src/core/client_config/lb_policy.h

/*
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#ifndef GRPC_INTERNAL_CORE_CLIENT_CONFIG_LB_POLICY_H
#define GRPC_INTERNAL_CORE_CLIENT_CONFIG_LB_POLICY_H

#include "src/core/client_config/subchannel.h"
#include "src/core/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_refcount refs;
};

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

  void (*shutdown)(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy);

  /** implement grpc_lb_policy_pick */
  int (*pick)(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
              grpc_pollset *pollset, grpc_metadata_batch *initial_metadata,
              grpc_subchannel **target, grpc_closure *on_complete);
  void (*cancel_pick)(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
                      grpc_subchannel **target);

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

  /** broadcast a transport op to all subchannels */
  void (*broadcast)(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
                    grpc_transport_op *op);

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

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

#ifdef GRPC_LB_POLICY_REFCOUNT_DEBUG
#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))
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);
#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))
void grpc_lb_policy_ref(grpc_lb_policy *policy);
void grpc_lb_policy_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);

/** Start shutting down (fail any pending picks) */
void grpc_lb_policy_shutdown(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy);

/** Given initial metadata in \a initial_metadata, find an appropriate
    target for this rpc, and 'return' it by calling \a on_complete after setting
    \a target.
    Picking can be asynchronous. Any IO should be done under \a pollset. */
int grpc_lb_policy_pick(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
                        grpc_pollset *pollset,
                        grpc_metadata_batch *initial_metadata,
                        grpc_subchannel **target, grpc_closure *on_complete);

void grpc_lb_policy_cancel_pick(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
                                grpc_subchannel **target);

void grpc_lb_policy_broadcast(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy,
                              grpc_transport_op *op);

void grpc_lb_policy_exit_idle(grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy);

void grpc_lb_policy_notify_on_state_change(grpc_exec_ctx *exec_ctx,
                                           grpc_lb_policy *policy,
                                           grpc_connectivity_state *state,
                                           grpc_closure *closure);

grpc_connectivity_state grpc_lb_policy_check_connectivity(
    grpc_exec_ctx *exec_ctx, grpc_lb_policy *policy);

#endif /* GRPC_INTERNAL_CORE_CONFIG_LB_POLICY_H */