[C&P] Fix Unbonding slashed suppliers (#891)

## Summary

This PR fixes a few bugs that were dependent to each other:
1. Gracefully unbond suppliers that have 0upokt due to off-stake
slashing.
2. Fix access to an expired session tree on the realy miner
3. Fix proof block hash seed used on chain.

@okdas , One case I didn't manage to reproduce is having the application
module account go to `0`. Please tell me if you encounter it again.

## Issue

- #841 

## Type of change

Select one or more from the following:

- [ ] New feature, functionality or library
- [ ] Consensus breaking; add the `consensus-breaking` label if so. See
#791 for details
- [x] Bug fix
- [ ] Code health or cleanup
- [ ] Documentation
- [ ] Other (specify)

## Testing

- [ ] **Documentation**: `make docusaurus_start`; only needed if you
make doc changes
- [x] **Unit Tests**: `make go_develop_and_test`
- [x] **LocalNet E2E Tests**: `make test_e2e`
- [ ] **DevNet E2E Tests**: Add the `devnet-test-e2e` label to the PR.

## Sanity Checklist

- [x] I have tested my changes using the available tooling
- [x] I have commented my code
- [x] I have performed a self-review of my own code; both comments &
source code

pkg/relayer/session/sessiontree.go

@@ -3,6 +3,7 @@ package session
 import (
 	"bytes"
 	"crypto/sha256"
+	"fmt"
 	"os"
 	"path/filepath"
 	"sync"
@@ -12,6 +13,7 @@ import (
 	"github.com/pokt-network/smt/kvstore/pebble"
 
 	"github.com/pokt-network/poktroll/pkg/crypto/protocol"
+	"github.com/pokt-network/poktroll/pkg/polylog"
 	"github.com/pokt-network/poktroll/pkg/relayer"
 	sessiontypes "github.com/pokt-network/poktroll/x/session/types"
 )
@@ -27,6 +29,8 @@ var _ relayer.SessionTree = (*sessionTree)(nil)
 // default value for this should be -1, implying "unlimited".
 // Ref discussion: https://github.com/pokt-network/poktroll/pull/755#discussion_r1737287860
 type sessionTree struct {
+	logger polylog.Logger
+
 	// sessionMu is a mutex used to protect sessionTree operations from concurrent access.
 	sessionMu *sync.Mutex
 
@@ -273,8 +277,15 @@ func (st *sessionTree) Delete() error {
 	// This was intentionally removed to lower the IO load.
 	// When the database is closed, it is deleted it from disk right away.
 
-	if err := st.treeStore.Stop(); err != nil {
-		return err
+	if st.treeStore != nil {
+		if err := st.treeStore.Stop(); err != nil {
+			return err
+		}
+	} else {
+		st.logger.With(
+			"claim_root", fmt.Sprintf("%x", st.GetClaimRoot()),
+			"session_id", st.GetSessionHeader().SessionId,
+		).Info().Msg("KVStore is already stopped")
 	}
 
 	// Delete the KVStore from disk

x/proof/keeper/msg_server_submit_proof.go

@@ -257,13 +257,13 @@ func (k Keeper) ProofRequirementForClaim(ctx context.Context, claim *types.Claim
 	}
 
 	// Hash of block when proof submission is allowed.
-	earliestProofCommitBlockHash, err := k.getEarliestSupplierProofCommitBlockHash(ctx, claim)
+	proofRequirementSeedBlockHash, err := k.getProofRequirementSeedBlockHash(ctx, claim)
 	if err != nil {
 		return requirementReason, err
 	}
 
 	// The probability that a proof is required.
-	proofRequirementSampleValue, err := claim.GetProofRequirementSampleValue(earliestProofCommitBlockHash)
+	proofRequirementSampleValue, err := claim.GetProofRequirementSampleValue(proofRequirementSeedBlockHash)
 	if err != nil {
 		return requirementReason, err
 	}
@@ -292,9 +292,9 @@ func (k Keeper) ProofRequirementForClaim(ctx context.Context, claim *types.Claim
 	return requirementReason, nil
 }
 
-// getEarliestSupplierProofCommitBlockHash returns the block hash of the earliest
-// block at which a claim may have its proof committed.
-func (k Keeper) getEarliestSupplierProofCommitBlockHash(
+// getProofRequirementSeedBlockHash returns the block hash of the seed block for
+// the proof requirement probabilistic check.
+func (k Keeper) getProofRequirementSeedBlockHash(
 	ctx context.Context,
 	claim *types.Claim,
 ) (blockHash []byte, err error) {
@@ -318,5 +318,7 @@ func (k Keeper) getEarliestSupplierProofCommitBlockHash(
 		supplierOperatorAddress,
 	)
 
-	return k.sessionKeeper.GetBlockHash(ctx, earliestSupplierProofCommitHeight), nil
+	// The proof requirement seed block is the last block of the session, and it is
+	// the block that is before the earliest block at which a proof can be committed.
+	return k.sessionKeeper.GetBlockHash(ctx, earliestSupplierProofCommitHeight-1), nil
 }

x/supplier/keeper/unbond_suppliers.go

@@ -54,15 +54,20 @@ func (k Keeper) EndBlockerUnbondSuppliers(ctx context.Context) error {
 			return err
 		}
 
-		// Send the coins from the supplier pool back to the supplier.
-		if err = k.bankKeeper.SendCoinsFromModuleToAccount(
-			ctx, suppliertypes.ModuleName, ownerAddress, []cosmostypes.Coin{*supplier.Stake},
-		); err != nil {
-			logger.Error(fmt.Sprintf(
-				"could not send %s coins from module %s to account %s due to %s",
-				supplier.Stake.String(), suppliertypes.ModuleName, ownerAddress, err,
-			))
-			return err
+		// If the supplier stake is 0 due to slashing, then do not move 0 coins
+		// to its account.
+		// Coin#IsPositive returns false if the coin is 0.
+		if supplier.Stake.IsPositive() {
+			// Send the coins from the supplier pool back to the supplier.
+			if err = k.bankKeeper.SendCoinsFromModuleToAccount(
+				ctx, suppliertypes.ModuleName, ownerAddress, []cosmostypes.Coin{*supplier.Stake},
+			); err != nil {
+				logger.Error(fmt.Sprintf(
+					"could not send %s coins from module %s to account %s due to %s",
+					supplier.Stake.String(), suppliertypes.ModuleName, ownerAddress, err,
+				))
+				return err
+			}
 		}
 
 		// Remove the supplier from the store.