no initial set
+	p := NewPoA()
+
+	// Attempt to add the validator
+	_, err := p.AddValidator(proposalAddress, proposalKey, 1)
+	uassert.NoError(t, err)
+
+	// Make sure the validator is added
+	if !p.IsValidator(proposalAddress) || p.validators.Size() != 1 {
+		t.Fatal("address is not validator")
+	}
+}
+
+func TestPoA_RemoveValidator_Invalid(t *testing.T) {
+	t.Parallel()
+
+	t.Run("proposed removal not in set", func(t *testing.T) {
+		t.Parallel()
+
+		var (
+			proposalAddress = testutils.TestAddress("caller")
+			initialSet      = generateTestValidators(1)
+		)
+
+		initialSet[0].Address = proposalAddress
+
+		// Create the protocol with an initial set
+		p := NewPoA(WithInitialSet(initialSet))
+
+		// Attempt to remove the validator
+		_, err := p.RemoveValidator(testutils.TestAddress("totally random"))
+		uassert.ErrorIs(t, err, validators.ErrValidatorMissing)
+	})
+}
+
+func TestPoA_RemoveValidator(t *testing.T) {
+	t.Parallel()
+
+	var (
+		proposalAddress = testutils.TestAddress("caller")
+		initialSet      = generateTestValidators(1)
+	)
+
+	initialSet[0].Address = proposalAddress
+
+	// Create the protocol with an initial set
+	p := NewPoA(WithInitialSet(initialSet))
+
+	// Attempt to remove the validator
+	_, err := p.RemoveValidator(proposalAddress)
+	urequire.NoError(t, err)
+
+	// Make sure the validator is removed
+	if p.IsValidator(proposalAddress) || p.validators.Size() != 0 {
+		t.Fatal("address is validator")
+	}
+}
+
+func TestPoA_GetValidator(t *testing.T) {
+	t.Parallel()
+
+	t.Run("validator not in set", func(t *testing.T) {
+		t.Parallel()
+
+		// Create the protocol with no initial set
+		p := NewPoA()
+
+		// Attempt to get the voting power
+		_, err := p.GetValidator(testutils.TestAddress("caller"))
+		uassert.ErrorIs(t, err, validators.ErrValidatorMissing)
+	})
+
+	t.Run("validator fetched", func(t *testing.T) {
+		t.Parallel()
+
+		var (
+			address     = testutils.TestAddress("caller")
+			pubKey      = "public-key"
+			votingPower = uint64(10)
+
+			initialSet = generateTestValidators(1)
+		)
+
+		initialSet[0].Address = address
+		initialSet[0].PubKey = pubKey
+		initialSet[0].VotingPower = votingPower
+
+		// Create the protocol with an initial set
+		p := NewPoA(WithInitialSet(initialSet))
+
+		// Get the validator
+		val, err := p.GetValidator(address)
+		urequire.NoError(t, err)
+
+		// Validate the address
+		if val.Address != address {
+			t.Fatal("invalid address")
+		}
+
+		// Validate the voting power
+		if val.VotingPower != votingPower {
+			t.Fatal("invalid voting power")
+		}
+
+		// Validate the public key
+		if val.PubKey != pubKey {
+			t.Fatal("invalid public key")
+		}
+	})
+}
+
+func TestPoA_GetValidators(t *testing.T) {
+	t.Parallel()
+
+	t.Run("empty set", func(t *testing.T) {
+		t.Parallel()
+
+		// Create the protocol with no initial set
+		p := NewPoA()
+
+		// Attempt to get the voting power
+		vals := p.GetValidators()
+
+		if len(vals) != 0 {
+			t.Fatal("validator set is not empty")
+		}
+	})
+
+	t.Run("validator set fetched", func(t *testing.T) {
+		t.Parallel()
+
+		initialSet := generateTestValidators(10)
+
+		// Create the protocol with an initial set
+		p := NewPoA(WithInitialSet(initialSet))
+
+		// Get the validator set
+		vals := p.GetValidators()
+
+		if len(vals) != len(initialSet) {
+			t.Fatal("returned validator set mismatch")
+		}
+
+		for _, val := range vals {
+			for _, initialVal := range initialSet {
+				if val.Address != initialVal.Address {
+					continue
+				}
+
+				// Validate the voting power
+				uassert.Equal(t, val.VotingPower, initialVal.VotingPower)
+
+				// Validate the public key
+				uassert.Equal(t, val.PubKey, initialVal.PubKey)
+			}
+		}
+	})
+}

examples/gno.land/p/sys/validators/gno.mod

@@ -0,0 +1 @@
+module gno.land/p/sys/validators