Make execution trace configurable via env variable

We want to make the execution trace cache size configurable as SPs
may want to disable it while exchanges may want to crank it up.

We were also are going with intuition for this value, so having
ability to change it without a new build would help.

Fixes: https://github.com/filecoin-project/lotus/issues/10584

chain/stmgr/execute.go

@@ -131,6 +131,7 @@ func (sm *StateManager) ExecutionTrace(ctx context.Context, ts *types.TipSet) (c
 	tsKey := ts.Key()
 
 	// check if we have the trace for this tipset in the cache
+	if defaultExecTraceCacheSize > 0 {
 		sm.execTraceCacheLock.Lock()
 		if entry, ok := sm.execTraceCache.Get(tsKey); ok {
 			// we have to make a deep copy since caller can modify the invocTrace
@@ -140,6 +141,7 @@ func (sm *StateManager) ExecutionTrace(ctx context.Context, ts *types.TipSet) (c
 			return entry.postStateRoot, invocTraceCopy, nil
 		}
 		sm.execTraceCacheLock.Unlock()
+	}
 
 	var invocTrace []*api.InvocResult
 	st, err := sm.ExecutionTraceWithMonitor(ctx, ts, &InvocationTracer{trace: &invocTrace})
@@ -147,11 +149,13 @@ func (sm *StateManager) ExecutionTrace(ctx context.Context, ts *types.TipSet) (c
 		return cid.Undef, nil, err
 	}
 
+	if defaultExecTraceCacheSize > 0 {
 		invocTraceCopy := makeDeepCopy(invocTrace)
 
 		sm.execTraceCacheLock.Lock()
 		sm.execTraceCache.Add(tsKey, tipSetCacheEntry{st, invocTraceCopy})
 		sm.execTraceCacheLock.Unlock()
+	}
 
 	return st, invocTrace, nil
 }

chain/stmgr/stmgr.go

@@ -3,6 +3,8 @@ package stmgr
 import (
 	"context"
 	"fmt"
+	"os"
+	"strconv"
 	"sync"
 
 	lru "github.com/hashicorp/golang-lru/v2"
@@ -40,8 +42,7 @@ import (
 const LookbackNoLimit = api.LookbackNoLimit
 const ReceiptAmtBitwidth = 3
 
-const execTraceCacheSize = 16
+var defaultExecTraceCacheSize = 16
-
 var log = logging.Logger("statemgr")
 
 type StateManagerAPI interface {
@@ -74,6 +75,17 @@ func (m *migrationResultCache) keyForMigration(root cid.Cid) dstore.Key {
 	return dstore.NewKey(kStr)
 }
 
+func init() {
+	if s := os.Getenv("LOTUS_EXEC_TRACE_CACHE"); s != "" {
+		letc, err := strconv.Atoi(s)
+		if err != nil {
+			log.Errorf("failed to parse 'LOTUS_EXEC_TRACE_CACHE' env var: %s", err)
+		}
+
+		defaultExecTraceCacheSize = letc
+	}
+}
+
 func (m *migrationResultCache) Get(ctx context.Context, root cid.Cid) (cid.Cid, bool, error) {
 	k := m.keyForMigration(root)
 
@@ -200,10 +212,15 @@ func NewStateManager(cs *store.ChainStore, exec Executor, sys vm.SyscallBuilder,
 		}
 	}
 
-	execTraceCache, err := lru.NewARC[types.TipSetKey, tipSetCacheEntry](execTraceCacheSize)
+	log.Debugf("execTraceCache size: %d", defaultExecTraceCacheSize)
+	var execTraceCache *lru.ARCCache[types.TipSetKey, tipSetCacheEntry]
+	var err error
+	if defaultExecTraceCacheSize > 0 {
+		execTraceCache, err = lru.NewARC[types.TipSetKey, tipSetCacheEntry](defaultExecTraceCacheSize)
 		if err != nil {
 			return nil, err
 		}
+	}
 
 	return &StateManager{
 		networkVersions:   networkVersions,