feat(engine): run sync state root if not enough parallelism (#20127)

Author: shekhirin

crates/engine/primitives/src/config.rs

@@ -97,7 +97,7 @@ pub struct TreeConfig {
     state_provider_metrics: bool,
     /// Cross-block cache size in bytes.
     cross_block_cache_size: u64,
-    /// Whether the host has enough parallelism to run state root task.
+    /// Whether the host has enough parallelism to run state root in parallel.
     has_enough_parallelism: bool,
     /// Whether multiproof task should chunk proof targets.
     multiproof_chunking_enabled: bool,
@@ -385,12 +385,17 @@ impl TreeConfig {
         self
     }
 
-    /// Setter for has enough parallelism.
+    /// Setter for whether or not the host has enough parallelism to run state root in parallel.
     pub const fn with_has_enough_parallelism(mut self, has_enough_parallelism: bool) -> Self {
         self.has_enough_parallelism = has_enough_parallelism;
         self
     }
 
+    /// Whether or not the host has enough parallelism to run state root in parallel.
+    pub const fn has_enough_parallelism(&self) -> bool {
+        self.has_enough_parallelism
+    }
+
     /// Setter for state provider metrics.
     pub const fn with_state_provider_metrics(mut self, state_provider_metrics: bool) -> Self {
         self.state_provider_metrics = state_provider_metrics;

crates/engine/tree/src/tree/payload_validator.rs

@@ -857,7 +857,7 @@ where
     /// Note: Use state root task only if prefix sets are empty, otherwise proof generation is
     /// too expensive because it requires walking all paths in every proof.
     const fn plan_state_root_computation(&self) -> StateRootStrategy {
-        if self.config.state_root_fallback() {
+        if self.config.state_root_fallback() || !self.config.has_enough_parallelism() {
             StateRootStrategy::Synchronous
         } else if self.config.use_state_root_task() {
             StateRootStrategy::StateRootTask