Add ext_terminate (#5234)

With this patch forward this will be the only way for
a contract to destroy itself. This patch therefore changes
the semantics of all other contract initiated balance
transfers to fail if they would bring the caller below the
existential deposit.

substrate/frame/contracts/src/wasm/runtime.rs

@@ -41,6 +41,9 @@ enum SpecialTrap {
 	Return(Vec<u8>),
 	/// Signals that trap was generated because the contract exhausted its gas limit.
 	OutOfGas,
+	/// Signals that a trap was generated in response to a succesful call to the
+	/// `ext_terminate` host function.
+	Termination,
 }
 
 /// Can only be used for one call.
@@ -83,14 +86,20 @@ pub(crate) fn to_execution_result<E: Ext>(
 				status: STATUS_SUCCESS,
 				data,
 			})
-		}
+		},
+		Some(SpecialTrap::Termination) => {
+			return Ok(ExecReturnValue {
+				status: STATUS_SUCCESS,
+				data: Vec::new(),
+			})
+		},
 		Some(SpecialTrap::OutOfGas) => {
 			return Err(ExecError {
 				reason: "ran out of gas during contract execution".into(),
 				buffer: runtime.scratch_buf,
 			})
-		}
-		_ => (),
+		},
+		None => (),
 	}
 
 	// Check the exact type of the error.
@@ -443,6 +452,9 @@ define_env!(Env, <E: Ext>,
 	// changes made by the called contract are reverted. The scratch buffer is filled with the
 	// output data returned by the called contract, even in the case of a failure status.
 	//
+	// This call fails if it would bring the calling contract below the existential deposit.
+	// In order to destroy a contract `ext_terminate` must be used.
+	//
 	// If the contract traps during execution or otherwise fails to complete successfully, then
 	// this function clears the scratch buffer and returns 0x0100. As with a failure status, any
 	// state changes made by the called contract are reverted.
@@ -523,6 +535,9 @@ define_env!(Env, <E: Ext>,
 	// of the newly instantiated contract. In the case of a failure status, the scratch buffer is
 	// cleared.
 	//
+	// This call fails if it would bring the calling contract below the existential deposit.
+	// In order to destroy a contract `ext_terminate` must be used.
+	//
 	// If the contract traps during execution or otherwise fails to complete successfully, then
 	// this function clears the scratch buffer and returns 0x0100. As with a failure status, any
 	// state changes made by the called contract are reverted.
@@ -601,6 +616,30 @@ define_env!(Env, <E: Ext>,
 		}
 	},
 
+	// Remove the calling account and transfer remaining balance.
+	//
+	// This function never returns. Either the termination was successful and the
+	// execution of the destroyed contract is halted. Or it failed during the termination
+	// which is considered fatal and results in a trap + rollback.
+	//
+	// - beneficiary_ptr: a pointer to the address of the beneficiary account where all
+	//   where all remaining funds of the caller are transfered.
+	//   Should be decodable as an `T::AccountId`. Traps otherwise.
+	// - beneficiary_len: length of the address buffer.
+	ext_terminate(
+		ctx,
+		beneficiary_ptr: u32,
+		beneficiary_len: u32
+	) => {
+		let beneficiary: <<E as Ext>::T as frame_system::Trait>::AccountId =
+			read_sandbox_memory_as(ctx, beneficiary_ptr, beneficiary_len)?;
+
+		if let Ok(_) = ctx.ext.terminate(&beneficiary, ctx.gas_meter) {
+			ctx.special_trap = Some(SpecialTrap::Termination);
+		}
+		Err(sp_sandbox::HostError)
+	},
+
 	// Save a data buffer as a result of the execution, terminate the execution and return a
 	// successful result to the caller.
 	//