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diff --git a/src/lib/subsys/subsys.h b/src/lib/subsys/subsys.h
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+/* Copyright (c) 2003-2004, Roger Dingledine
+ * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
+ * Copyright (c) 2007-2020, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+/**
+ * @file subsys.h
+ * @brief Types used to declare a subsystem.
+ **/
+
+#ifndef TOR_SUBSYS_T
+#define TOR_SUBSYS_T
+
+#include <stdbool.h>
+
+struct pubsub_connector_t;
+struct config_format_t;
+struct smartlist_t;
+
+/**
+ * A subsystem is a part of Tor that is initialized, shut down, configured,
+ * and connected to other parts of Tor.
+ *
+ * All callbacks are optional -- if a callback is set to NULL, the subsystem
+ * manager will treat it as a no-op.
+ *
+ * You should use c99 named-field initializers with this structure, for
+ * readability and safety. (There are a lot of functions here, all of them
+ * optional, and many of them with similar signatures.)
+ *
+ * See @ref initialization for more information about initialization and
+ * shutdown in Tor.
+ *
+ * To make a new subsystem, you declare a const instance of this type, and
+ * include it on the list in subsystem_list.c.  The code that manages these
+ * subsystems is in subsysmgr.c.
+ **/
+typedef struct subsys_fns_t {
+  /**
+   * The name of this subsystem.  It should be a programmer-readable
+   * identifier.
+   **/
+  const char *name;
+
+  /**
+   * The file in which the subsystem object is declared. Used for debugging.
+   **/
+  const char *location;
+
+  /**
+   * Whether this subsystem is supported -- that is, whether it is compiled
+   * into Tor.  For most subsystems, this should be true.
+   **/
+  bool supported;
+
+  /**
+   * The 'initialization level' for the subsystem.  It should run from -100
+   * through +100.  The subsystems are initialized from lowest level to
+   * highest, and shut down from highest level to lowest.
+   **/
+  int level;
+
+  /**
+   * Initialize any global components of this subsystem.
+   *
+   * This function MAY rely on any lower-level subsystem being initialized.
+   *
+   * This function MUST NOT rely on any runtime configuration information;
+   * it is only for global state or pre-configuration state.
+   *
+   * (If you need to do any setup that depends on configuration, you'll need
+   * to declare a configuration callback instead. (Not yet designed))
+   *
+   * This function MUST NOT have any parts that can fail.
+   **/
+  int (*initialize)(void);
+
+  /**
+   * Connect a subsystem to the message dispatch system.
+   *
+   * This function should use the macros in @refdir{lib/pubsub} to register a
+   * set of messages that this subsystem may publish, and may subscribe to.
+   *
+   * See pubsub_macros.h for more information, and for examples.
+   **/
+  int (*add_pubsub)(struct pubsub_connector_t *);
+
+  /**
+   * Perform any necessary pre-fork cleanup.  This function may not fail.
+   *
+   * On Windows (and any other platforms without fork()), this function will
+   * never be invoked.  Otherwise it is used when we are about to start
+   * running as a background daemon, or when we are about to run a unit test
+   * in a subprocess.  Unlike the subsys_fns_t.postfork callback, it is run
+   * from the parent process.
+   *
+   * Note that we do not invoke this function when the child process's only
+   * purpose is to call exec() and run another program.
+   */
+  void (*prefork)(void);
+
+  /**
+   * Perform any necessary post-fork setup. This function may not fail.
+   *
+   * On Windows (and any other platforms without fork()), this function will
+   * never be invoked.  Otherwise it is used when we are about to start
+   * running as a background daemon, or when we are about to run a unit test
+   * in a subprocess.  Unlike the subsys_fns_t.prefork callback, it is run
+   * from the child process.
+   *
+   * Note that we do not invoke this function when the child process's only
+   * purpose is to call exec() and run another program.
+   */
+  void (*postfork)(void);
+
+  /**
+   * Free any thread-local resources held by this subsystem. Called before
+   * the thread exits.
+   *
+   * This function is not allowed to fail.
+   *
+   * \bug Note that this callback is currently buggy: See \ticket{32103}.
+   */
+  void (*thread_cleanup)(void);
+
+  /**
+   * Free all resources held by this subsystem.
+   *
+   * This function is not allowed to fail.
+   *
+   * Subsystems are shut down when Tor is about to exit or return control to
+   * an embedding program. This callback must return the process to a state
+   * such that subsys_fns_t.init will succeed if invoked again.
+   **/
+  void (*shutdown)(void);
+
+  /**
+   * A config_format_t describing all of the torrc fields owned by this
+   * subsystem.
+   *
+   * This object, if present, is registered in a confmgr_t for Tor's options,
+   * and used to parse option fields from the command line and torrc file.
+   **/
+  const struct config_format_t *options_format;
+
+  /**
+   * A config_format_t describing all of the DataDir/state fields owned by
+   * this subsystem.
+   *
+   * This object, if present, is registered in a confmgr_t for Tor's state,
+   * and used to parse state fields from the DataDir/state file.
+   **/
+  const struct config_format_t *state_format;
+
+  /**
+   * Receive an options object as defined by options_format. Return 0
+   * on success, -1 on failure.
+   *
+   * It is safe to store the pointer to the object until set_options()
+   * is called again.
+   *
+   * This function is only called after all the validation code defined
+   * by subsys_fns_t.options_format has passed.
+   **/
+  int (*set_options)(void *);
+
+  /* XXXX Add an implementation for options_act_reversible() later in this
+   * branch. */
+
+  /**
+   * Receive a state object as defined by state_format. Return 0 on success,
+   * -1 on failure.
+   *
+   * It is safe to store the pointer to the object; set_state() is only
+   * called on startup.
+   *
+   * This function is only called after all the validation code defined
+   * by subsys_fns_t.state_format has passed.
+   *
+   * This function will only be called once per invocation of Tor, since
+   * Tor does not reload its state while it is running.
+   **/
+  int (*set_state)(void *);
+
+  /**
+   * Update any information that needs to be stored in the provided state
+   * object (as defined by state_format).  Return 0 on success, -1 on failure.
+   *
+   * The object provided here will be the same one as provided earlier to
+   * set_state().  This method is called when we are about to save the state
+   * to disk.
+   **/
+  int (*flush_state)(void *);
+
+  /**
+   * Return a list of metrics store of this subsystem. This is called
+   * every time a request arrives on the MetricsPort.
+   *
+   * The list MUST contain metrics_store_t object and contains entries so it
+   * can be formatted for the metrics port.
+   *
+   * This can return NULL or be NULL.
+   **/
+  const struct smartlist_t *(*get_metrics)(void);
+} subsys_fns_t;
+
+#ifndef COCCI
+/**
+ * Macro to declare a subsystem's location.
+ **/
+#define SUBSYS_DECLARE_LOCATION() \
+  .location = __FILE__
+#endif /* !defined(COCCI) */
+
+/**
+ * Lowest allowed subsystem level.
+ **/
+#define MIN_SUBSYS_LEVEL -100
+/**
+ * Highest allowed subsystem level.
+ **/
+#define MAX_SUBSYS_LEVEL 100
+
+/**
+ * All tor "libraries" (in src/libs) should have a subsystem level equal to or
+ * less than this value.
+ */
+#define SUBSYS_LEVEL_LIBS -10
+
+#endif /* !defined(TOR_SUBSYS_T) */