Optimize event serialization with pre-allocated buffer (#2794)

* Optimize event serialization with pre-allocated buffer

- Added event_buffer field to LlmpEventManager
- Used to_slice instead of to_allocvec
- Pre-allocated buffer size is 4KB

Fixes #1082

* Fallback to to_allocvec in case of event_buffer overflow

Also combined the shared logic between compressed & uncompressed event
firing while keeping the same behavior

* Made the initial event_buffer size to a const

Also removed the unnecessary event_buffer.clear(), since we are already
resizing it

libafl/src/events/llmp/mgr.rs

@@ -45,6 +45,9 @@ use crate::{
     Error, HasMetadata,
 };
 
+/// Default initial capacity of the event buffer - 4KB
+const INITIAL_EVENT_BUFFER_SIZE: usize = 1024 * 4;
+
 /// An [`EventManager`] that forwards all events to other attached fuzzers on shared maps or via tcp,
 /// using low-level message passing, `llmp`.
 pub struct LlmpEventManager<EMH, S, SP>
@@ -75,6 +78,7 @@ where
     should_serialize_cnt: usize,
     pub(crate) time_ref: Option<Handle<TimeObserver>>,
     phantom: PhantomData<S>,
+    event_buffer: Vec<u8>,
 }
 
 impl LlmpEventManager<(), NopState<NopInput>, NopShMemProvider> {
@@ -165,6 +169,7 @@ impl<EMH> LlmpEventManagerBuilder<EMH> {
             time_ref,
             phantom: PhantomData,
             custom_buf_handlers: vec![],
+            event_buffer: Vec::with_capacity(INITIAL_EVENT_BUFFER_SIZE),
         })
     }
 
@@ -199,6 +204,7 @@ impl<EMH> LlmpEventManagerBuilder<EMH> {
             time_ref,
             phantom: PhantomData,
             custom_buf_handlers: vec![],
+            event_buffer: Vec::with_capacity(INITIAL_EVENT_BUFFER_SIZE),
         })
     }
 
@@ -233,6 +239,7 @@ impl<EMH> LlmpEventManagerBuilder<EMH> {
             time_ref,
             phantom: PhantomData,
             custom_buf_handlers: vec![],
+            event_buffer: Vec::with_capacity(INITIAL_EVENT_BUFFER_SIZE),
         })
     }
 
@@ -265,6 +272,7 @@ impl<EMH> LlmpEventManagerBuilder<EMH> {
             time_ref,
             phantom: PhantomData,
             custom_buf_handlers: vec![],
+            event_buffer: Vec::with_capacity(INITIAL_EVENT_BUFFER_SIZE),
         })
     }
 }
@@ -409,6 +417,7 @@ where
             + EvaluatorObservers<E, Self, <S::Corpus as Corpus>::Input, S>
             + Evaluator<E, Self, <S::Corpus as Corpus>::Input, S>,
     {
+        println!("Got event in client: {} from {:?}", event.name(), client_id);
         if !self.hooks.pre_exec_all(state, client_id, &event)? {
             return Ok(());
         }
@@ -512,44 +521,56 @@ where
             true
         }
     }
-
-    #[cfg(feature = "llmp_compression")]
     fn fire(
         &mut self,
         _state: &mut Self::State,
         event: Event<<Self::State as UsesInput>::Input>,
     ) -> Result<(), Error> {
-        let serialized = postcard::to_allocvec(&event)?;
+        #[cfg(feature = "llmp_compression")]
         let flags = LLMP_FLAG_INITIALIZED;
 
-        match self.compressor.maybe_compress(&serialized) {
-            Some(comp_buf) => {
-                self.llmp.send_buf_with_flags(
-                    LLMP_TAG_EVENT_TO_BOTH,
-                    flags | LLMP_FLAG_COMPRESSED,
-                    &comp_buf,
-                )?;
+        self.event_buffer.resize(self.event_buffer.capacity(), 0);
+
+        // Serialize the event, reallocating event_buffer if needed
+        let written_len = match postcard::to_slice(&event, &mut self.event_buffer) {
+            Ok(written) => written.len(),
+            Err(postcard::Error::SerializeBufferFull) => {
+                let serialized = postcard::to_allocvec(&event)?;
+                self.event_buffer = serialized;
+                self.event_buffer.len()
             }
-            None => {
-                self.llmp.send_buf(LLMP_TAG_EVENT_TO_BOTH, &serialized)?;
+            Err(e) => return Err(Error::from(e)),
+        };
+
+        #[cfg(feature = "llmp_compression")]
+        {
+            match self
+                .compressor
+                .maybe_compress(&self.event_buffer[..written_len])
+            {
+                Some(comp_buf) => {
+                    self.llmp.send_buf_with_flags(
+                        LLMP_TAG_EVENT_TO_BOTH,
+                        flags | LLMP_FLAG_COMPRESSED,
+                        &comp_buf,
+                    )?;
+                }
+                None => {
+                    self.llmp
+                        .send_buf(LLMP_TAG_EVENT_TO_BOTH, &self.event_buffer[..written_len])?;
+                }
             }
         }
+
+        #[cfg(not(feature = "llmp_compression"))]
+        {
+            self.llmp
+                .send_buf(LLMP_TAG_EVENT_TO_BOTH, &self.event_buffer[..written_len]);
+        }
+
         self.last_sent = current_time();
-
         Ok(())
     }
-
-    #[cfg(not(feature = "llmp_compression"))]
-    fn fire(
-        &mut self,
-        _state: &mut Self::State,
-        event: Event<<Self::State as UsesInput>::Input>,
-    ) -> Result<(), Error> {
-        let serialized = postcard::to_allocvec(&event)?;
-        self.llmp.send_buf(LLMP_TAG_EVENT_TO_BOTH, &serialized)?;
-        Ok(())
-    }
-
     fn serialize_observers<OT>(&mut self, observers: &OT) -> Result<Option<Vec<u8>>, Error>
     where
         OT: ObserversTuple<Self::Input, Self::State> + Serialize,