types: NewValidatorSet doesn't panic on empty valz list (#2938)

* types: NewValidatorSet doesn't panic on empty valz list

* changelog

CHANGELOG_PENDING.md

@@ -30,3 +30,5 @@ program](https://hackerone.com/tendermint).
 - [consensus] [\#2871](https://github.com/tendermint/tendermint/issues/2871) Remove *ProposalHeartbeat* infrastructure as it serves no real purpose
 
 ### BUG FIXES:
+- [types] \#2938 Fix regression in v0.26.4 where we panic on empty
+  genDoc.Validators

types/validator_set.go

@@ -29,10 +29,10 @@ type ValidatorSet struct {
 	totalVotingPower int64
 }
 
+// NewValidatorSet initializes a ValidatorSet by copying over the
+// values from `valz`, a list of Validators. If valz is nil or empty,
+// the new ValidatorSet will have an empty list of Validators.
 func NewValidatorSet(valz []*Validator) *ValidatorSet {
-	if valz != nil && len(valz) == 0 {
-		panic("validator set initialization slice cannot be an empty slice (but it can be nil)")
-	}
 	validators := make([]*Validator, len(valz))
 	for i, val := range valz {
 		validators[i] = val.Copy()

types/validator_set_test.go

@@ -17,9 +17,12 @@ import (
 )
 
 func TestValidatorSetBasic(t *testing.T) {
-	assert.Panics(t, func() { NewValidatorSet([]*Validator{}) })
+	// empty or nil validator lists are allowed,
+	// but attempting to IncrementAccum on them will panic.
+	vset := NewValidatorSet([]*Validator{})
+	assert.Panics(t, func() { vset.IncrementAccum(1) })
 
-	vset := NewValidatorSet(nil)
+	vset = NewValidatorSet(nil)
 	assert.Panics(t, func() { vset.IncrementAccum(1) })
 
 	assert.EqualValues(t, vset, vset.Copy())