[#105] processing: Add docs

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

processing/doc.go

@@ -1,2 +1,22 @@
-// Processing contract description.
+/*
+Processing contract is a contract deployed in NeoFS main chain.
+
+Processing contract pays for all multi signature transaction executions when notary
+service enabled in main chain. Notary service prepares multi signed transaction,
+however they should contain side chain GAS to be executed. It is inconvenient to
+ask Alphabet nodes to pay for these transactions: nodes can change over time,
+some nodes will spend side chain GAS faster, it creates economic instability.
+
+Processing contract exists to solve this issue. At the Withdraw invocation of
+NeoFS contract, user pays fee directly to this contract. This fee is used to
+pay for Cheque invocation of NeoFS contract that returns main chain GAS back
+to the user. Address of the Processing contract is uses as the first signer in
+the multi signature transaction. Therefore NeoVM executes Verify method of the
+contract and if invocation is verified, then Processing contract pays for the
+execution.
+
+Contract notifications
+
+Processing contract does not produce notifications to process.
+*/
 package processing

processing/processing_contract.go

@@ -18,6 +18,7 @@ const (
 	multiaddrMethod = "alphabetAddress"
 )
 
+// OnNEP17Payment is a callback for NEP-17 compatible native GAS contract.
 func OnNEP17Payment(from interop.Hash160, amount int, data interface{}) {
 	caller := runtime.GetCallingScriptHash()
 	if !common.BytesEqual(caller, []byte(gas.Hash)) {
@@ -50,6 +51,8 @@ func _deploy(data interface{}, isUpdate bool) {
 	runtime.Log("processing contract initialized")
 }
 
+// Migrate method updates contract source code and manifest. Can be invoked
+// only by contract owner.
 func Migrate(script []byte, manifest []byte, data interface{}) bool {
 	ctx := storage.GetReadOnlyContext()
 
@@ -64,6 +67,8 @@ func Migrate(script []byte, manifest []byte, data interface{}) bool {
 	return true
 }
 
+// Verify method returns true if transaction contains valid multi signature of
+// Alphabet nodes of the Inner Ring.
 func Verify() bool {
 	ctx := storage.GetContext()
 	neofsContractAddr := storage.Get(ctx, neofsContractKey).(interop.Hash160)
@@ -72,6 +77,7 @@ func Verify() bool {
 	return runtime.CheckWitness(multiaddr)
 }
 
+// Version returns version of the contract.
 func Version() int {
 	return version
 }