srml-contract: update ext_random(_seed) (#2635)

* Initial implementation.

* Rename random_seed to random

* Update rustdocs

* Update COMPLEXITY.md

* Fix comment.

* Limit the size of subject.

* Bump the runtime version.

* Fix doc

* Update node/runtime/src/lib.rs

Co-Authored-By: thiolliere <gui.thiolliere@gmail.com>

substrate/srml/contract/src/wasm/runtime.rs

@@ -527,9 +527,19 @@ define_env!(Env, <E: Ext>,
 		Ok(())
 	},
 
-	// Load the latest block RNG seed into the scratch buffer
-	ext_random_seed(ctx) => {
-		ctx.scratch_buf = ctx.ext.random_seed().encode();
+	// Stores the random number for the current block for the given subject into the scratch
+	// buffer.
+	//
+	// The data is encoded as T::Hash. The current contents of the scratch buffer are
+	// overwritten.
+	ext_random(ctx, subject_ptr: u32, subject_len: u32) => {
+		// The length of a subject can't exceed `max_subject_len`.
+		if subject_len > ctx.schedule.max_subject_len {
+			return Err(sandbox::HostError);
+		}
+
+		let subject_buf = read_sandbox_memory(ctx, subject_ptr, subject_len)?;
+		ctx.scratch_buf = ctx.ext.random(&subject_buf).encode();
 		Ok(())
 	},