ThreadPlan.h

Go to the documentation of this file.

//===-- ThreadPlan.h --------------------------------------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef liblldb_ThreadPlan_h_
#define liblldb_ThreadPlan_h_

#include <mutex>
#include <string>

#include "lldb/Target/Process.h"
#include "lldb/Target/StopInfo.h"
#include "lldb/Target/Target.h"
#include "lldb/Target/Thread.h"
#include "lldb/Target/ThreadPlanTracer.h"
#include "lldb/Utility/UserID.h"
#include "lldb/lldb-private.h"

namespace lldb_private {

//  ThreadPlan:
//  This is the pure virtual base class for thread plans.
//
//  The thread plans provide the "atoms" of behavior that
//  all the logical process control, either directly from commands or through
//  more complex composite plans will rely on.
//
//  Plan Stack:
//
//  The thread maintaining a thread plan stack, and you program the actions of a
//  particular thread
//  by pushing plans onto the plan stack.
//  There is always a "Current" plan, which is the top of the plan stack,
//  though in some cases
//  a plan may defer to plans higher in the stack for some piece of information
//  (let us define that the plan stack grows downwards).
//
//  The plan stack is never empty, there is always a Base Plan which persists
//  through the life
//  of the running process.
//
//
//  Creating Plans:
//
//  The thread plan is generally created and added to the plan stack through the
//  QueueThreadPlanFor... API
//  in lldb::Thread.  Those API's will return the plan that performs the named
//  operation in a manner
//  appropriate for the current process.  The plans in lldb/source/Target are
//  generic
//  implementations, but a Process plugin can override them.
//
//  ValidatePlan is then called.  If it returns false, the plan is unshipped.
//  This is a little
//  convenience which keeps us from having to error out of the constructor.
//
//  Then the plan is added to the plan stack.  When the plan is added to the
//  plan stack its DidPush
//  will get called.  This is useful if a plan wants to push any additional
//  plans as it is constructed,
//  since you need to make sure you're already on the stack before you push
//  additional plans.
//
//  Completed Plans:
//
//  When the target process stops the plans are queried, among other things, for
//  whether their job is done.
//  If it is they are moved from the plan stack to the Completed Plan stack in
//  reverse order from their position
//  on the plan stack (since multiple plans may be done at a given stop.)  This
//  is used primarily so that
//  the lldb::Thread::StopInfo for the thread can be set properly.  If one plan
//  pushes another to achieve part of
//  its job, but it doesn't want that sub-plan to be the one that sets the
//  StopInfo, then call SetPrivate on the
//  sub-plan when you create it, and the Thread will pass over that plan in
//  reporting the reason for the stop.
//
//  Discarded plans:
//
//  Your plan may also get discarded, i.e. moved from the plan stack to the
//  "discarded plan stack".  This can
//  happen, for instance, if the plan is calling a function and the function
//  call crashes and you want
//  to unwind the attempt to call.  So don't assume that your plan will always
//  successfully stop.  Which leads to:
//
//  Cleaning up after your plans:
//
//  When the plan is moved from the plan stack its WillPop method is always
//  called, no matter why.  Once it is
//  moved off the plan stack it is done, and won't get a chance to run again.
//  So you should
//  undo anything that affects target state in this method.  But be sure to
//  leave the plan able to correctly
//  fill the StopInfo, however.
//  N.B. Don't wait to do clean up target state till the destructor, since that
//  will usually get called when
//  the target resumes, and you want to leave the target state correct for new
//  plans in the time between when
//  your plan gets unshipped and the next resume.
//
//  Thread State Checkpoint:
//
//  Note that calling functions on target process (ThreadPlanCallFunction) changes
//  current thread state. The function can be called either by direct user demand or
//  internally, for example lldb allocates memory on device to calculate breakpoint
//  condition expression - on Linux it is performed by calling mmap on device.
//  ThreadStateCheckpoint saves Thread state (stop info and completed
//  plan stack) to restore it after completing function call.
//
//  Over the lifetime of the plan, various methods of the ThreadPlan are then
//  called in response to changes of state in
//  the process we are debugging as follows:
//
//  Resuming:
//
//  When the target process is about to be restarted, the plan's WillResume
//  method is called,
//  giving the plan a chance to prepare for the run.  If WillResume returns
//  false, then the
//  process is not restarted.  Be sure to set an appropriate error value in the
//  Process if
//  you have to do this.  Note, ThreadPlans actually implement DoWillResume,
//  WillResume wraps that call.
//
//  Next the "StopOthers" method of all the threads are polled, and if one
//  thread's Current plan
//  returns "true" then only that thread gets to run.  If more than one returns
//  "true" the threads that want to run solo
//  get run one by one round robin fashion.  Otherwise all are let to run.
//
//  Note, the way StopOthers is implemented, the base class implementation just
//  asks the previous plan.  So if your plan
//  has no opinion about whether it should run stopping others or not, just
//  don't implement StopOthers, and the parent
//  will be asked.
//
//  Finally, for each thread that is running, it run state is set to the return
//  of RunState from the
//  thread's Current plan.
//
//  Responding to a stop:
//
//  When the target process stops, the plan is called in the following stages:
//
//  First the thread asks the Current Plan if it can handle this stop by calling
//  PlanExplainsStop.
//  If the Current plan answers "true" then it is asked if the stop should
//  percolate all the way to the
//  user by calling the ShouldStop method.  If the current plan doesn't explain
//  the stop, then we query up
//  the plan stack for a plan that does explain the stop.  The plan that does
//  explain the stop then needs to
//  figure out what to do about the plans below it in the stack.  If the stop is
//  recoverable, then the plan that
//  understands it can just do what it needs to set up to restart, and then
//  continue.
//  Otherwise, the plan that understood the stop should call DiscardPlanStack to
//  clean up the stack below it.
//  Note, plans actually implement DoPlanExplainsStop, the result is cached in
//  PlanExplainsStop so the DoPlanExplainsStop
//  itself will only get called once per stop.
//
//  Master plans:
//
//  In the normal case, when we decide to stop, we will  collapse the plan stack
//  up to the point of the plan that understood
//  the stop reason.  However, if a plan wishes to stay on the stack after an
//  event it didn't directly handle
//  it can designate itself a "Master" plan by responding true to IsMasterPlan,
//  and then if it wants not to be
//  discarded, it can return false to OkayToDiscard, and it and all its dependent
//  plans will be preserved when
//  we resume execution.
//
//  The other effect of being a master plan is that when the Master plan is done
//  , if it has set "OkayToDiscard" to false,
//  then it will be popped & execution will stop and return to the user.
//  Remember that if OkayToDiscard is false, the
//  plan will be popped and control will be given to the next plan above it on
//  the stack  So setting OkayToDiscard to
//  false means the user will regain control when the MasterPlan is completed.
//
//  Between these two controls this allows things like: a MasterPlan/DontDiscard
//  Step Over to hit a breakpoint, stop and
//  return control to the user, but then when the user continues, the step out
//  succeeds.
//  Even more tricky, when the breakpoint is hit, the user can continue to step
//  in/step over/etc, and finally when they
//  continue, they will finish up the Step Over.
//
//  FIXME: MasterPlan & OkayToDiscard aren't really orthogonal.  MasterPlan
//  designation means that this plan controls
//  it's fate and the fate of plans below it.  OkayToDiscard tells whether the
//  MasterPlan wants to stay on the stack.  I
//  originally thought "MasterPlan-ness" would need to be a fixed characteristic
//  of a ThreadPlan, in which case you needed
//  the extra control.  But that doesn't seem to be true.  So we should be able
//  to convert to only MasterPlan status to mean
//  the current "MasterPlan/DontDiscard".  Then no plans would be MasterPlans by
//  default, and you would set the ones you
//  wanted to be "user level" in this way.
//
//
//  Actually Stopping:
//
//  If a plan says responds "true" to ShouldStop, then it is asked if it's job
//  is complete by calling
//  MischiefManaged.  If that returns true, the plan is popped from the plan
//  stack and added to the
//  Completed Plan Stack.  Then the next plan in the stack is asked if it
//  ShouldStop, and  it returns "true",
//  it is asked if it is done, and if yes popped, and so on till we reach a plan
//  that is not done.
//
//  Since you often know in the ShouldStop method whether your plan is complete,
//  as a convenience you can call
//  SetPlanComplete and the ThreadPlan implementation of MischiefManaged will
//  return "true", without your having
//  to redo the calculation when your sub-classes MischiefManaged is called.  If
//  you call SetPlanComplete, you can
//  later use IsPlanComplete to determine whether the plan is complete.  This is
//  only a convenience for sub-classes,
//  the logic in lldb::Thread will only call MischiefManaged.
//
//  One slightly tricky point is you have to be careful using SetPlanComplete in
//  PlanExplainsStop because you
//  are not guaranteed that PlanExplainsStop for a plan will get called before
//  ShouldStop gets called.  If your sub-plan
//  explained the stop and then popped itself, only your ShouldStop will get
//  called.
//
//  If ShouldStop for any thread returns "true", then the WillStop method of the
//  Current plan of
//  all threads will be called, the stop event is placed on the Process's public
//  broadcaster, and
//  control returns to the upper layers of the debugger.
//
//  Reporting the stop:
//
//  When the process stops, the thread is given a StopReason, in the form of a
//  StopInfo object.  If there is a completed
//  plan corresponding to the stop, then the "actual" stop reason can be
//  suppressed, and instead a StopInfoThreadPlan
//  object will be cons'ed up from the top completed plan in the stack.
//  However, if the plan doesn't want to be
//  the stop reason, then it can call SetPlanComplete and pass in "false" for
//  the "success" parameter.  In that case,
//  the real stop reason will be used instead.  One exapmle of this is the
//  "StepRangeStepIn" thread plan.  If it stops
//  because of a crash or breakpoint hit, it wants to unship itself, because it
//  isn't so useful to have step in keep going
//  after a breakpoint hit.  But it can't be the reason for the stop or no-one
//  would see that they had hit a breakpoint.
//
//  Cleaning up the plan stack:
//
//  One of the complications of MasterPlans is that you may get past the limits
//  of a plan without triggering it to clean
//  itself up.  For instance, if you are doing a MasterPlan StepOver, and hit a
//  breakpoint in a called function, then
//  step over enough times to step out of the initial StepOver range, each of
//  the step overs will explain the stop &
//  take themselves off the stack, but control would never be returned to the
//  original StepOver.  Eventually, the user
//  will continue, and when that continue stops, the old stale StepOver plan
//  that was left on the stack will get woken
//  up and notice it is done. But that can leave junk on the stack for a while.
//  To avoid that, the plans implement a
//  "IsPlanStale" method, that can check whether it is relevant anymore.  On
//  stop, after the regular plan negotiation,
//  the remaining plan stack is consulted and if any plan says it is stale, it
//  and the plans below it are discarded from
//  the stack.
//
//  Automatically Resuming:
//
//  If ShouldStop for all threads returns "false", then the target process will
//  resume.  This then cycles back to
//  Resuming above.
//
//  Reporting eStateStopped events when the target is restarted:
//
//  If a plan decides to auto-continue the target by returning "false" from
//  ShouldStop, then it will be asked
//  whether the Stopped event should still be reported.  For instance, if you
//  hit a breakpoint that is a User set
//  breakpoint, but the breakpoint callback said to continue the target process,
//  you might still want to inform
//  the upper layers of lldb that the stop had happened.
//  The way this works is every thread gets to vote on whether to report the
//  stop.  If all votes are eVoteNoOpinion,
//  then the thread list will decide what to do (at present it will pretty much
//  always suppress these stopped events.)
//  If there is an eVoteYes, then the event will be reported regardless of the
//  other votes.  If there is an eVoteNo
//  and no eVoteYes's, then the event won't be reported.
//
//  One other little detail here, sometimes a plan will push another plan onto
//  the plan stack to do some part of
//  the first plan's job, and it would be convenient to tell that plan how it
//  should respond to ShouldReportStop.
//  You can do that by setting the stop_vote in the child plan when you create
//  it.
//
//  Suppressing the initial eStateRunning event:
//
//  The private process running thread will take care of ensuring that only one
//  "eStateRunning" event will be
//  delivered to the public Process broadcaster per public eStateStopped event.
//  However there are some cases
//  where the public state of this process is eStateStopped, but a thread plan
//  needs to restart the target, but
//  doesn't want the running event to be publicly broadcast.  The obvious
//  example of this is running functions
//  by hand as part of expression evaluation.  To suppress the running event
//  return eVoteNo from ShouldReportStop,
//  to force a running event to be reported return eVoteYes, in general though
//  you should return eVoteNoOpinion
//  which will allow the ThreadList to figure out the right thing to do.
//  The run_vote argument to the constructor works like stop_vote, and is a way
//  for a plan to instruct a sub-plan
//  on how to respond to ShouldReportStop.
//

class ThreadPlan : public std::enable_shared_from_this<ThreadPlan>,
                   public UserID {
public:
  typedef enum { eAllThreads, eSomeThreads, eThisThread } ThreadScope;

  // We use these enums so that we can cast a base thread plan to it's real
  // type without having to resort to dynamic casting.
  typedef enum {
    eKindGeneric,
    eKindNull,
    eKindBase,
    eKindCallFunction,
    eKindPython,
    eKindStepInstruction,
    eKindStepOut,
    eKindStepOverBreakpoint,
    eKindStepOverRange,
    eKindStepInRange,
    eKindRunToAddress,
    eKindStepThrough,
    eKindStepUntil,
    eKindTestCondition

  } ThreadPlanKind;

  // Constructors and Destructors
  ThreadPlan(ThreadPlanKind kind, const char *name, Thread &thread,
             Vote stop_vote, Vote run_vote);

  virtual ~ThreadPlan();

  /// Returns the name of this thread plan.
  ///
  /// \return
  ///   A const char * pointer to the thread plan's name.
  const char *GetName() const { return m_name.c_str(); }

  /// Returns the Thread that is using this thread plan.
  ///
  /// \return
  ///   A  pointer to the thread plan's owning thread.
  Thread &GetThread() { return m_thread; }

  const Thread &GetThread() const { return m_thread; }

  Target &GetTarget() { return m_thread.GetProcess()->GetTarget(); }

  const Target &GetTarget() const { return m_thread.GetProcess()->GetTarget(); }

  /// Print a description of this thread to the stream \a s.
  /// \a thread.
  ///
  /// \param[in] s
  ///    The stream to which to print the description.
  ///
  /// \param[in] level
  ///    The level of description desired.  Note that eDescriptionLevelBrief
  ///    will be used in the stop message printed when the plan is complete.
  virtual void GetDescription(Stream *s, lldb::DescriptionLevel level) = 0;

  /// Returns whether this plan could be successfully created.
  ///
  /// \param[in] error
  ///    A stream to which to print some reason why the plan could not be
  ///    created.
  ///    Can be NULL.
  ///
  /// \return
  ///   \b true if the plan should be queued, \b false otherwise.
  virtual bool ValidatePlan(Stream *error) = 0;

  bool TracerExplainsStop() {
    if (!m_tracer_sp)
      return false;
    else
      return m_tracer_sp->TracerExplainsStop();
  }

  lldb::StateType RunState();

  bool PlanExplainsStop(Event *event_ptr);

  virtual bool ShouldStop(Event *event_ptr) = 0;

  virtual bool ShouldAutoContinue(Event *event_ptr) { return false; }

  // Whether a "stop class" event should be reported to the "outside world".
  // In general if a thread plan is active, events should not be reported.

  virtual Vote ShouldReportStop(Event *event_ptr);

  virtual Vote ShouldReportRun(Event *event_ptr);

  virtual void SetStopOthers(bool new_value);

  virtual bool StopOthers();

  // This is the wrapper for DoWillResume that does generic ThreadPlan logic,
  // then calls DoWillResume.
  bool WillResume(lldb::StateType resume_state, bool current_plan);

  virtual bool WillStop() = 0;

  bool IsMasterPlan() { return m_is_master_plan; }

  bool SetIsMasterPlan(bool value) {
    bool old_value = m_is_master_plan;
    m_is_master_plan = value;
    return old_value;
  }

  virtual bool OkayToDiscard();

  void SetOkayToDiscard(bool value) { m_okay_to_discard = value; }

  // The base class MischiefManaged does some cleanup - so you have to call it
  // in your MischiefManaged derived class.
  virtual bool MischiefManaged();

  virtual void ThreadDestroyed() {
    // Any cleanup that a plan might want to do in case the thread goes away in
    // the middle of the plan being queued on a thread can be done here.
  }

  bool GetPrivate() { return m_plan_private; }

  void SetPrivate(bool input) { m_plan_private = input; }

  virtual void DidPush();

  virtual void WillPop();

  // This pushes a plan onto the plan stack of the current plan's thread.
  void PushPlan(lldb::ThreadPlanSP &thread_plan_sp) {
    m_thread.PushPlan(thread_plan_sp);
  }

  ThreadPlanKind GetKind() const { return m_kind; }

  bool IsPlanComplete();

  void SetPlanComplete(bool success = true);

  virtual bool IsPlanStale() { return false; }

  bool PlanSucceeded() { return m_plan_succeeded; }

  virtual bool IsBasePlan() { return false; }

  lldb::ThreadPlanTracerSP &GetThreadPlanTracer() { return m_tracer_sp; }

  void SetThreadPlanTracer(lldb::ThreadPlanTracerSP new_tracer_sp) {
    m_tracer_sp = new_tracer_sp;
  }

  void DoTraceLog() {
    if (m_tracer_sp && m_tracer_sp->TracingEnabled())
      m_tracer_sp->Log();
  }

  // Some thread plans hide away the actual stop info which caused any
  // particular stop.  For instance the ThreadPlanCallFunction restores the
  // original stop reason so that stopping and calling a few functions won't
  // lose the history of the run. This call can be implemented to get you back
  // to the real stop info.
  virtual lldb::StopInfoSP GetRealStopInfo() { return m_thread.GetStopInfo(); }

  // If the completion of the thread plan stepped out of a function, the return
  // value of the function might have been captured by the thread plan
  // (currently only ThreadPlanStepOut does this.) If so, the ReturnValueObject
  // can be retrieved from here.

  virtual lldb::ValueObjectSP GetReturnValueObject() {
    return lldb::ValueObjectSP();
  }

  // If the thread plan managing the evaluation of a user expression lives
  // longer than the command that instigated the expression (generally because
  // the expression evaluation hit a breakpoint, and the user regained control
  // at that point) a subsequent process control command step/continue/etc.
  // might complete the expression evaluations.  If so, the result of the
  // expression evaluation will show up here.

  virtual lldb::ExpressionVariableSP GetExpressionVariable() {
    return lldb::ExpressionVariableSP();
  }

  // If a thread plan stores the state before it was run, then you might want
  // to restore the state when it is done.  This will do that job. This is
  // mostly useful for artificial plans like CallFunction plans.

  virtual bool RestoreThreadState() {
    // Nothing to do in general.
    return true;
  }

  virtual bool IsVirtualStep() { return false; }

  virtual bool SetIterationCount(size_t count) {
    if (m_takes_iteration_count) {
      // Don't tell me to do something 0 times...
      if (count == 0)
        return false;
      m_iteration_count = count;
    }
    return m_takes_iteration_count;
  }

  virtual size_t GetIterationCount() {
    if (!m_takes_iteration_count)
      return 0;
    else
      return m_iteration_count;
  }

protected:
  // Classes that inherit from ThreadPlan can see and modify these

  virtual bool DoWillResume(lldb::StateType resume_state, bool current_plan) {
    return true;
  }

  virtual bool DoPlanExplainsStop(Event *event_ptr) = 0;

  // This gets the previous plan to the current plan (for forwarding requests).
  // This is mostly a formal requirement, it allows us to make the Thread's
  // GetPreviousPlan protected, but only friend ThreadPlan to thread.

  ThreadPlan *GetPreviousPlan() { return m_thread.GetPreviousPlan(this); }

  // This forwards the private Thread::GetPrivateStopInfo which is generally
  // what ThreadPlan's need to know.

  lldb::StopInfoSP GetPrivateStopInfo() {
    return m_thread.GetPrivateStopInfo();
  }

  void SetStopInfo(lldb::StopInfoSP stop_reason_sp) {
    m_thread.SetStopInfo(stop_reason_sp);
  }

  void CachePlanExplainsStop(bool does_explain) {
    m_cached_plan_explains_stop = does_explain ? eLazyBoolYes : eLazyBoolNo;
  }

  LazyBool GetCachedPlanExplainsStop() const {
    return m_cached_plan_explains_stop;
  }

  virtual lldb::StateType GetPlanRunState() = 0;

  bool IsUsuallyUnexplainedStopReason(lldb::StopReason);

  Status m_status;
  Thread &m_thread;
  Vote m_stop_vote;
  Vote m_run_vote;
  bool m_takes_iteration_count;
  bool m_could_not_resolve_hw_bp;
  int32_t m_iteration_count = 1;

private:
  // For ThreadPlan only
  static lldb::user_id_t GetNextID();

  ThreadPlanKind m_kind;
  std::string m_name;
  std::recursive_mutex m_plan_complete_mutex;
  LazyBool m_cached_plan_explains_stop;
  bool m_plan_complete;
  bool m_plan_private;
  bool m_okay_to_discard;
  bool m_is_master_plan;
  bool m_plan_succeeded;

  lldb::ThreadPlanTracerSP m_tracer_sp;

private:
  DISALLOW_COPY_AND_ASSIGN(ThreadPlan);
};

// ThreadPlanNull:
// Threads are assumed to always have at least one plan on the plan stack. This
// is put on the plan stack when a thread is destroyed so that if you
// accidentally access a thread after it is destroyed you won't crash. But
// asking questions of the ThreadPlanNull is definitely an error.

class ThreadPlanNull : public ThreadPlan {
public:
  ThreadPlanNull(Thread &thread);
  ~ThreadPlanNull() override;

  void GetDescription(Stream *s, lldb::DescriptionLevel level) override;

  bool ValidatePlan(Stream *error) override;

  bool ShouldStop(Event *event_ptr) override;

  bool MischiefManaged() override;

  bool WillStop() override;

  bool IsBasePlan() override { return true; }

  bool OkayToDiscard() override { return false; }

  const Status &GetStatus() { return m_status; }

protected:
  bool DoPlanExplainsStop(Event *event_ptr) override;

  lldb::StateType GetPlanRunState() override;

  DISALLOW_COPY_AND_ASSIGN(ThreadPlanNull);
};

} // namespace lldb_private

#endif // liblldb_ThreadPlan_h_