/*
 * Copyright (C) 2014, United States Government, as represented by the
 * Administrator of the National Aeronautics and Space Administration.
 * All rights reserved.
 *
 * The Java Pathfinder core (jpf-core) platform is licensed under the
 * Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 * 
 *        http://www.apache.org/licenses/LICENSE-2.0. 
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and 
 * limitations under the License.
 */
package gov.nasa.jpf.vm;

/**
 * SharednessPolicy is a configured policy that is used to detect data races for objects that are accessible from concurrent
 * threads.
 *
 * Its major purpose is to detect operations that need CGs in order to expose races, and introduce such CGs in a way that
 * minimizes state space overhead. This was previously implemented in various different places (ThreadInfo, FieldInstruction etc.)
 * and controlled by a number of "vm.por.*" properties. Strictly speaking, the default implementation does not do classic partial
 * order reduction, it merely tries to reduce states associated with shared objects based on information that was collected by
 * previous execution. All configuration therefore is now done through "vm.shared.*" properties that are loaded in
 * SharednessPolicy implementations.
 *
 * The interface of this class, which is used by field and array element accessors (GETx, PUTx, xASTORE, xALOAD, Field
 * reflection), revolves around two concepts:
 *
 * (1) OBJECT SHAREDNESS - this is supposed to be precise, i.e. a property associated with ElementInfo instances that is set if
 * such an instance is in fact used by two different threads. As such, it has to be updated from the respective accessors (e.g.
 * GETx, PUTx). Detecting sharedness is the responsibility of the SharednessPolicy, storing it is done by means of ElementInfo
 * flags (which could also be set explicitly by listeners or peers).
 *
 * Once an object is marked as shared, respective field and element access operations can lead to races and hence it has to be
 * determined if a CG should be registered to re-execute such instructions. In order to minimize superfluous states, the default
 * policies filter out a number of access specific conditions such as immutable objects, and access point state such as number of
 * runnable threads and especially LOCK PROTECTION (which should normally happen for well designed concurrent programs). Detecting
 * lock protection is field specific and is done by keeping track of (intersections of) lock sets held by the accessing thread.
 * The corresponding FieldLockInfo objects are stored in the respective ElementInfo. The default FieldLockInfo is semi-conservative
 * - even if there is a non-empty lock set it treats the access as unprotected for a certain number of times. If the set becomes
 * empty the object/field is permanently marked as unprotected. If the threshold is reached with a non-empty set, the object/field
 * is henceforth treated as protected (a warning is issued should the lock set subsequently become empty).  *
 * If the access operation passes all filters, the SharednessPolicy registers a CG, i.e. the respective operation is re-executed
 * after performing a scheduling choice. Semantic actions (pushing operand values, changing field values) is done in the bottom
 * half of such operations. However, this can lead to cases in which sharedness is not detected due to non-overlapping lifetimes
 * of accessing threads. Note this does not mean that there is only one live thread at a time, only that threads don't have a CG
 * within their overlapping lifetime that would expose the race. This can lead to missed paths/errors.
 *
 * (2) OBJECT EXPOSURE - is a conservative measure to avoid such missed paths. It is used to introduce additional CGs when a
 * reference to an unshared object gets stored in a shared object, i.e. it is conceptually related to the object whose reference
 * gets stored (i.e. the field value), not to the object that holds the field. Exposure CGs are conservative since at this point
 * JPF only knows that the exposed object /could/ become shared in the future, not that it will. There are various different
 * degrees of conservatism that can be employed (transitive, only first object etc.), but exposure CGs can be a major contributor
 * to state explosion and hence should be minimized. Exposure CGs should break /after/ the semantic action (e.g. field
 * assignment), so that it becomes visible to other threads. This leads to a tricky problem that the bottom half of related
 * accessors has to determine if the semantic action already took place (exposure CGs) or not (sharedness CG). The action is not
 * allowed to be re-executed in the bottom half since this could change the SUT program behavior. In order to determine execution
 * state, implementation mechanisms have to be aware of that there can be any number of transitions between the top and bottom
 * half, i.e. cannot rely on the current CG in the bottom half execution. Conceptually, the execution state is a StackFrame
 * attribute.
 *
 * Note that exposure CGs are not mandatory. Concrete SharednessPolicy implementations can either ignore them in bug finding mode,
 * or can replace them by means of re-execution.
 * 
 * Concrete SharednessPolicy implementations fall within a spectrum that is marked by two extremes: SEARCH GLOBAL and PATH LOCAL
 * behavior. Search global policies are mostly for bug finding and keep sharedness, lock protection and exposure information from
 * previously executed paths. This has two implications: (a) the search policy / execution order matters (e.g. leading to
 * different results when randomizing choice selection), (b) path replay based on CG traces can lead to different results (e.g.
 * not showing errors found in a previous search).
 *
 * The opposite mode is path local behavior, i.e. no sharednes/protection/exposure information from previous execution paths is
 * used. This should yield the same results for the same path, no matter of execution history. This mode requires the use of
 * persistent data structures for sharedness, lock info and exposure, and hence can be significantly more expensive. However, it
 * is required to guarantee path reply that preserves outcome.
 *
 * Although the two standard policy implementations (GlobalSharednessPolicy and PathSharednessPolicy) correspond to static
 * incarnations of these extremes, it should be noted that the SharednessPolicy interface strives to accommodate mixed and dynamic
 * modes, especially controlling re-execution with adaptive behavior. A primary use for this could be to avoid exposure CGs until
 * additional information becomes available that indicates object sharedness (e.g. non-overlapping thread access), then backtrack
 * to a common ancestor state and re-execute with added exposure CGs for the respective object/field.
 *
 * The motivation for this flexibility and the related implementation complexity/cost is that race detection based on field/array
 * access is a major contributor to state explosion. In many cases, suitable optimizations make the difference between running
 * into search constraints or finishing the search.
 */
public interface SharednessPolicy {
  
  /**
   * per application / SystemClassLoaderInfo specific initialization of this policy 
   */
  void initializeSharednessPolicy (VM vm, ApplicationContext appCtx);
  
  /**
   * initializeSharednessPolicy object specific sharedness data 
   */
  void initializeObjectSharedness (ThreadInfo allocThread, DynamicElementInfo ei);
  
  /**
   * initializeSharednessPolicy class specific sharedness data 
   */  
  void initializeClassSharedness (ThreadInfo allocThread, StaticElementInfo ei);
  
  
  boolean canHaveSharedObjectCG (ThreadInfo ti, Instruction insn, ElementInfo eiFieldOwner, FieldInfo fi);
  ElementInfo updateObjectSharedness (ThreadInfo ti, ElementInfo ei, FieldInfo fi);
  boolean setsSharedObjectCG  (ThreadInfo ti, Instruction insn, ElementInfo eiFieldOwner, FieldInfo fi);
  
  boolean canHaveSharedClassCG (ThreadInfo ti, Instruction insn, ElementInfo eiFieldOwner, FieldInfo fi);
  ElementInfo updateClassSharedness (ThreadInfo ti, ElementInfo ei, FieldInfo fi);
  boolean setsSharedClassCG  (ThreadInfo ti, Instruction insn, ElementInfo eiFieldOwner, FieldInfo fi);
  
  boolean canHaveSharedArrayCG (ThreadInfo ti, Instruction insn, ElementInfo eiArray, int idx);
  ElementInfo updateArraySharedness (ThreadInfo ti, ElementInfo eiArray, int idx);
  boolean setsSharedArrayCG  (ThreadInfo ti, Instruction insn, ElementInfo eiArray, int idx);
  
  boolean setsSharedObjectExposureCG (ThreadInfo ti, Instruction insn, ElementInfo eiFieldOwner, FieldInfo fi, ElementInfo eiExposed);
  boolean setsSharedClassExposureCG (ThreadInfo ti, Instruction insn, ElementInfo eiFieldOwner, FieldInfo fi, ElementInfo eiExposed);
  
  
  /**
   * give policy a chance to clean up referencing ThreadInfoSets upon
   * thread termination
   */
  void cleanupThreadTermination(ThreadInfo ti);
}