Merge pull request #10214 from ethereum/fallbackReturn

Allow fallback function to return data.

Changelog.md

@@ -1,5 +1,8 @@
 ### 0.7.6 (unreleased)
 
+Language Features:
+ * The fallback function can now also have a single ``calldata`` argument (equaling ``msg.data``) and return ``bytes memory`` (which will not be ABI-encoded but returned as-is).
+
 Compiler Features:
  * SMTChecker: Support named arguments in function calls.
  * SMTChecker: Support struct constructor.

docs/contracts/functions.rst

@@ -275,7 +275,10 @@ A contract can have at most one ``receive`` function, declared using
 ``receive() external payable { ... }``
 (without the ``function`` keyword).
 This function cannot have arguments, cannot return anything and must have
-``external`` visibility and ``payable`` state mutability. It is executed on a
+``external`` visibility and ``payable`` state mutability.
+It can be virtual, can override and can have modifiers.
+
+The receive function is executed on a
 call to the contract with empty calldata. This is the function that is executed
 on plain Ether transfers (e.g. via ``.send()`` or ``.transfer()``). If no such
 function exists, but a payable :ref:`fallback function <fallback-function>`
@@ -339,15 +342,22 @@ Below you can see an example of a Sink contract that uses function ``receive``.
 Fallback Function
 =================
 
-A contract can have at most one ``fallback`` function, declared using ``fallback () external [payable]``
+A contract can have at most one ``fallback`` function, declared using either ``fallback () external [payable]``
-(without the ``function`` keyword).
+or ``fallback (bytes calldata _input) external [payable] returns (bytes memory _output)``
-This function cannot have arguments, cannot return anything and must have ``external`` visibility.
+(both without the ``function`` keyword).
-It is executed on a call to the contract if none of the other
+This function must have ``external`` visibility. A fallback function can be virtual, can override
+and can have modifiers.
+
+The fallback function is executed on a call to the contract if none of the other
 functions match the given function signature, or if no data was supplied at
 all and there is no :ref:`receive Ether function <receive-ether-function>`.
 The fallback function always receives data, but in order to also receive Ether
 it must be marked ``payable``.
 
+If the version with parameters is used, ``_input`` will contain the full data sent to the contract
+(equal to ``msg.data``) and can return data in ``_output``. The returned data will not be
+ABI-encoded. Instead it will be returned without modifications (not even padding).
+
 In the worst case, if a payable fallback function is also used in
 place of a receive function, it can only rely on 2300 gas being
 available (see :ref:`receive Ether function <receive-ether-function>`
@@ -364,12 +374,11 @@ operations as long as there is enough gas passed on to it.
     to distinguish Ether transfers from interface confusions.
 
 .. note::
-    Even though the fallback function cannot have arguments, one can still use ``msg.data`` to retrieve
+    If you want to decode the input data, you can check the first four bytes
-    any payload supplied with the call.
+    for the function selector and then
-    After having checked the first four bytes of ``msg.data``,
     you can use ``abi.decode`` together with the array slice syntax to
     decode ABI-encoded data:
-    ``(c, d) = abi.decode(msg.data[4:], (uint256, uint256));``
+    ``(c, d) = abi.decode(_input[4:], (uint256, uint256));``
     Note that this should only be used as a last resort and
     proper functions should be used instead.
 

docs/grammar/Solidity.g4

@@ -84,7 +84,8 @@ contractBodyElement:
 	constructorDefinition
 	| functionDefinition
 	| modifierDefinition
-	| fallbackReceiveFunctionDefinition
+	| fallbackFunctionDefinition
+	| receiveFunctionDefinition
 	| structDefinition
 	| enumDefinition
 	| stateVariableDeclaration
@@ -189,9 +190,32 @@ locals[
 	(Semicolon | body=block);
 
 /**
- * Definitions of the special fallback and receive functions.
+ * Definition of the special fallback function.
  */
-fallbackReceiveFunctionDefinition
+fallbackFunctionDefinition
+locals[
+	boolean visibilitySet = false,
+	boolean mutabilitySet = false,
+	boolean virtualSet = false,
+	boolean overrideSpecifierSet = false,
+	boolean hasParameters = false
+]
+:
+	kind=Fallback LParen (parameterList { $hasParameters = true; } )? RParen
+	(
+		{!$visibilitySet}? External {$visibilitySet = true;}
+		| {!$mutabilitySet}? stateMutability {$mutabilitySet = true;}
+		| modifierInvocation
+		| {!$virtualSet}? Virtual {$virtualSet = true;}
+		| {!$overrideSpecifierSet}? overrideSpecifier {$overrideSpecifierSet = true;}
+	)*
+	( {$hasParameters}? Returns LParen returnParameters=parameterList RParen | {!$hasParameters}? )
+	(Semicolon | body=block);
+
+/**
+ * Definition of the special receive function.
+ */
+receiveFunctionDefinition
 locals[
 	boolean visibilitySet = false,
 	boolean mutabilitySet = false,
@@ -199,10 +223,10 @@ locals[
 	boolean overrideSpecifierSet = false
 ]
 :
-	kind=(Fallback | Receive) LParen RParen
+	kind=Receive LParen RParen
 	(
-		{!$visibilitySet}? visibility {$visibilitySet = true;}
+		{!$visibilitySet}? External {$visibilitySet = true;}
-		| {!$mutabilitySet}? stateMutability {$mutabilitySet = true;}
+		| {!$mutabilitySet}? Payable {$mutabilitySet = true;}
 		| modifierInvocation
 		| {!$virtualSet}? Virtual {$virtualSet = true;}
 		| {!$overrideSpecifierSet}? overrideSpecifier {$overrideSpecifierSet = true;}

libsolidity/analysis/OverrideChecker.cpp

@@ -394,6 +394,10 @@ bool OverrideProxy::OverrideComparator::operator<(OverrideComparator const& _oth
 	if (functionKind != _other.functionKind)
 		return *functionKind < *_other.functionKind;
 
+	// Parameters do not matter for non-regular functions.
+	if (functionKind != Token::Function)
+		return false;
+
 	if (!parameterTypes || !_other.parameterTypes)
 		return false;
 
@@ -574,16 +578,19 @@ void OverrideChecker::checkOverride(OverrideProxy const& _overriding, OverridePr
 		FunctionType const* functionType = _overriding.functionType();
 		FunctionType const* superType = _super.functionType();
 
-		solAssert(functionType->hasEqualParameterTypes(*superType), "Override doesn't have equal parameters!");
+		if (_overriding.functionKind() != Token::Fallback)
+		{
+			solAssert(functionType->hasEqualParameterTypes(*superType), "Override doesn't have equal parameters!");
 
-		if (!functionType->hasEqualReturnTypes(*superType))
+			if (!functionType->hasEqualReturnTypes(*superType))
-			overrideError(
+				overrideError(
-				_overriding,
+					_overriding,
-				_super,
+					_super,
-				4822_error,
+					4822_error,
-				"Overriding " + _overriding.astNodeName() + " return types differ.",
+					"Overriding " + _overriding.astNodeName() + " return types differ.",
-				"Overridden " + _overriding.astNodeName() + " is here:"
+					"Overridden " + _overriding.astNodeName() + " is here:"
-			);
+				);
+		}
 
 		// Stricter mutability is always okay except when super is Payable
 		if (

libsolidity/analysis/TypeChecker.cpp

@@ -1831,15 +1831,21 @@ void TypeChecker::typeCheckFallbackFunction(FunctionDefinition const& _function)
 		);
 	if (_function.visibility() != Visibility::External)
 		m_errorReporter.typeError(1159_error, _function.location(), "Fallback function must be defined as \"external\".");
-	if (!_function.returnParameters().empty())
+
+	if (!_function.returnParameters().empty() || !_function.parameters().empty())
 	{
-		if (_function.returnParameters().size() > 1 || *type(*_function.returnParameters().front()) != *TypeProvider::bytesMemory())
+		if (
-			m_errorReporter.typeError(5570_error, _function.returnParameterList()->location(), "Fallback function can only have a single \"bytes memory\" return value.");
+			_function.returnParameters().size() != 1 ||
-		else
+			*type(*_function.returnParameters().front()) != *TypeProvider::bytesMemory() ||
-			m_errorReporter.typeError(6151_error, _function.returnParameterList()->location(), "Return values for fallback functions are not yet implemented.");
+			_function.parameters().size() != 1 ||
+			*type(*_function.parameters().front()) != *TypeProvider::bytesCalldata()
+		)
+			m_errorReporter.typeError(
+				5570_error,
+				_function.returnParameterList()->location(),
+				"Fallback function either has to have the signature \"fallback()\" or \"fallback(bytes calldata) returns (bytes memory)\"."
+			);
 	}
-	if (!_function.parameters().empty())
-		m_errorReporter.typeError(3978_error, _function.parameterList().location(), "Fallback function cannot take parameters.");
 }
 
 void TypeChecker::typeCheckReceiveFunction(FunctionDefinition const& _function)

libsolidity/codegen/ContractCompiler.cpp

@@ -467,10 +467,21 @@ void ContractCompiler::appendFunctionSelector(ContractDefinition const& _contrac
 				appendCallValueCheck();
 
 			solAssert(fallback->isFallback(), "");
-			solAssert(FunctionType(*fallback).parameterTypes().empty(), "");
+			m_context.setStackOffset(0);
-			solAssert(FunctionType(*fallback).returnParameterTypes().empty(), "");
+
+			if (!FunctionType(*fallback).parameterTypes().empty())
+				m_context << u256(0) << Instruction::CALLDATASIZE;
+
 			fallback->accept(*this);
-			m_context << Instruction::STOP;
+
+			if (FunctionType(*fallback).returnParameterTypes().empty())
+				m_context << Instruction::STOP;
+			else
+			{
+				m_context << Instruction::DUP1 << Instruction::MLOAD << Instruction::SWAP1;
+				m_context << u256(0x20) << Instruction::ADD;
+				m_context << Instruction::RETURN;
+			}
 		}
 		else
 			m_context.appendRevert("Unknown signature and no fallback defined");
@@ -479,6 +490,7 @@ void ContractCompiler::appendFunctionSelector(ContractDefinition const& _contrac
 
 	for (auto const& it: interfaceFunctions)
 	{
+		m_context.setStackOffset(1);
 		FunctionTypePointer const& functionType = it.second;
 		solAssert(functionType->hasDeclaration(), "");
 		CompilerContext::LocationSetter locationSetter(m_context, functionType->declaration());
@@ -588,7 +600,9 @@ bool ContractCompiler::visit(FunctionDefinition const& _function)
 	// reserve additional slots: [retarg0] ... [retargm]
 
 	unsigned parametersSize = CompilerUtils::sizeOnStack(_function.parameters());
-	if (!_function.isConstructor())
+	if (_function.isFallback())
+		m_context.adjustStackOffset(static_cast<int>(parametersSize));
+	else if (!_function.isConstructor())
 		// adding 1 for return address.
 		m_context.adjustStackOffset(static_cast<int>(parametersSize) + 1);
 	for (ASTPointer<VariableDeclaration> const& variable: _function.parameters())
@@ -628,7 +642,8 @@ bool ContractCompiler::visit(FunctionDefinition const& _function)
 	unsigned const c_returnValuesSize = CompilerUtils::sizeOnStack(_function.returnParameters());
 
 	vector<int> stackLayout;
-	stackLayout.push_back(static_cast<int>(c_returnValuesSize)); // target of return address
+	if (!_function.isConstructor() && !_function.isFallback())
+		stackLayout.push_back(static_cast<int>(c_returnValuesSize)); // target of return address
 	stackLayout += vector<int>(c_argumentsSize, -1); // discard all arguments
 	for (size_t i = 0; i < c_returnValuesSize; ++i)
 		stackLayout.push_back(static_cast<int>(i));
@@ -639,7 +654,7 @@ bool ContractCompiler::visit(FunctionDefinition const& _function)
 			errinfo_sourceLocation(_function.location()) <<
 			errinfo_comment("Stack too deep, try removing local variables.")
 		);
-	while (stackLayout.back() != static_cast<int>(stackLayout.size() - 1))
+	while (!stackLayout.empty() && stackLayout.back() != static_cast<int>(stackLayout.size() - 1))
 		if (stackLayout.back() < 0)
 		{
 			m_context << Instruction::POP;

libsolidity/codegen/ir/IRGenerator.cpp

@@ -652,8 +652,16 @@ string IRGenerator::dispatchRoutine(ContractDefinition const& _contract)
 	{
 		string fallbackCode;
 		if (!fallback->isPayable())
-			fallbackCode += callValueCheck();
+			fallbackCode += callValueCheck() + "\n";
-		fallbackCode += m_context.enqueueFunctionForCodeGeneration(*fallback) + "() stop()";
+		if (fallback->parameters().empty())
+			fallbackCode += m_context.enqueueFunctionForCodeGeneration(*fallback) + "() stop()";
+		else
+		{
+			solAssert(fallback->parameters().size() == 1 && fallback->returnParameters().size() == 1, "");
+			fallbackCode += "let retval := " + m_context.enqueueFunctionForCodeGeneration(*fallback) + "(0, calldatasize())\n";
+			fallbackCode += "return(add(retval, 0x20), mload(retval))\n";
+
+		}
 
 		t("fallback", fallbackCode);
 	}

test/libsolidity/semanticTests/fallback/falback_return.sol

@@ -0,0 +1,18 @@
+contract A {
+    uint public x;
+    fallback () external {
+        if (x == 2) return;
+        x++;
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// ()
+// x() -> 1
+// ()
+// x() -> 2
+// ()
+// x() -> 2
+// ()
+// x() -> 2

test/libsolidity/semanticTests/fallback/fallback_argument.sol

@@ -0,0 +1,17 @@
+contract A {
+    uint public x;
+    fallback (bytes calldata _input) external returns (bytes memory) {
+        x = _input.length;
+        return "";
+    }
+    function f() public returns (bool, bytes memory) {
+        (bool success, bytes memory retval) = address(this).call("abc");
+        return (success, retval);
+    }
+}
+// ====
+// compileViaYul: also
+// EVMVersion: >=byzantium
+// ----
+// f() -> 0x01, 0x40, 0x00
+// x() -> 3

test/libsolidity/semanticTests/fallback/fallback_argument_to_storage.sol

@@ -0,0 +1,16 @@
+contract A {
+    bytes public x;
+    fallback (bytes calldata _input) external returns (bytes memory) {
+        x = _input;
+        return "";
+    }
+    function f() public returns (bool, bytes memory) {
+        (bool success, bytes memory retval) = address(this).call("abc");
+        return (success, retval);
+    }
+}
+// ====
+// EVMVersion: >=byzantium
+// ----
+// f() -> 0x01, 0x40, 0x00
+// x() -> 0x20, 3, "abc"

test/libsolidity/semanticTests/fallback/fallback_override.sol

@@ -0,0 +1,19 @@
+contract A {
+    fallback (bytes calldata _input) virtual external returns (bytes memory) {
+        return _input;
+    }
+}
+contract B is A {
+    fallback (bytes calldata _input) override external returns (bytes memory) {
+        return "xyz";
+    }
+    function f() public returns (bool, bytes memory) {
+        (bool success, bytes memory retval) = address(this).call("abc");
+        return (success, retval);
+    }
+}
+// ====
+// EVMVersion: >=byzantium
+// compileViaYul: also
+// ----
+// f() -> 0x01, 0x40, 0x03, 0x78797a0000000000000000000000000000000000000000000000000000000000

test/libsolidity/semanticTests/fallback/fallback_override2.sol

@@ -0,0 +1,18 @@
+contract A {
+    fallback (bytes calldata _input) virtual external returns (bytes memory) {
+        return _input;
+    }
+}
+contract B is A {
+    fallback () override external {
+    }
+    function f() public returns (bool, bytes memory) {
+        (bool success, bytes memory retval) = address(this).call("abc");
+        return (success, retval);
+    }
+}
+// ====
+// EVMVersion: >=byzantium
+// compileViaYul: also
+// ----
+// f() -> 1, 0x40, 0x00

test/libsolidity/semanticTests/fallback/fallback_override_multi.sol

@@ -0,0 +1,22 @@
+contract A {
+    fallback (bytes calldata _input) virtual external returns (bytes memory) {
+        return _input;
+    }
+}
+contract B {
+    fallback (bytes calldata _input) virtual external returns (bytes memory) {
+        return "xyz";
+    }
+}
+contract C is B, A {
+    fallback () external override (B, A) {}
+    function f() public returns (bool, bytes memory) {
+        (bool success, bytes memory retval) = address(this).call("abc");
+        return (success, retval);
+    }
+}
+// ====
+// EVMVersion: >=byzantium
+// compileViaYul: also
+// ----
+// f() -> 0x01, 0x40, 0x00

test/libsolidity/semanticTests/fallback/fallback_return_data.sol

@@ -0,0 +1,14 @@
+contract A {
+    fallback (bytes calldata _input) external returns (bytes memory) {
+        return _input;
+    }
+    function f() public returns (bool, bytes memory) {
+        (bool success, bytes memory retval) = address(this).call("abc");
+        return (success, retval);
+    }
+}
+// ====
+// compileViaYul: also
+// EVMVersion: >=byzantium
+// ----
+// f() -> 0x01, 0x40, 0x03, 0x6162630000000000000000000000000000000000000000000000000000000000

test/libsolidity/syntaxTests/fallback/arguments.sol

@@ -2,4 +2,4 @@ contract C {
     fallback(uint256) external {}
 }
 // ----
-// TypeError 3978: (25-34): Fallback function cannot take parameters.
+// TypeError 5570: (44-44): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".

test/libsolidity/syntaxTests/fallback/fallback_duplicate_returns.sol

@@ -0,0 +1,6 @@
+contract C {
+    fallback(bytes calldata _input) external returns (bytes memory _output) {}
+    fallback() external {}
+}
+// ----
+// DeclarationError 7301: (96-118): Only one fallback function is allowed.

test/libsolidity/syntaxTests/fallback/fallback_duplicate_returns_inheritance.sol

@@ -0,0 +1,9 @@
+contract C {
+    fallback(bytes calldata _input) external returns (bytes memory _output) {}
+}
+contract D is C {
+    fallback() external {}
+}
+// ----
+// TypeError 9456: (116-138): Overriding function is missing "override" specifier.
+// TypeError 4334: (17-91): Trying to override non-virtual function. Did you forget to add "virtual"?

test/libsolidity/syntaxTests/fallback/fallback_duplicate_returns_override.sol

@@ -0,0 +1,7 @@
+contract C {
+    fallback(bytes calldata _input) external virtual returns (bytes memory _output) {}
+}
+contract D is C {
+    fallback() external override {}
+}
+// ----

test/libsolidity/syntaxTests/fallback/fallback_wrong_data_location.sol

@@ -0,0 +1,13 @@
+contract D {
+    fallback(bytes memory) external returns (bytes memory) {}
+}
+contract E {
+    fallback(bytes memory) external returns (bytes calldata) {}
+}
+contract F {
+    fallback(bytes calldata) external returns (bytes calldata) {}
+}
+// ----
+// TypeError 5570: (57-71): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".
+// TypeError 5570: (134-150): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".
+// TypeError 5570: (215-231): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".

test/libsolidity/syntaxTests/fallback/inheritance_multi_base.sol

@@ -0,0 +1,20 @@
+contract A {
+    fallback (bytes calldata _input) external returns (bytes memory) {
+        return _input;
+    }
+}
+contract B {
+    fallback (bytes calldata _input) external returns (bytes memory) {
+        return "xyz";
+    }
+}
+contract C is B, A {
+     function f() public returns (bool, bytes memory) {
+        (bool success, bytes memory retval) = address(this).call("abc");
+        return (success, retval);
+    }
+}
+// ====
+// EVMVersion: >=byzantium
+// ----
+// TypeError 6480: (229-420): Derived contract must override function "". Two or more base classes define function with same name and parameter types.

test/libsolidity/syntaxTests/fallback/no_input_no_output.sol

@@ -0,0 +1,9 @@
+contract C {
+    fallback() external returns (bytes memory _output) {}
+}
+contract D {
+    fallback(bytes calldata _input) external {}
+}
+// ----
+// TypeError 5570: (45-67): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".
+// TypeError 5570: (131-131): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".

test/libsolidity/syntaxTests/fallback/return_value_number.sol

@@ -2,4 +2,4 @@ contract C {
     fallback() external returns (bytes memory, bytes memory) {}
 }
 // ----
-// TypeError 5570: (45-73): Fallback function can only have a single "bytes memory" return value.
+// TypeError 5570: (45-73): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".

test/libsolidity/syntaxTests/fallback/return_value_type.sol

@@ -2,4 +2,4 @@ contract C {
     fallback() external returns (uint256) {}
 }
 // ----
-// TypeError 5570: (45-54): Fallback function can only have a single "bytes memory" return value.
+// TypeError 5570: (45-54): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".

test/libsolidity/syntaxTests/fallback/return_value_unsupported.sol

@@ -2,4 +2,4 @@ contract C {
     fallback() external returns (bytes memory) {}
 }
 // ----
-// TypeError 6151: (45-59): Return values for fallback functions are not yet implemented.
+// TypeError 5570: (45-59): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".

test/libsolidity/syntaxTests/fallback/returns.sol

@@ -0,0 +1,4 @@
+contract C {
+    fallback(bytes calldata _input) external returns (bytes memory _output) {}
+}
+// ----

test/libsolidity/syntaxTests/nameAndTypeResolution/075_fallback_function_with_arguments.sol

@@ -3,4 +3,4 @@ contract C {
     fallback(uint a) external { x = 2; }
 }
 // ----
-// TypeError 3978: (37-45): Fallback function cannot take parameters.
+// TypeError 5570: (55-55): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".

test/libsolidity/syntaxTests/nameAndTypeResolution/077_fallback_function_with_return_parameters.sol

@@ -2,4 +2,4 @@ contract C {
     fallback() external returns (uint) { }
 }
 // ----
-// TypeError 5570: (45-51): Fallback function can only have a single "bytes memory" return value.
+// TypeError 5570: (45-51): Fallback function either has to have the signature "fallback()" or "fallback(bytes calldata) returns (bytes memory)".