fix: types: ensure .Amount is non-nil in Coin.Validate()

This change fixes a scenario in which Coin.Validate() would
panic when given a nil Amount. While here, added a fuzz test
along with unit/regression tests.

Fixes #15690

CHANGELOG.md

@@ -166,6 +166,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ### Bug Fixes
 
+* (types) [#15691](https://github.com/cosmos/cosmos-sdk/pull/15691) Make Coin.Validate() check that .Amount is not nil
 * (types) [#15433](https://github.com/cosmos/cosmos-sdk/pull/15433) Allow disabling of account address caches (for printing bech32 account addresses).
 * (x/auth) [#15059](https://github.com/cosmos/cosmos-sdk/pull/15059) `ante.CountSubKeys` returns 0 when passing a nil `Pubkey`.
 * (x/capability) [#15030](https://github.com/cosmos/cosmos-sdk/pull/15030) Prevent `x/capability` from consuming `GasMeter` gas during `InitMemStore`

types/coin.go

@@ -2,6 +2,7 @@ package types
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"regexp"
 	"sort"
@@ -44,6 +45,10 @@ func (coin Coin) Validate() error {
 		return err
 	}
 
+	if coin.Amount.IsNil() {
+		return errors.New("amount is nil")
+	}
+
 	if coin.Amount.IsNegative() {
 		return fmt.Errorf("negative coin amount: %v", coin.Amount)
 	}

types/coin_test.go

@@ -1276,6 +1276,40 @@ func (s *coinTestSuite) TestMarshalJSONCoins() {
 	}
 }
 
+func (s *coinTestSuite) TestCoinValidate() {
+	testCases := []struct {
+		name    string
+		coin    sdk.Coin
+		wantErr string
+	}{
+		{"nil coin: nil Amount", sdk.Coin{}, "invalid denom"},
+		{"non-blank coin, nil Amount", sdk.Coin{Denom: "atom"}, "nil Amount"},
+		{"valid coin", sdk.Coin{Denom: "atom", Amount: math.NewInt(100)}, ""},
+		{"valid coin negative", sdk.Coin{Denom: "atom", Amount: math.NewInt(-999)}, ""},
+	}
+
+	for _, tc := range testCases {
+		tc := tc
+		t := s.T()
+		t.Run(tc.name, func(t *testing.T) {
+			err := tc.coin.Validate()
+			if tc.wantErr == "" {
+				if err != nil {
+					t.Errorf("Unexpected error: %v", err)
+				}
+				return
+			} else {
+				if err == nil {
+					t.Error("Expected an error")
+				} else if !strings.Contains(err.Error(), tc.wantErr) {
+					t.Errorf("Error mismatch\n\tGot:  %q\nWant: %q", err, tc.wantErr)
+				}
+			}
+		})
+	}
+
+}
+
 func (s *coinTestSuite) TestCoinAminoEncoding() {
 	cdc := codec.NewLegacyAmino()
 	c := sdk.NewInt64Coin(testDenom1, 5)

types/fuzz_test.go

@@ -0,0 +1,24 @@
+package types
+
+import (
+	"testing"
+
+	"github.com/cosmos/cosmos-sdk/codec"
+)
+
+func FuzzCoinUnmarshalJSON(f *testing.F) {
+	if testing.Short() {
+		f.Skip()
+	}
+
+	cdc := codec.NewLegacyAmino()
+	f.Add(`{"denom":"atom","amount":"1000"}`)
+	f.Add(`{"denom":"atom","amount":"-1000"}`)
+	f.Add(`{"denom":"uatom","amount":"1000111111111111111111111"}`)
+	f.Add(`{"denom":"mu","amount":"0"}`)
+
+	f.Fuzz(func(t *testing.T, jsonBlob string) {
+		var c Coin
+		_ = cdc.UnmarshalJSON([]byte(jsonBlob), &c)
+	})
+}