Break out of quantification loop if there is no forward progress (#560)

This fixes infinite loops when we loop over an internal node that does not have any forward progress. Also included is an optimization to only emit the check/break instructions if we have a case that might result in an infinite loop (possibly non-progressing inner node + unlimited quantification)

Sources/_StringProcessing/ByteCodeGen.swift

@@ -543,7 +543,12 @@ fileprivate extension Compiler.ByteCodeGen {
           decrement %minTrips and fallthrough
 
       loop-body:
+        <if can't guarantee forward progress && extraTrips = nil>:
+          mov currentPosition %pos
         evaluate the subexpression
+        <if can't guarantee forward progress && extraTrips = nil>:
+          if %pos is currentPosition:
+            goto exit
         goto min-trip-count control block
 
       exit-policy control block:
@@ -646,7 +651,28 @@ fileprivate extension Compiler.ByteCodeGen {
     //   <subexpression>
     //   branch min-trip-count
     builder.label(loopBody)
+
+    // if we aren't sure if the child node will have forward progress and
+    // we have an unbounded quantification
+    let startPosition: PositionRegister?
+    let emitPositionChecking =
+      (!optimizationsEnabled || !child.guaranteesForwardProgress) &&
+      extraTrips == nil
+
+    if emitPositionChecking {
+      startPosition = builder.makePositionRegister()
+      builder.buildMoveCurrentPosition(into: startPosition!)
+    } else {
+      startPosition = nil
+    }
     try emitNode(child)
+    if emitPositionChecking {
+      // in all quantifier cases, no matter what minTrips or extraTrips is,
+      // if we have a successful non-advancing match, branch to exit because it
+      // can match an arbitrary number of times
+      builder.buildCondBranch(to: exit, ifSamePositionAs: startPosition!)
+    }
+
     if minTrips <= 1 {
       // fallthrough
     } else {
@@ -832,3 +858,42 @@ fileprivate extension Compiler.ByteCodeGen {
     return nil
   }
 }
+
+extension DSLTree.Node {
+  var guaranteesForwardProgress: Bool {
+    switch self {
+    case .orderedChoice(let children):
+      return children.allSatisfy { $0.guaranteesForwardProgress }
+    case .concatenation(let children):
+      return children.contains(where: { $0.guaranteesForwardProgress })
+    case .capture(_, _, let node, _):
+      return node.guaranteesForwardProgress
+    case .nonCapturingGroup(let kind, let child):
+      switch kind.ast {
+      case .lookahead, .negativeLookahead, .lookbehind, .negativeLookbehind:
+        return false
+      default: return child.guaranteesForwardProgress
+      }
+    case .atom(let atom):
+      switch atom {
+      case .changeMatchingOptions, .assertion: return false
+      default: return true
+      }
+    case .trivia, .empty:
+      return false
+    case .quotedLiteral(let string):
+      return !string.isEmpty
+    case .convertedRegexLiteral(let node, _):
+      return node.guaranteesForwardProgress
+    case .consumer, .matcher:
+      // Allow zero width consumers and matchers
+     return false
+    case .customCharacterClass:
+      return true
+    case .quantification(let amount, _, let child):
+      let (atLeast, _) = amount.ast.bounds
+      return atLeast ?? 0 > 0 && child.guaranteesForwardProgress
+    default: return false
+    }
+  }
+}

Sources/_StringProcessing/Engine/Backtracking.swift

@@ -32,15 +32,18 @@ extension Processor {
     // The int registers store values that can be relevant to
     // backtracking, such as the number of trips in a quantification.
     var intRegisters: [Int]
+    // Same with position registers
+    var posRegisters: [Input.Index]
 
     var destructure: (
       pc: InstructionAddress,
       pos: Position?,
       stackEnd: CallStackAddress,
       captureEnds: [_StoredCapture],
-      intRegisters: [Int]
+      intRegisters: [Int],
+      PositionRegister: [Input.Index]
     ) {
-      (pc, pos, stackEnd, captureEnds, intRegisters)
+      (pc, pos, stackEnd, captureEnds, intRegisters, posRegisters)
     }
   }
 
@@ -53,7 +56,8 @@ extension Processor {
       pos: addressOnly ? nil : currentPosition,
       stackEnd: .init(callStack.count),
       captureEnds: storedCaptures,
-      intRegisters: registers.ints)
+      intRegisters: registers.ints,
+      posRegisters: registers.positions)
   }
 }
 

Sources/_StringProcessing/Engine/InstPayload.swift

@@ -284,10 +284,10 @@ extension Instruction.Payload {
     interpretPair()
   }
 
-  init(pos: PositionRegister, pos2: PositionRegister) {
-    self.init(pos, pos2)
+  init(addr: InstructionAddress, position: PositionRegister) {
+    self.init(addr, position)
   }
-  var pairedPosPos: (PositionRegister, PositionRegister) {
+  var pairedAddrPos: (InstructionAddress, PositionRegister) {
     interpretPair()
   }
 

Sources/_StringProcessing/Engine/Instruction.swift

@@ -37,6 +37,14 @@ extension Instruction {
     ///
     case moveImmediate
 
+    /// Move the current position into a register
+    ///
+    ///     moveCurrentPosition(into: PositionRegister)
+    ///
+    /// Operands:
+    ///   - Position register to move into
+    case moveCurrentPosition
+
     // MARK: General Purpose: Control flow
 
     /// Branch to a new instruction
@@ -57,6 +65,16 @@ extension Instruction {
     ///
     case condBranchZeroElseDecrement
 
+    /// Conditionally branch if the current position is the same as the register
+    ///
+    ///     condBranch(
+    ///       to: InstAddr, ifSamePositionAs: PositionRegister)
+    ///
+    /// Operands:
+    ///   - Instruction address to branch to, if the position in the register is the same as currentPosition
+    ///   - Position register to check against
+    case condBranchSamePosition
+
     // TODO: Function calls
 
     // MARK: - Matching

Sources/_StringProcessing/Engine/MEBuilder.swift

@@ -32,6 +32,7 @@ extension MEProgram {
     var nextIntRegister = IntRegister(0)
     var nextCaptureRegister = CaptureRegister(0)
     var nextValueRegister = ValueRegister(0)
+    var nextPositionRegister = PositionRegister(0)
 
     // Special addresses or instructions
     var failAddressToken: AddressToken? = nil
@@ -105,6 +106,14 @@ extension MEProgram.Builder {
     fixup(to: t)
   }
 
+  mutating func buildCondBranch(
+    to t: AddressToken,
+    ifSamePositionAs r: PositionRegister
+  ) {
+    instructions.append(.init(.condBranchSamePosition, .init(position: r)))
+    fixup(to: t)
+  }
+
   mutating func buildSave(_ t: AddressToken) {
     instructions.append(.init(.save))
     fixup(to: t)
@@ -211,6 +220,10 @@ extension MEProgram.Builder {
       .init(value: value, capture: capture)))
   }
 
+  mutating func buildMoveCurrentPosition(into r: PositionRegister) {
+    instructions.append(.init(.moveCurrentPosition, .init(position: r)))
+  }
+
   mutating func buildBackreference(
     _ cap: CaptureRegister
   ) {
@@ -257,7 +270,8 @@ extension MEProgram.Builder {
       switch inst.opcode {
       case .condBranchZeroElseDecrement:
         payload = .init(addr: addr, int: inst.payload.int)
-
+      case .condBranchSamePosition:
+        payload = .init(addr: addr, position: inst.payload.position)
       case .branch, .save, .saveAddress, .clearThrough:
         payload = .init(addr: addr)
 
@@ -281,6 +295,7 @@ extension MEProgram.Builder {
     regInfo.sequences = sequences.count
     regInfo.ints = nextIntRegister.rawValue
     regInfo.values = nextValueRegister.rawValue
+    regInfo.positions = nextPositionRegister.rawValue
     regInfo.bitsets = asciiBitsets.count
     regInfo.consumeFunctions = consumeFunctions.count
     regInfo.assertionFunctions = assertionFunctions.count
@@ -421,6 +436,12 @@ extension MEProgram.Builder {
     return r
   }
 
+  mutating func makePositionRegister() -> PositionRegister {
+    let r = nextPositionRegister
+    defer { nextPositionRegister.rawValue += 1 }
+    return r
+  }
+
   // TODO: A register-mapping helper struct, which could release
   // registers without monotonicity required
 

Sources/_StringProcessing/Engine/Processor.swift

@@ -245,7 +245,7 @@ extension Processor {
   }
 
   mutating func signalFailure() {
-    guard let (pc, pos, stackEnd, capEnds, intRegisters) =
+    guard let (pc, pos, stackEnd, capEnds, intRegisters, posRegisters) =
             savePoints.popLast()?.destructure
     else {
       state = .fail
@@ -259,6 +259,7 @@ extension Processor {
     callStack.removeLast(callStack.count - stackEnd.rawValue)
     storedCaptures = capEnds
     registers.ints = intRegisters
+    registers.positions = posRegisters
   }
 
   mutating func abort(_ e: Error? = nil) {
@@ -315,7 +316,10 @@ extension Processor {
 
       registers[reg] = int
       controller.step()
-
+    case .moveCurrentPosition:
+      let reg = payload.position
+      registers[reg] = currentPosition
+      controller.step()
     case .branch:
       controller.pc = payload.addr
 
@@ -327,7 +331,13 @@ extension Processor {
         registers[int] -= 1
         controller.step()
       }
-
+    case .condBranchSamePosition:
+      let (addr, pos) = payload.pairedAddrPos
+      if registers[pos] == currentPosition {
+        controller.pc = addr
+      } else {
+        controller.step()
+      }
     case .save:
       let resumeAddr = payload.addr
       let sp = makeSavePoint(resumeAddr)

Sources/_StringProcessing/Engine/Registers.swift

@@ -47,6 +47,8 @@ extension Processor {
     var ints: [Int]
 
     var values: [Any]
+
+    var positions: [Input.Index]
   }
 }
 
@@ -66,6 +68,12 @@ extension Processor.Registers {
       values[i.rawValue] = newValue
     }
   }
+  subscript(_ i: PositionRegister) -> Input.Index {
+    get { positions[i.rawValue] }
+    set {
+      positions[i.rawValue] = newValue
+    }
+  }
   subscript(_ i: ElementRegister) -> Input.Element {
     elements[i.rawValue]
   }
@@ -89,6 +97,8 @@ extension Processor.Registers {
 }
 
 extension Processor.Registers {
+  static let sentinelIndex = "".startIndex
+
   init(
     _ program: MEProgram,
     _ sentinel: String.Index
@@ -120,11 +130,15 @@ extension Processor.Registers {
 
     self.values = Array(
       repeating: SentinelValue(), count: info.values)
+    self.positions = Array(
+      repeating: Processor.Registers.sentinelIndex,
+      count: info.positions)
   }
 
   mutating func reset(sentinel: Input.Index) {
     self.ints._setAll(to: 0)
     self.values._setAll(to: SentinelValue())
+    self.positions._setAll(to: Processor.Registers.sentinelIndex)
   }
 }
 

Tests/RegexTests/CompileTests.swift

@@ -208,4 +208,40 @@ extension RegexTests {
     expectProgram(for: "[abc]", semanticLevel: .unicodeScalar, doesNotContain: [.matchBitset])
     expectProgram(for: "[abc]", semanticLevel: .unicodeScalar, contains: [.consumeBy])
   }
+
+  func testQuantificationForwardProgressCompile() {
+    // Unbounded quantification + non forward progressing inner nodes
+    // Expect to emit the position checking instructions
+    expectProgram(for: #"(?:(?=a)){1,}"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:\b)*"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:(?#comment))+"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:|)+"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:\w|)+"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:\w|(?i-i:))+"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:\w|(?#comment))+"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:\w|(?#comment)(?i-i:))+"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:\w|(?i))+"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
+
+    // Bounded quantification, don't emit position checking
+    expectProgram(for: #"(?:(?=a)){1,4}"#, doesNotContain: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:\b)?"#, doesNotContain: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:(?#comment)){,4}"#, doesNotContain: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:|){,4}"#, doesNotContain: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:\w|){,4}"#, doesNotContain: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:\w|(?i-i:)){,4}"#, doesNotContain: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:\w|(?#comment)){,4}"#, doesNotContain: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:\w|(?#comment)(?i-i:)){,4}"#, doesNotContain: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(?:\w|(?i)){,4}"#, doesNotContain: [.moveCurrentPosition, .condBranchSamePosition])
+
+    // Inner node is a quantification that does not guarantee forward progress
+    expectProgram(for: #"(a*)*"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(a?)*"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(a{,5})*"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"((\b){,4})*"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"((\b){1,4})*"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"((|){1,4})*"#, contains: [.moveCurrentPosition, .condBranchSamePosition])
+    // Inner node is a quantification that guarantees forward progress
+    expectProgram(for: #"(a+)*"#, doesNotContain: [.moveCurrentPosition, .condBranchSamePosition])
+    expectProgram(for: #"(a{1,})*"#, doesNotContain: [.moveCurrentPosition, .condBranchSamePosition])
+  }
 }

Tests/RegexTests/MatchTests.swift

@@ -1895,5 +1895,31 @@ extension RegexTests {
       XCTAssertEqual(matches.count, 3)
     }
   }
-}
 
+  func expectCompletion(regex: String, in target: String) {
+    let expectation = XCTestExpectation(description: "Run the given regex to completion")
+    Task.init {
+      let r = try! Regex(regex)
+      let val = target.matches(of: r).isEmpty
+      expectation.fulfill()
+      return val
+    }
+    wait(for: [expectation], timeout: 3.0)
+  }
+
+  func testQuantificationForwardProgress() {
+    expectCompletion(regex: #"(?:(?=a)){1,}"#, in: "aa")
+    expectCompletion(regex: #"(?:\b)+"#, in: "aa")
+    expectCompletion(regex: #"(?:(?#comment))+"#, in: "aa")
+    expectCompletion(regex: #"(?:|)+"#, in: "aa")
+    expectCompletion(regex: #"(?:\w|)+"#, in: "aa")
+    expectCompletion(regex: #"(?:\w|(?i-i:))+"#, in: "aa")
+    expectCompletion(regex: #"(?:\w|(?#comment))+"#, in: "aa")
+    expectCompletion(regex: #"(?:\w|(?#comment)(?i-i:))+"#, in: "aa")
+    expectCompletion(regex: #"(?:\w|(?i))+"#, in: "aa")
+    expectCompletion(regex: #"(a*)*"#, in: "aa")
+    expectCompletion(regex: #"(a?)*"#, in: "aa")
+    expectCompletion(regex: #"(a{,4})*"#, in: "aa")
+    expectCompletion(regex: #"((|)+)*"#, in: "aa")
+  }
+}