package com.google.inject.internal;

import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.Iterables;
import com.google.common.collect.ListMultimap;
import com.google.inject.Injector;
import com.google.inject.Key;
import com.google.inject.Provider;
import com.google.inject.ProvisionException;
import com.google.inject.Scope;
import com.google.inject.Scopes;
import com.google.inject.Singleton;
import com.google.inject.internal.CycleDetectingLock.CycleDetectingLockFactory;
import com.google.inject.spi.Dependency;
import com.google.inject.spi.Message;
import java.util.Formatter;
import java.util.List;

/**
 * One instance per {@link Injector}. Also see {@code @}{@link Singleton}.
 *
 * <p>Introduction from the author: Implementation of this class seems unreasonably complicated at
 * the first sight. I fully agree with you, that the beast below is very complex and it's hard to
 * reason on how does it work or not. Still I want to assure you that hundreds(?) of hours were
 * thrown into making this code simple, while still maintaining Singleton contract.
 *
 * <p>Anyway, why is it so complex? Singleton scope does not seem to be that unique. 1) Guice has
 * never truly expected to be used in multi threading environment with many Injectors working
 * alongside each other. There is almost no code with Guice that propagates state between threads.
 * And Singleton scope is The exception. 2) Guice supports circular dependencies and thus manages
 * proxy objects. There is no interface that allows user defined Scopes to create proxies, it is
 * expected to be done by Guice. Singleton scope needs to be able to detect circular dependencies
 * spanning several threads, therefore Singleton scope needs to be able to create these proxies. 3)
 * To make things worse, Guice has a very tricky definition for a binding resolution when Injectors
 * are in in a parent/child relationship. And Scope does not have access to this information by
 * design, the only real action that Scope can do is to call or not to call a creator. 4) There is
 * no readily available code in Guice that can detect a potential deadlock, and no code for handling
 * dependency cycles spanning several threads. This is significantly harder as all the dependencies
 * in a thread at runtime can be represented with a list, where in a multi threaded environment we
 * have more complex dependency trees. 5) Guice has a pretty strong contract regarding Garbage
 * Collection, which often prevents us from linking objects directly. So simple domain specific code
 * can not be written and intermediary id objects need to be managed. 6) Guice is relatively fast
 * and we should not make things worse. We're trying our best to optimize synchronization for speed
 * and memory. Happy path should be almost as fast as in a single threaded solution and should not
 * take much more memory. 7) Error message generation in Guice was not meant to be used like this
 * and to work around its APIs we need a lot of code. Additional complexity comes from inherent data
 * races as message is only generated when failure occurs on proxy object generation. Things get
 * ugly pretty fast.
 *
 * @see #scope(Key, Provider)
 * @see CycleDetectingLock
 * @author timofeyb (Timothy Basanov)
 */
public class SingletonScope implements Scope {

  /** A sentinel value representing null. */
  private static final Object NULL = new Object();

  /**
   * Allows us to detect when circular proxies are necessary. It's only used during singleton
   * instance initialization, after initialization direct access through volatile field is used.
   *
   * <p>NB: Factory uses {@link Key}s as a user locks ids, different injectors can share them.
   * Cycles are detected properly as cycle detection does not rely on user locks ids, but error
   * message generated could be less than ideal.
   */
  // TODO(user): we may use one factory per injector tree for optimization reasons
  private static final CycleDetectingLockFactory<Key<?>> cycleDetectingLockFactory =
      new CycleDetectingLockFactory<Key<?>>();

  /**
   * Provides singleton scope with the following properties: - creates no more than one instance per
   * Key as a creator is used no more than once, - result is cached and returned quickly on
   * subsequent calls, - exception in a creator is not treated as instance creation and is not
   * cached, - creates singletons in parallel whenever possible, - waits for dependent singletons to
   * be created even across threads and when dependencies are shared as long as no circular
   * dependencies are detected, - returns circular proxy only when circular dependencies are
   * detected, - aside from that, blocking synchronization is only used for proxy creation and
   * initialization,
   *
   * @see CycleDetectingLockFactory
   */
  @Override
  public <T> Provider<T> scope(final Key<T> key, final Provider<T> creator) {
    /**
     * Locking strategy: - volatile instance: double-checked locking for quick exit when scope is
     * initialized, - constructionContext: manipulations with proxies list or instance
     * initialization - creationLock: singleton instance creation, -- allows to guarantee only one
     * instance per singleton, -- special type of a lock, that prevents potential deadlocks, --
     * guards constructionContext for all operations except proxy creation
     */
    return new Provider<T>() {
      /**
       * The lazily initialized singleton instance. Once set, this will either have type T or will
       * be equal to NULL. Would never be reset to null.
       */
      volatile Object instance;

      /**
       * Circular proxies are used when potential deadlocks are detected. Guarded by itself.
       * ConstructionContext is not thread-safe, so each call should be synchronized.
       */
      final ConstructionContext<T> constructionContext = new ConstructionContext<>();

      /** For each binding there is a separate lock that we hold during object creation. */
      final CycleDetectingLock<Key<?>> creationLock = cycleDetectingLockFactory.create(key);

      /**
       * The singleton provider needs a reference back to the injector, in order to get ahold of
       * InternalContext during instantiation.
       */
      final /* @Nullable */ InjectorImpl injector;

      {
        // If we are getting called by Scoping
        if (creator instanceof ProviderToInternalFactoryAdapter) {
          injector = ((ProviderToInternalFactoryAdapter) creator).getInjector();
        } else {
          injector = null;
        }
      }

      @SuppressWarnings("DoubleCheckedLocking")
      @Override
      public T get() {
        // cache volatile variable for the usual case of already initialized object
        final Object initialInstance = instance;
        if (initialInstance == null) {
          // instance is not initialized yet

          // first, store the current InternalContext in a map, so that if there is a circular
          // dependency error, we can use the InternalContext objects to create a complete
          // error message.
          // Handle injector being null, which can happen when users call Scoping.scope themselves
          final InternalContext context = injector == null ? null : injector.getLocalContext();
          // acquire lock for current binding to initialize an instance
          final ListMultimap<Thread, Key<?>> locksCycle =
              creationLock.lockOrDetectPotentialLocksCycle();

          if (locksCycle.isEmpty()) {
            // this thread now owns creation of an instance
            try {
              // intentionally reread volatile variable to prevent double initialization
              if (instance == null) {
                // creator throwing an exception can cause circular proxies created in
                // different thread to never be resolved, just a warning
                T provided = creator.get();
                Object providedNotNull = provided == null ? NULL : provided;

                // scope called recursively can initialize instance as a side effect
                if (instance == null) {
                  // instance is still not initialized, so we can proceed

                  // don't remember proxies created by Guice on circular dependency
                  // detection within the same thread; they are not real instances to cache
                  if (Scopes.isCircularProxy(provided)) {
                    return provided;
                  }

                  synchronized (constructionContext) {
                    // guarantee thread-safety for instance and proxies initialization
                    instance = providedNotNull;
                    constructionContext.setProxyDelegates(provided);
                  }
                } else {
                  // safety assert in case instance was initialized
                  Preconditions.checkState(
                      instance == providedNotNull,
                      "Singleton is called recursively returning different results");
                }
              }
            } catch (RuntimeException e) {
              // something went wrong, be sure to clean a construction context
              // this helps to prevent potential memory leaks in circular proxies list
              synchronized (constructionContext) {
                constructionContext.finishConstruction();
              }
              throw e;
            } finally {
              // always release our creation lock, even on failures
              creationLock.unlock();
            }
          } else {
            if (context == null) {
              throw new ProvisionException(
                  ImmutableList.of(createCycleDependenciesMessage(locksCycle, null)));
            }
            // potential deadlock detected, creation lock is not taken by this thread
            synchronized (constructionContext) {
              // guarantee thread-safety for instance and proxies initialization
              if (instance == null) {
                // creating a proxy to satisfy circular dependency across several threads
                Dependency<?> dependency =
                    Preconditions.checkNotNull(
                        context.getDependency(), "internalContext.getDependency()");
                Class<?> rawType = dependency.getKey().getTypeLiteral().getRawType();

                try {
                  @SuppressWarnings("unchecked")
                  T proxy =
                      (T) constructionContext.createProxy(context.getInjectorOptions(), rawType);
                  return proxy;
                } catch (InternalProvisionException e) {
                  // best effort to create a rich error message
                  Message proxyCreationError = Iterables.getOnlyElement(e.getErrors());
                  Message cycleDependenciesMessage =
                      createCycleDependenciesMessage(locksCycle, proxyCreationError);
                  // adding stack trace generated by us in addition to a standard one
                  throw new ProvisionException(
                      ImmutableList.of(cycleDependenciesMessage, proxyCreationError));
                }
              }
            }
          }

          // at this point we're sure that singleton was initialized,
          // reread volatile variable to catch all corner cases

          // caching volatile variable to minimize number of reads performed
          final Object initializedInstance = instance;
          Preconditions.checkState(
              initializedInstance != null,
              "Internal error: Singleton is not initialized contrary to our expectations");
          @SuppressWarnings("unchecked")
          T initializedTypedInstance = (T) initializedInstance;
          return initializedInstance == NULL ? null : initializedTypedInstance;
        } else {
          // singleton is already initialized and local cache can be used
          @SuppressWarnings("unchecked")
          T typedInitialIntance = (T) initialInstance;
          return initialInstance == NULL ? null : typedInitialIntance;
        }
      }

      /**
       * Helper method to create beautiful and rich error descriptions. Best effort and slow. Tries
       * its best to provide dependency information from injectors currently available in a global
       * internal context.
       *
       * <p>The main thing being done is creating a list of Dependencies involved into lock cycle
       * across all the threads involved. This is a structure we're creating:
       *
       * <pre>
       * { Current Thread, C.class, B.class, Other Thread, B.class, C.class, Current Thread }
       * To be inserted in the beginning by Guice: { A.class, B.class, C.class }
       * </pre>
       *
       * When we're calling Guice to create A and it fails in the deadlock while trying to create C,
       * which is being created by another thread, which waits for B. List would be reversed before
       * printing it to the end user.
       */
      private Message createCycleDependenciesMessage(
          ListMultimap<Thread, Key<?>> locksCycle, /* @Nullable */ Message proxyCreationError) {
        // this is the main thing that we'll show in an error message,
        // current thread is populate by Guice
        StringBuilder sb = new StringBuilder();
        Formatter fmt = new Formatter(sb);
        fmt.format("Encountered circular dependency spanning several threads.");
        if (proxyCreationError != null) {
          fmt.format(" %s", proxyCreationError.getMessage());
        }
        fmt.format("%n");
        for (Thread lockedThread : locksCycle.keySet()) {
          List<Key<?>> lockedKeys = locksCycle.get(lockedThread);
          fmt.format("%s is holding locks the following singletons in the cycle:%n", lockedThread);
          for (Key<?> lockedKey : lockedKeys) {
            fmt.format("%s%n", Errors.convert(lockedKey));
          }
          for (StackTraceElement traceElement : lockedThread.getStackTrace()) {
            fmt.format("\tat %s%n", traceElement);
          }
        }
        fmt.close();
        return new Message(Thread.currentThread(), sb.toString());
      }

      @Override
      public String toString() {
        return String.format("%s[%s]", creator, Scopes.SINGLETON);
      }
    };
  }

  @Override
  public String toString() {
    return "Scopes.SINGLETON";
  }
}
