Add xcm fuzzer (#3869)

* Extend xcm-simulator with a fuzzer for xcm exec

This commit adds a very basic fuzzer that fuzzes the xcm exectution,
in particular the process_instruction function.

* Update cargo.toml in xcm-simulator-fuzzer

* Add xcm-fuzzer to honggfuzz workflow

* Update Cargo.lock

* Update xcm/xcm-simulator/fuzzer/Cargo.toml so honggfuzz shows up on top

Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>

* Update relay_chain.rs in xcm-fuzzer

* Use MAX_XCM_DECODE_DEPTH instead of hardcoded decode limit in xcm-fuzzer

* Add comment on how to generate coverage report in xcm-fuzzer

* fix warnings, fmt, and unused result

* fix compiler

Co-authored-by: Vincent Ulitzsch <vincent.ulitzsch@srlabs.de>
Co-authored-by: Vincent Ulitzsch <vincent@srlabs.de>
Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>
Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>

polkadot/Cargo.lock

@@ -121,6 +121,12 @@ dependencies = [
  "num-traits",
 ]
 
+[[package]]
+name = "arbitrary"
+version = "1.0.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "577b08a4acd7b99869f863c50011b01eb73424ccc798ecd996f2e24817adfca7"
+
 [[package]]
 name = "arrayref"
 version = "0.3.6"
@@ -2603,6 +2609,17 @@ dependencies = [
  "hmac 0.8.1",
 ]
 
+[[package]]
+name = "honggfuzz"
+version = "0.5.54"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bea09577d948a98a5f59b7c891e274c4fb35ad52f67782b3d0cb53b9c05301f1"
+dependencies = [
+ "arbitrary",
+ "lazy_static",
+ "memmap",
+]
+
 [[package]]
 name = "hostname"
 version = "0.3.1"
@@ -4043,6 +4060,16 @@ version = "2.4.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "308cc39be01b73d0d18f82a0e7b2a3df85245f84af96fdddc5d202d27e47b86a"
 
+[[package]]
+name = "memmap"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6585fd95e7bb50d6cc31e20d4cf9afb4e2ba16c5846fc76793f11218da9c475b"
+dependencies = [
+ "libc",
+ "winapi 0.3.9",
+]
+
 [[package]]
 name = "memmap2"
 version = "0.2.0"
@@ -11886,6 +11913,30 @@ dependencies = [
  "xcm-simulator",
 ]
 
+[[package]]
+name = "xcm-simulator-fuzzer"
+version = "0.9.9"
+dependencies = [
+ "frame-support",
+ "frame-system",
+ "honggfuzz",
+ "pallet-balances",
+ "pallet-xcm",
+ "parity-scale-codec",
+ "polkadot-core-primitives",
+ "polkadot-parachain",
+ "polkadot-runtime-parachains",
+ "scale-info",
+ "sp-core",
+ "sp-io",
+ "sp-runtime",
+ "sp-std",
+ "xcm",
+ "xcm-builder",
+ "xcm-executor",
+ "xcm-simulator",
+]
+
 [[package]]
 name = "yamux"
 version = "0.9.0"