src/core/ext/client_channel/lb_policy.h

/*
 *
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#ifndef GRPC_CORE_EXT_CLIENT_CHANNEL_LB_POLICY_H
#define GRPC_CORE_EXT_CLIENT_CHANNEL_LB_POLICY_H

#include "src/core/ext/client_channel/subchannel.h"
#include "src/core/lib/iomgr/polling_entity.h"
#include "src/core/lib/transport/connectivity_state.h"

/** A load balancing policy: specified by a vtable and a struct (which
    is expected to be extended to contain some parameters) */
typedef struct grpc_lb_policy grpc_lb_policy;
typedef struct grpc_lb_policy_vtable grpc_lb_policy_vtable;

typedef void (*grpc_lb_completion)(void *cb_arg, grpc_subchannel *subchannel,
                                   grpc_status_code status, const char *errmsg);

struct grpc_lb_policy {
  const grpc_lb_policy_vtable *vtable;
  gpr_atm ref_pair;
  /* owned pointer to interested parties in load balancing decisions */
  grpc_pollset_set *interested_parties;
  /* combiner under which lb_policy actions take place */
  grpc_combiner *combiner;
};

/** Extra arguments for an LB pick */
typedef struct grpc_lb_policy_pick_args {
  /** Initial metadata associated with the picking call. */
  grpc_metadata_batch *initial_metadata;
  /** Bitmask used for selective cancelling. See \a
   * grpc_lb_policy_cancel_picks() and \a GRPC_INITIAL_METADATA_* in
   * grpc_types.h */
  uint32_t initial_metadata_flags;
  /** Storage for LB token in \a initial_metadata, or NULL if not used */
  grpc_linked_mdelem *lb_token_mdelem_storage;
  /** Deadline for the call to the LB server */
  gpr_timespec deadline;
} grpc_lb_policy_pick_args;

struct grpc_lb_policy_vtable {
  void (*destroy)(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy);
  void (*shutdown_locked)(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy);

  /** \see grpc_lb_policy_pick */
  int (*pick_locked)(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
                     const grpc_lb_policy_pick_args *pick_args,
                     grpc_connected_subchannel **target, void **user_data,
                     grpc_closure *on_complete);

  /** \see grpc_lb_policy_cancel_pick */
  void (*cancel_pick_locked)(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
                             grpc_connected_subchannel **target,
                             grpc_error *error);

  /** \see grpc_lb_policy_cancel_picks */
  void (*cancel_picks_locked)(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
                              uint32_t initial_metadata_flags_mask,
                              uint32_t initial_metadata_flags_eq,
                              grpc_error *error);

  /** \see grpc_lb_policy_ping_one */
  void (*ping_one_locked)(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
                          grpc_closure *closure);

  /** Try to enter a READY connectivity state */
  void (*exit_idle_locked)(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy);

  /** check the current connectivity of the lb_policy */
  grpc_connectivity_state (*check_connectivity_locked)(
      grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
      grpc_error **connectivity_error);

  /** call notify when the connectivity state of a channel changes from *state.
      Updates *state with the new state of the policy. Calling with a NULL \a
      state cancels the subscription.  */
  void (*notify_on_state_change_locked)(grpc_exec_ctx *exec_ctx,
                                        grpc_lb_policy *policy,
                                        grpc_connectivity_state *state,
                                        grpc_closure *closure);
};

/*#define GRPC_LB_POLICY_REFCOUNT_DEBUG*/
#ifdef GRPC_LB_POLICY_REFCOUNT_DEBUG

/* Strong references: the policy will shutdown when they reach zero */
#define GRPC_LB_POLICY_REF(p, r) \
  grpc_lb_policy_ref((p), __FILE__, __LINE__, (r))
#define GRPC_LB_POLICY_UNREF(exec_ctx, p, r) \
  grpc_lb_policy_unref((exec_ctx), (p), __FILE__, __LINE__, (r))

/* Weak references: they don't prevent the shutdown of the LB policy. When no
 * strong references are left but there are still weak ones, shutdown is called.
 * Once the weak reference also reaches zero, the LB policy is destroyed. */
#define GRPC_LB_POLICY_WEAK_REF(p, r) \
  grpc_lb_policy_weak_ref((p), __FILE__, __LINE__, (r))
#define GRPC_LB_POLICY_WEAK_UNREF(exec_ctx, p, r) \
  grpc_lb_policy_weak_unref((exec_ctx), (p), __FILE__, __LINE__, (r))
void grpc_lb_policy_ref(grpc_lb_policy *policy, const char *file, int line,
                        const char *reason);
void grpc_lb_policy_unref(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
                          const char *file, int line, const char *reason);
void grpc_lb_policy_weak_ref(grpc_lb_policy *policy, const char *file, int line,
                             const char *reason);
void grpc_lb_policy_weak_unref(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
                               const char *file, int line, const char *reason);
#else
#define GRPC_LB_POLICY_REF(p, r) grpc_lb_policy_ref((p))
#define GRPC_LB_POLICY_UNREF(cl, p, r) grpc_lb_policy_unref((cl), (p))
#define GRPC_LB_POLICY_WEAK_REF(p, r) grpc_lb_policy_weak_ref((p))
#define GRPC_LB_POLICY_WEAK_UNREF(cl, p, r) grpc_lb_policy_weak_unref((cl), (p))
void grpc_lb_policy_ref(grpc_lb_policy *policy);
void grpc_lb_policy_unref(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy);
void grpc_lb_policy_weak_ref(grpc_lb_policy *policy);
void grpc_lb_policy_weak_unref(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy);
#endif

/** called by concrete implementations to initialize the base struct */
void grpc_lb_policy_init(grpc_lb_policy *policy,
                         const grpc_lb_policy_vtable *vtable,
                         grpc_combiner *combiner);

/** Finds an appropriate subchannel for a call, based on \a pick_args.

    \a target will be set to the selected subchannel, or NULL on failure.
    Upon success, \a user_data will be set to whatever opaque information
    may need to be propagated from the LB policy, or NULL if not needed.

    If the pick succeeds and a result is known immediately, a non-zero
    value will be returned.  Otherwise, \a on_complete will be invoked
    once the pick is complete with its error argument set to indicate
    success or failure.

    Any IO should be done under the \a interested_parties \a grpc_pollset_set
    in the \a grpc_lb_policy struct. */
int grpc_lb_policy_pick_locked(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
                               const grpc_lb_policy_pick_args *pick_args,
                               grpc_connected_subchannel **target,
                               void **user_data, grpc_closure *on_complete);

/** Perform a connected subchannel ping (see \a grpc_connected_subchannel_ping)
    against one of the connected subchannels managed by \a policy. */
void grpc_lb_policy_ping_one_locked(grpc_exec_ctx *exec_ctx,
                                    grpc_lb_policy *policy,
                                    grpc_closure *closure);

/** Cancel picks for \a target.
    The \a on_complete callback of the pending picks will be invoked with \a
    *target set to NULL. */
void grpc_lb_policy_cancel_pick_locked(grpc_exec_ctx *exec_ctx,
                                       grpc_lb_policy *policy,
                                       grpc_connected_subchannel **target,
                                       grpc_error *error);

/** Cancel all pending picks for which their \a initial_metadata_flags (as given
    in the call to \a grpc_lb_policy_pick) matches \a initial_metadata_flags_eq
    when AND'd with \a initial_metadata_flags_mask */
void grpc_lb_policy_cancel_picks_locked(grpc_exec_ctx *exec_ctx,
                                        grpc_lb_policy *policy,
                                        uint32_t initial_metadata_flags_mask,
                                        uint32_t initial_metadata_flags_eq,
                                        grpc_error *error);

/** Try to enter a READY connectivity state */
void grpc_lb_policy_exit_idle_locked(grpc_exec_ctx *exec_ctx,
                                     grpc_lb_policy *policy);

/* Call notify when the connectivity state of a channel changes from \a *state.
 * Updates \a *state with the new state of the policy */
void grpc_lb_policy_notify_on_state_change_locked(
    grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
    grpc_connectivity_state *state, grpc_closure *closure);

grpc_connectivity_state grpc_lb_policy_check_connectivity_locked(
    grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
    grpc_error **connectivity_error);

#endif /* GRPC_CORE_EXT_CLIENT_CHANNEL_LB_POLICY_H */