[codec/conformance] handle not enough data gracefully

codec/src/conformance.rs

@@ -11,29 +11,47 @@ use rand::{Rng, SeedableRng};
 use rand_chacha::ChaCha8Rng;
 use std::{fmt::Debug, marker::PhantomData};
 
-/// Size of the random buffer used for generating arbitrary values.
-///
-/// This should be large enough to generate complex nested types.
-const ARBITRARY_BUFFER_SIZE: usize = 4096;
+/// Initial size of the random buffer used for generating arbitrary values.
+const INITIAL_BUFFER_SIZE: usize = 4096;
+
+/// Maximum buffer size to try before giving up (16 MB).
+const MAX_BUFFER_SIZE: usize = 16 * 1024 * 1024;
 
 /// Generate a deterministic value of type `T` using the given seed.
 ///
 /// Uses the [`arbitrary`] crate with a seeded ChaCha RNG for reproducible
-/// value generation.
+/// value generation. If the initial buffer is insufficient, the buffer size
+/// is doubled until generation succeeds or the maximum size is reached.
 pub fn generate_value<T>(seed: u64) -> T
 where
     T: for<'a> Arbitrary<'a>,
 {
-    // Create a seeded RNG
-    let mut rng = ChaCha8Rng::seed_from_u64(seed);
+    let mut buffer_size = INITIAL_BUFFER_SIZE;
+    loop {
+        // Create a seeded RNG
+        let mut rng = ChaCha8Rng::seed_from_u64(seed);
 
-    // Generate random bytes for the Unstructured input
-    let mut buffer = vec![0u8; ARBITRARY_BUFFER_SIZE];
-    rng.fill(&mut buffer[..]);
+        // Generate random bytes for the Unstructured input
+        let mut buffer = vec![0u8; buffer_size];
+        rng.fill(&mut buffer[..]);
 
-    // Generate the arbitrary value
-    let mut unstructured = Unstructured::new(&buffer);
-    T::arbitrary(&mut unstructured).expect("failed to generate arbitrary value")
+        // Try to generate the arbitrary value
+        let mut unstructured = Unstructured::new(&buffer);
+        match T::arbitrary(&mut unstructured) {
+            Ok(value) => return value,
+            Err(arbitrary::Error::NotEnoughData) => {
+                // Give up if we've already tried the maximum size
+                if buffer_size >= MAX_BUFFER_SIZE {
+                    panic!(
+                        "failed to generate arbitrary value: NotEnoughData with {buffer_size} bytes"
+                    );
+                }
+                // Double the buffer size (capped at MAX) and retry
+                buffer_size = (buffer_size * 2).min(MAX_BUFFER_SIZE);
+            }
+            Err(e) => panic!("failed to generate arbitrary value: {e}"),
+        }
+    }
 }
 
 /// Marker type for codec conformance testing.