Add fuzzer for the compact custom codec implementation from PR #6720 (#7091)

* Add fuzzer for the compact custom codec implementation introduced in PR #6720.

This commit adds a fuzzing harness for the custom compact encoding/decoding
introduced in PR #6720.

* Update primitives/npos-elections/fuzzer/src/compact.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Update Cargo.lock: Add changes in elections-fuzzer

* Change indentation from spaces to tabs

Co-authored-by: Vincent Ulitzsch <vincent@srlabs.de>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/Cargo.lock

@@ -8250,6 +8250,7 @@ name = "sp-npos-elections-fuzzer"
 version = "2.0.0-alpha.5"
 dependencies = [
  "honggfuzz",
+ "parity-scale-codec",
  "rand 0.7.3",
  "sp-npos-elections",
  "sp-runtime",

substrate/primitives/npos-elections/fuzzer/Cargo.toml

@@ -19,6 +19,7 @@ sp-std = { version = "2.0.0-rc6", path = "../../std" }
 sp-runtime = { version = "2.0.0-rc6", path = "../../runtime" }
 honggfuzz = "0.5"
 rand = { version = "0.7.3", features = ["std", "small_rng"] }
+codec = { package = "parity-scale-codec", version = "1.0.0", default-features = false, features = ["derive"] }
 
 [[bin]]
 name = "reduce"
@@ -27,3 +28,7 @@ path = "src/reduce.rs"
 [[bin]]
 name = "balance_solution"
 path = "src/balance_solution.rs"
+
+[[bin]]
+name = "compact"
+path = "src/compact.rs"

substrate/primitives/npos-elections/fuzzer/src/compact.rs

+use honggfuzz::fuzz;
+use sp_npos_elections::generate_solution_type;
+use sp_npos_elections::sp_arithmetic::Percent;
+use sp_runtime::codec::{Encode, Error};
+
+fn main() {
+	generate_solution_type!(#[compact] pub struct InnerTestSolutionCompact::<u32, u32, Percent>(16));
+	loop {
+		fuzz!(|fuzzer_data: &[u8]| {
+			let result_decoded: Result<InnerTestSolutionCompact, Error> =
+				<InnerTestSolutionCompact as codec::Decode>::decode(&mut &fuzzer_data[..]);
+			// Ignore errors as not every random sequence of bytes can be decoded as InnerTestSolutionCompact
+			if let Ok(decoded) = result_decoded {
+				// Decoding works, let's re-encode it and compare results.
+				let reencoded: std::vec::Vec<u8> = decoded.encode();
+				// The reencoded value may or may not be equal to the original fuzzer output. However, the
+				// original decoder should be optimal (in the sense that there is no shorter encoding of
+				// the same object). So let's see if the fuzzer can find something shorter:
+				if fuzzer_data.len() < reencoded.len() {
+					panic!("fuzzer_data.len() < reencoded.len()");
+				}
+				// The reencoded value should definitely be decodable (if unwrap() fails that is a valid
+				// panic/finding for the fuzzer):
+				let decoded2: InnerTestSolutionCompact =
+					<InnerTestSolutionCompact as codec::Decode>::decode(
+						&mut reencoded.as_slice(),
+					).unwrap();
+				// And it should be equal to the original decoded object (resulting from directly
+				// decoding fuzzer_data):
+				assert_eq!(decoded, decoded2);
+			}
+		});
+	}
+}