core: fix slow block flag to be disabled by default

Previously, --debug.logslowblock defaulted to 2 seconds (enabled)
and setting 0 would disable logging. This was confusing.

Now:
- Disabled by default (flag must be explicitly set)
- Value 0 logs ALL blocks
- Positive value filters by execution time threshold

cmd/utils/flags.go

@@ -692,8 +692,8 @@ var (
 	}
 	LogSlowBlockFlag = &cli.DurationFlag{
 		Name:     "debug.logslowblock",
-		Usage:    "Block execution time threshold beyond which detailed statistics will be logged (0 means disable)",
-		Value:    ethconfig.Defaults.SlowBlockThreshold,
+		Usage:    "Block execution time threshold beyond which detailed statistics will be logged (0 logs all blocks)",
+		Value:    0,
 		Category: flags.LoggingCategory,
 	}
 
@@ -2351,8 +2351,12 @@ func MakeChain(ctx *cli.Context, stack *node.Node, readonly bool) (*core.BlockCh
 		// Enable state size tracking if enabled
 		StateSizeTracking: ctx.Bool(StateSizeTrackingFlag.Name),
 
-		// Configure the slow block statistic logger
-		SlowBlockThreshold: ctx.Duration(LogSlowBlockFlag.Name),
+		// Configure the slow block statistic logger (disabled by default)
+		SlowBlockThreshold: ethconfig.Defaults.SlowBlockThreshold,
+	}
+	// Only enable slow block logging if the flag was explicitly set
+	if ctx.IsSet(LogSlowBlockFlag.Name) {
+		options.SlowBlockThreshold = ctx.Duration(LogSlowBlockFlag.Name)
 	}
 	if options.ArchiveMode && !options.Preimages {
 		options.Preimages = true

core/blockchain.go

@@ -214,7 +214,8 @@ type BlockChainConfig struct {
 	StateSizeTracking bool
 
 	// SlowBlockThreshold is the block execution time threshold beyond which
-	// detailed statistics will be logged.
+	// detailed statistics will be logged. Negative value means disabled (default),
+	// zero logs all blocks, positive value filters blocks by execution time.
 	SlowBlockThreshold time.Duration
 }
 

core/blockchain_stats_test.go

@@ -266,10 +266,20 @@ func TestLogSlowBlockThreshold(t *testing.T) {
 	// Reset buffer
 	buf.Reset()
 
-	// Test with zero threshold (disabled)
+	// Test with zero threshold (logs all blocks)
 	stats.logSlow(block, 0)
 
+	if buf.Len() == 0 {
+		t.Errorf("Expected output for zero threshold (logs all), got nothing")
+	}
+
+	// Reset buffer
+	buf.Reset()
+
+	// Test with negative threshold (disabled)
+	stats.logSlow(block, -1)
+
 	if buf.Len() > 0 {
-		t.Errorf("Expected no output for zero threshold, got: %s", buf.String())
+		t.Errorf("Expected no output for negative threshold (disabled), got: %s", buf.String())
 	}
 }

eth/ethconfig/config.go

@@ -75,7 +75,7 @@ var Defaults = Config{
 	RPCTxFeeCap:             1, // 1 ether
 	TxSyncDefaultTimeout:    20 * time.Second,
 	TxSyncMaxTimeout:        1 * time.Minute,
-	SlowBlockThreshold:      time.Second * 2,
+	SlowBlockThreshold:      -1, // Disabled by default; set via --debug.logslowblock flag
 	RangeLimit:              0,
 }
 
@@ -130,8 +130,9 @@ type Config struct {
 	// presence of these blocks for every new peer connection.
 	RequiredBlocks map[uint64]common.Hash `toml:"-"`
 
-	// SlowBlockThreshold is the block execution speed threshold (Mgas/s)
-	// below which detailed statistics are logged.
+	// SlowBlockThreshold is the block execution time threshold beyond which
+	// detailed statistics are logged. Negative means disabled (default), zero
+	// logs all blocks, positive filters by execution time.
 	SlowBlockThreshold time.Duration `toml:",omitempty"`
 
 	// Database options