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Scala example source code file (SynchronizedSymbols.scala)

This example Scala source code file (SynchronizedSymbols.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Scala source code examples by using tags.

All credit for the original source code belongs to; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Scala tags/keywords

classsymbol, collection, io, long, modulesymbol, position, reflection, synchronizedclasssymbol, synchronizedtermsymbol, synchronizedtypesymbol, termname, type, typename

The SynchronizedSymbols.scala Scala example source code

package scala
package reflect
package runtime

import scala.collection.{ immutable, mutable }
import scala.reflect.internal.Flags._

private[reflect] trait SynchronizedSymbols extends internal.Symbols { self: SymbolTable =>

  private lazy val atomicIds = new java.util.concurrent.atomic.AtomicInteger(0)
  override protected def nextId() = atomicIds.incrementAndGet()

  private lazy val atomicExistentialIds = new java.util.concurrent.atomic.AtomicInteger(0)
  override protected def nextExistentialId() = atomicExistentialIds.incrementAndGet()

  private lazy val _recursionTable = mkThreadLocalStorage(immutable.Map.empty[Symbol, Int])
  override def recursionTable = _recursionTable.get
  override def recursionTable_=(value: immutable.Map[Symbol, Int]) = _recursionTable.set(value)

  // Set the fields which point companions at one another.  Returns the module.
  override def connectModuleToClass(m: ModuleSymbol, moduleClass: ClassSymbol): ModuleSymbol =
    gilSynchronized { super.connectModuleToClass(m, moduleClass) }

  override def newFreeTermSymbol(name: TermName, value: => Any, flags: Long = 0L, origin: String = null): FreeTermSymbol =
    new FreeTermSymbol(name, value, origin) with SynchronizedTermSymbol initFlags flags

  override def newFreeTypeSymbol(name: TypeName, flags: Long = 0L, origin: String = null): FreeTypeSymbol =
    new FreeTypeSymbol(name, origin) with SynchronizedTypeSymbol initFlags flags

  override protected def makeNoSymbol: NoSymbol = new NoSymbol with SynchronizedSymbol

  trait SynchronizedSymbol extends Symbol {

    /** (Things written in this comment only applies to runtime reflection. Compile-time reflection,
     *  especially across phases and runs, is somewhat more complicated, but we won't be touching it,
     *  because at the moment we only care about synchronizing runtime reflection).
     *  As it has been noted on multiple occasions, generally speaking, reflection artifacts aren't thread-safe.
     *  Reasons for that differ from artifact to artifact. In some cases it's quite bad (e.g. types use a number
     *  of non-concurrent compiler caches, so we need to serialize certain operations on types in order to make
     *  sure that things stay deterministic). However, in case of symbols there's hope, because it's only during
     *  initializaton that symbols are thread-unsafe. After everything's set up, symbols become immutable
     *  (sans a few deterministic caches that can be populated simultaneously by multiple threads) and therefore thread-safe.
     *  Note that by saying "symbols become immutable" I mean literally that. In a very common case of PackageClassSymbol's,
     *  even when a symbol finishes its initialization and becomes immutable, its info forever remains mutable.
     *  Therefore even if we no longer need to synchronize a PackageClassSymbol after it's initialized, we still have to take
     *  care of its ClassInfoType (or, more precisely, of the underlying Scope), but that's done elsewhere, and
     *  here we don't need to worry about that.
     *  Okay, so now we simply check `Symbol.isInitialized` and if it's true, then everything's fine? Haha, nope!
     *  The thing is that some completers call sym.setInfo when still in-flight and then proceed with initialization
     *  (e.g. see LazyPackageType). Consequently, setInfo sets _validTo to current period, which means that after
     *  a call to setInfo isInitialized will start returning true. Unfortunately, this doesn't mean that info becomes
     *  ready to be used, because subsequent initialization might change the info.
     *  Therefore we need to somehow distinguish between initialized and really initialized symbol states.
     *  Okay, let's do it on per-completer basis. We have seven kinds of completers to worry about:
     *    1) LazyPackageType that initializes packages and their underlying package classes
     *    2) TopClassCompleter that initializes top-level Scala-based class-module companion pairs of static definitions
     *    3) LazyTypeRef and LazyTypeRefAndAlias set up by TopClassCompleter that initialize (transitive members) of top-level classes/modules
     *    4) FromJavaClassCompleter that does the same for both top-level and non-toplevel Java-based classes/modules
     *    5) Fully-initialized signatures of non-class/module Java-based reflection artifacts
     *    6) Importing completer that transfers metadata from one universe to another
     *    7) Signatures of special symbols such as roots and symbolsNotPresentInBytecode
     *  The mechanisms underlying completion are quite complex, and it'd be only natural to suppose that over time we're going to overlook something.
     *  Wrt isThreadsafe we could have two wrong situations: false positives (isThreadsafe = true, but the symbol isn't actually threadsafe)
     *  and false negatives (isThreadsafe = false, but the symbol is actually threadsafe). However, even though both are wrong, only the former
     *  is actively malicious. Indeed, false positives might lead to races, inconsistent state and crashes, while the latter would only cause
     *  `initialize` to be called and a gil to be taken on every potentially auto-initializable operation. Unpleasant yes, but still robust.
     *  What makes me hopeful is that:
     *    1) By default (e.g. if some new completion mechanism gets introduced for a special flavor of symbols and we forget to call markCompleted)
     *       isThreadsafe is always in false negative state, which is unpleasant but safe.
     *    2) Calls to `markCompleted` which are the only potential source of erroneous behavior are few and are relatively easy to place:
     *       just put them just before your completer's `complete` returns, and you should be fine.
     *  upd. Actually, there's another problem of not keeping initialization mask up-to-date. If we're not careful enough,
     *  then it might so happen that getting a certain flag that the compiler assumes to be definitively set will spuriously
     *  return isThreadsafe(purpose = FlagsOp(<flag>)) = false and that will lead to spurious auto-initialization,
     *  which will cause an SO or a cyclic reference or some other crash. I've done my best to go through all possible completers
     *  and call `markFlagsCompleted` where appropriate, but again over time something might be overlooked, so to guard against that
     *  I'm only considering TopLevelPickledFlags to be sources of potential initialization. This ensures that such system flags as
     *  isMethod, isModule or isPackage are never going to auto-initialize.
    override def isThreadsafe(purpose: SymbolOps) = {
      if (isCompilerUniverse) false
      else if (_initialized) true
      else purpose.isFlagRelated && (_initializationMask & purpose.mask & TopLevelPickledFlags) == 0

    /** Communicates with completers declared in scala.reflect.runtime.SymbolLoaders
     *  about the status of initialization of the underlying symbol.
     *  Unfortunately, it's not as easy as just introducing the `markThreadsafe` method that would be called
     *  by the completers when they are really done (as opposed to `setInfo` that, as mentioned above, doesn't mean anything).
     *  Since we also want to auto-initialize symbols when certain methods are being called (`Symbol.hasFlag` for example),
     *  we need to track the identity of the initializer, so as to block until initialization is complete if the caller
     *  comes from a different thread, but to skip auto-initialization if we're the initializing thread.
     *  Just a volatile var is fine, because:
     *    1) Status can only be changed in a single-threaded fashion (this is enforced by gilSynchronized
     *       that effecively guards `Symbol.initialize`), which means that there can't be update conflicts.
     *    2) If someone reads a stale value of status, then the worst thing that might happen is that this someone
     *       is going to spuriously call `initialize`, which is either a gil-protected operation (if the symbol isn't inited yet)
     *       or a no-op (if the symbol is already inited), and that is fine in both cases.
     *  upd. It looks like we also need to keep track of a mask of initialized flags to make sure
     *  that normal symbol initialization routines don't trigger auto-init in Symbol.flags-related routines (e.g. Symbol.getFlag).
     *  Due to the same reasoning as above, a single volatile var is enough for to store the mask.
    @volatile private[this] var _initialized = false
    @volatile private[this] var _initializationMask = TopLevelPickledFlags
    override def markFlagsCompleted(mask: Long): this.type = { _initializationMask = _initializationMask & ~mask; this }
    override def markAllCompleted(): this.type = { _initializationMask = 0L; _initialized = true; this }

    def gilSynchronizedIfNotThreadsafe[T](body: => T): T = {
      // TODO: debug and fix the race that doesn't allow us uncomment this optimization
      // if (isCompilerUniverse || isThreadsafe(purpose = AllOps)) body
      // else gilSynchronized { body }
      gilSynchronized { body }

    override def validTo = gilSynchronizedIfNotThreadsafe { super.validTo }
    override def info = gilSynchronizedIfNotThreadsafe { }
    override def rawInfo: Type = gilSynchronizedIfNotThreadsafe { super.rawInfo }
    override def typeSignature: Type = gilSynchronizedIfNotThreadsafe { super.typeSignature }
    override def typeSignatureIn(site: Type): Type = gilSynchronizedIfNotThreadsafe { super.typeSignatureIn(site) }

    override def typeParams: List[Symbol] = gilSynchronizedIfNotThreadsafe {
      if (isCompilerUniverse) super.typeParams
      else {
        if (isMonomorphicType) Nil
        else {
          // analogously to the "info" getter, here we allow for two completions:
          //   one: sourceCompleter to LazyType, two: LazyType to completed type
          if (validTo == NoPeriod)
            rawInfo load this
          if (validTo == NoPeriod)
            rawInfo load this

    override def unsafeTypeParams: List[Symbol] = gilSynchronizedIfNotThreadsafe {
      if (isCompilerUniverse) super.unsafeTypeParams
      else {
        if (isMonomorphicType) Nil
        else rawInfo.typeParams

// ------ creators -------------------------------------------------------------------

    override protected def createAbstractTypeSymbol(name: TypeName, pos: Position, newFlags: Long): AbstractTypeSymbol =
      new AbstractTypeSymbol(this, pos, name) with SynchronizedTypeSymbol initFlags newFlags

    override protected def createAliasTypeSymbol(name: TypeName, pos: Position, newFlags: Long): AliasTypeSymbol =
      new AliasTypeSymbol(this, pos, name) with SynchronizedTypeSymbol initFlags newFlags

    override protected def createTypeSkolemSymbol(name: TypeName, origin: AnyRef, pos: Position, newFlags: Long): TypeSkolem =
      new TypeSkolem(this, pos, name, origin) with SynchronizedTypeSymbol initFlags newFlags

    override protected def createClassSymbol(name: TypeName, pos: Position, newFlags: Long): ClassSymbol =
      new ClassSymbol(this, pos, name) with SynchronizedClassSymbol initFlags newFlags

    override protected def createModuleClassSymbol(name: TypeName, pos: Position, newFlags: Long): ModuleClassSymbol =
      new ModuleClassSymbol(this, pos, name) with SynchronizedModuleClassSymbol initFlags newFlags

    override protected def createPackageClassSymbol(name: TypeName, pos: Position, newFlags: Long): PackageClassSymbol =
      new PackageClassSymbol(this, pos, name) with SynchronizedModuleClassSymbol initFlags newFlags

    override protected def createRefinementClassSymbol(pos: Position, newFlags: Long): RefinementClassSymbol =
      new RefinementClassSymbol(this, pos) with SynchronizedClassSymbol initFlags newFlags

    override protected def createImplClassSymbol(name: TypeName, pos: Position, newFlags: Long): ClassSymbol =
      new ClassSymbol(this, pos, name) with ImplClassSymbol with SynchronizedClassSymbol initFlags newFlags

    override protected def createPackageObjectClassSymbol(pos: Position, newFlags: Long): PackageObjectClassSymbol =
      new PackageObjectClassSymbol(this, pos) with SynchronizedClassSymbol initFlags newFlags

    override protected def createMethodSymbol(name: TermName, pos: Position, newFlags: Long): MethodSymbol =
      new MethodSymbol(this, pos, name) with SynchronizedMethodSymbol initFlags newFlags

    override protected def createModuleSymbol(name: TermName, pos: Position, newFlags: Long): ModuleSymbol =
      new ModuleSymbol(this, pos, name) with SynchronizedTermSymbol initFlags newFlags

    override protected def createPackageSymbol(name: TermName, pos: Position, newFlags: Long): ModuleSymbol =
      createModuleSymbol(name, pos, newFlags)

    override protected def createValueParameterSymbol(name: TermName, pos: Position, newFlags: Long) =
      new TermSymbol(this, pos, name) with SynchronizedTermSymbol initFlags newFlags

    override protected def createValueMemberSymbol(name: TermName, pos: Position, newFlags: Long) =
      new TermSymbol(this, pos, name) with SynchronizedTermSymbol initFlags newFlags

// ------- subclasses ---------------------------------------------------------------------

  trait SynchronizedTermSymbol extends SynchronizedSymbol

  trait SynchronizedMethodSymbol extends MethodSymbol with SynchronizedTermSymbol {
    // we can keep this lock fine-grained, because it's just a cache over asSeenFrom, which makes deadlocks impossible
    // unfortunately we cannot elide this lock, because the cache depends on `pre`
    private lazy val typeAsMemberOfLock = new Object
    override def typeAsMemberOf(pre: Type): Type = gilSynchronizedIfNotThreadsafe { typeAsMemberOfLock.synchronized { super.typeAsMemberOf(pre) } }

  trait SynchronizedModuleSymbol extends ModuleSymbol with SynchronizedTermSymbol

  trait SynchronizedTypeSymbol extends TypeSymbol with SynchronizedSymbol {
    // unlike with typeConstructor, a lock is necessary here, because tpe calculation relies on
    // temporarily assigning NoType to tpeCache to detect cyclic reference errors
    private lazy val tpeLock = new Object
    override def tpe_* : Type = gilSynchronizedIfNotThreadsafe { tpeLock.synchronized { super.tpe_* } }

  trait SynchronizedClassSymbol extends ClassSymbol with SynchronizedTypeSymbol

  trait SynchronizedModuleClassSymbol extends ModuleClassSymbol with SynchronizedClassSymbol

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