First implementation for signal handling and multi-threading:

safe shutdown and signal deliverence to all threads in 
the manager process


server-tools/instance-manager/Makefile.am:
  added listener.{h,cc} (listener thread)
  and thread_repository.{h,cc} - 
  class to handle threads && signals
server-tools/instance-manager/log.h:
  minor fixes
server-tools/instance-manager/manager.cc:
  unnecessary error checking was removed 
  added implementation for signal thread
server-tools/instance-manager/manager.h:
  header guards're added
server-tools/instance-manager/mysqlmanager.cc:
  unnecessary error checking was removed
server-tools/instance-manager/options.cc:
  minor fixes
server-tools/instance-manager/options.h:
  removed wrong declaration

server-tools/instance-manager/Makefile.am

@@ -15,8 +15,10 @@ liboptions_a_SOURCES= otpions.h options.cc
 
 bin_PROGRAMS = mysqlmanager
 
-#AM_CXXFLAGS= -Wformat=2 -W -Wall -Wformat-security
-mysqlmanager_SOURCES= mysqlmanager.cc manager.h manager.cc log.h log.cc
+mysqlmanager_SOURCES= mysqlmanager.cc manager.h manager.cc log.h log.cc \
+		      listener.h listener.cc \
+		      thread_repository.h thread_repository.cc
+
 mysqlmanager_LDADD= liboptions.a \
 		    $(top_builddir)/mysys/libmysys.a \
 		    $(top_builddir)/strings/libmystrings.a \

server-tools/instance-manager/listener.cc

+#include "listener.h"
+#include "thread_repository.h"
+#include "log.h"
+
+C_MODE_START
+
+pthread_handler_decl(listener, arg)
+{
+  Thread_info info(pthread_self());
+  Thread_repository &thread_repository=
+    ((Listener_thread_args *) arg)->thread_repository;
+  thread_repository.register_thread(&info);
+
+  while (true)
+  {
+    log_info("listener is alive");
+    sleep(2);
+    if (thread_repository.is_shutdown())
+      break;
+  }
+  log_info("listener(): shutdown requested, exiting...");
+
+  thread_repository.unregister_thread(&info);
+  return 0;
+}
+
+C_MODE_END
+
+#if 0
+  while (true)
+  {
+  }
+  /*
+    Dummy manager implementation: listens on a UNIX socket and
+    starts echo server in a dedicated thread for each accepted connection.
+    Enough to test startup/shutdown/options/logging of the instance manager.
+  */
+
+  int fd= socket(AF_UNIX, SOCK_STREAM, 0);
+  
+  if (!fd)
+    die("socket(): failed");
+
+  struct sockaddr_un address;
+  bzero(&address, sizeof(address));
+  address.sun_family= AF_UNIX;
+  strcpy(address.sun_path, socket_path);
+  int opt= 1;
+
+  if (unlink(socket_path) ||
+      bind(fd, (struct sockaddr *) &address, sizeof(address)) ||
+      setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt)))
+    die("unlink | bind | setsockopt failed");
+  
+  if (listen(fd, 5))
+    die("listen() failed");
+  
+  int client_fd;
+  while ((client_fd= accept(fd, 0, 0)) != -1);
+  {
+    printf("accepted\n");
+    const char *message= "\n10hel";
+    send(client_fd, message, strlen(message), 0);
+
+    int sleep_seconds= argc > 1 && atoi(argv[1]) ? atoi(argv[1]) : 1;
+    printf("sleeping %d seconds\n", sleep_seconds);
+    sleep(sleep_seconds);
+    close(client_fd);
+  }
+  printf("accept(): failed\n");
+  close(fd);
+#endif

server-tools/instance-manager/listener.h

+#ifndef INCLUDES_MYSQL_INSTANCE_MANAGER_LISTENER_H
+#define INCLUDES_MYSQL_INSTANCE_MANAGER_LISTENER_H
+/* Copyright (C) 2000 MySQL AB & MySQL Finland AB & TCX DataKonsult AB
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
+
+#ifdef __GNUC__
+#pragma interface
+#endif
+
+#include <my_global.h>
+#include <my_pthread.h>
+
+C_MODE_START
+
+pthread_handler_decl(listener, arg);
+
+C_MODE_END
+
+class Thread_repository;
+
+struct Listener_thread_args
+{
+  Thread_repository &thread_repository;
+  const char *socket_file_name;
+
+  Listener_thread_args(Thread_repository &thread_repository_arg,
+                       const char *socket_file_name_arg) :
+    thread_repository(thread_repository_arg),
+    socket_file_name(socket_file_name_arg) {}
+};
+
+#endif

server-tools/instance-manager/log.h

@@ -73,7 +73,7 @@ void print_error(const char *format, ...)
 void log_init();
 
 
-/* initialize logs for daemon application */
+/* print information to the error log and eixt(1) */  
 
 void die(const char *format, ...)
 #ifdef __GNUC__

server-tools/instance-manager/manager.cc

@@ -14,57 +14,52 @@
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
 
+#include "manager.h"
+
 #include <my_global.h>
+#include <signal.h>
+
+#include "thread_repository.h"
+#include "listener.h"
 #include "log.h"
 
-//#include "mysql_connection.h"
 
 void manager(const char *socket_file_name)
 {
-  while (true)
-  {
-    log_info("manager is alive");
-    sleep(2);
-  }
-#if 0
-  /*
-    Dummy manager implementation: listens on a UNIX socket and
-    starts echo server in a dedicated thread for each accepted connection.
-    Enough to test startup/shutdown/options/logging of the instance manager.
-  */
+  Thread_repository thread_repository;
+  Listener_thread_args listener_args(thread_repository, socket_file_name);
 
-  int fd= socket(AF_UNIX, SOCK_STREAM, 0);
-  
-  if (!fd)
-    die("socket(): failed");
+  /* write pid file */
 
-  struct sockaddr_un address;
-  bzero(&address, sizeof(address));
-  address.sun_family= AF_UNIX;
-  strcpy(address.sun_path, socket_path);
-  int opt= 1;
+  /* block signals */
+  sigset_t mask;
+  sigemptyset(&mask);
+  sigaddset(&mask, SIGINT);
+  sigaddset(&mask, SIGTERM);
+  sigaddset(&mask, SIGHUP);
 
-  if (unlink(socket_path) ||
-      bind(fd, (struct sockaddr *) &address, sizeof(address)) ||
-      setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt)))
-    die("unlink | bind | setsockopt failed");
-  
-  if (listen(fd, 5))
-    die("listen() failed");
-  
-  int client_fd;
-  while ((client_fd= accept(fd, 0, 0)) != -1);
+  /* all new threads will inherite this signal mask */
+  pthread_sigmask(SIG_BLOCK, &mask, NULL);
   {
-    printf("accepted\n");
-    const char *message= "\n10hel";
-    send(client_fd, message, strlen(message), 0);
+    /* create the listener */
+    pthread_t listener_thd_id;
+    pthread_attr_t listener_thd_attr;
 
-    int sleep_seconds= argc > 1 && atoi(argv[1]) ? atoi(argv[1]) : 1;
-    printf("sleeping %d seconds\n", sleep_seconds);
-    sleep(sleep_seconds);
-    close(client_fd);
+    if (pthread_attr_init(&listener_thd_attr))
+      die("manager(): pthread_attr_init(listener) failed");
+    if (pthread_attr_setdetachstate(&listener_thd_attr,
+                                    PTHREAD_CREATE_DETACHED))
+      die("manager(): pthread_attr_setdetachstate(listener) failed");
+    if (pthread_create(&listener_thd_id, &listener_thd_attr, listener,
+                       &listener_args))
+      die("manager(): pthread_create(listener) failed");
   }
-  printf("accept(): failed\n");
-  close(fd);
-#endif
+  /*
+    To work nicely with LinuxThreads, the signal thread is the first thread
+    in the process.
+  */
+  int signo;
+  sigwait(&mask, &signo);
+  thread_repository.deliver_shutdown();
+  /* don't pthread_exit to kill all threads who did not shut down in time */
 }

server-tools/instance-manager/manager.h

+#ifndef INCLUDES_MYSQL_INSTANCE_MANAGER_MANAGER_H
+#define INCLUDES_MYSQL_INSTANCE_MANAGER_MANAGER_H
 /* Copyright (C) 2000 MySQL AB & MySQL Finland AB & TCX DataKonsult AB
 
    This program is free software; you can redistribute it and/or modify
@@ -15,3 +17,5 @@
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
 
 void manager(const char *socket_file_name);
+
+#endif // INCLUDES_MYSQL_INSTANCE_MANAGER_MANAGER_H

server-tools/instance-manager/mysqlmanager.cc

@@ -106,13 +106,12 @@ static void daemonize(const char *log_file_name)
   case 0:                                       // child, fork ok
     int fd;
     /*
-      Become a session leader: setsid should succeed because child is
+      Become a session leader: setsid must succeed because child is
       guaranteed not to be a process group leader (it belongs to the 
       process group of the parent.) 
       The goal is not to have a controlling terminal.
     */
-    if (setsid() == -1)
-      die("daemonize(): setsid() failed, %s", strerror(errno)); 
+    setsid();
     /*
       As we now don't have a controlling terminal we will not receive
       tty-related signals - no need to ignore them.
@@ -193,19 +192,21 @@ static void angel(const Options &options)
   sigset_t zeromask;                            // to sigsuspend in parent
   struct sigaction sa_chld, sa_term;
   struct sigaction sa_chld_out, sa_term_out, sa_int_out, sa_hup_out;
-  if (sigemptyset(&zeromask) ||
-      sigemptyset(&sa_chld.sa_mask) ||
-      sigemptyset(&sa_term.sa_mask))            // how can it fail?
-    die("angel(): sigemptyset() failed, %s", strerror(errno));
+
+  sigemptyset(&zeromask);
+  sigemptyset(&sa_chld.sa_mask);
+  sigemptyset(&sa_term.sa_mask);
+
   sa_chld.sa_handler= reap_child;
   sa_chld.sa_flags= SA_NOCLDSTOP;
   sa_term.sa_handler= terminate;
   sa_term.sa_flags= 0;
-  if (sigaction(SIGCHLD, &sa_chld, &sa_chld_out) == -1 ||
-      sigaction(SIGTERM, &sa_term, &sa_term_out) == -1 ||
-      sigaction(SIGINT, &sa_term, &sa_int_out) == -1 ||
-      sigaction(SIGHUP, &sa_term, &sa_hup_out) == -1)
-    die("angel(): sigaction() failed, %s", strerror(errno));
+
+  /* sigaction can fail only on wrong arguments */
+  sigaction(SIGCHLD, &sa_chld, &sa_chld_out);
+  sigaction(SIGTERM, &sa_term, &sa_term_out);
+  sigaction(SIGINT, &sa_term, &sa_int_out);
+  sigaction(SIGHUP, &sa_term, &sa_hup_out);
 
   /* spawn a child */
 spawn:
@@ -218,22 +219,16 @@ static void angel(const Options &options)
       restore default actions for signals to let the manager work with
       signals as he wishes
     */ 
-    if (sigaction(SIGCHLD, &sa_chld_out, 0) == -1 ||
-        sigaction(SIGTERM, &sa_term_out, 0) == -1 ||
-        sigaction(SIGINT, &sa_int_out, 0) == -1 ||
-        sigaction(SIGHUP, &sa_hup_out, 0) == -1)
-      die("angel(): child failed to restore old signal actions, %s",
-          strerror(errno));
+    sigaction(SIGCHLD, &sa_chld_out, 0);
+    sigaction(SIGTERM, &sa_term_out, 0);
+    sigaction(SIGINT, &sa_int_out, 0);
+    sigaction(SIGHUP, &sa_hup_out, 0);
 
     manager(options.socket_file_name);
   default:                                    // parent, success
     while (child_status == CHILD_OK && is_terminated == 0)
-    {
-      errno= 0;
       sigsuspend(&zeromask);
-      if (errno != EINTR)
-        die("angel(): sigsuspend failed, %s", strerror(errno));
-    }
+
     if (is_terminated)
       log_info("angel got signal %d (%s), exiting",
                is_terminated, sys_siglist[is_terminated]);

server-tools/instance-manager/options.cc

@@ -14,16 +14,16 @@
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
 
+#ifdef __GNUC__
+#pragma implementation 
+#endif
+
 #include "options.h"
 
 #include <my_global.h>
 #include <my_sys.h>
 #include <my_getopt.h>
 
-#ifdef __GNUC__
-#pragma implementation 
-#endif
-
 
 #define QUOTE2(x) #x
 #define QUOTE(x) QUOTE2(x)
@@ -121,7 +121,3 @@ void Options::load(int argc, char **argv)
     exit(rc);
 }
 
-void init_mysys_library(const char *progname)
-{
-  MY_INIT((char *) progname);
-}

server-tools/instance-manager/options.h

@@ -35,6 +35,4 @@ struct Options
   static void load(int argc, char **argv);
 };
 
-void init_mysys_library(const char *progname);
-
 #endif // INCLUDES_MYSQL_INSTANCE_MANAGER_OPTIONS_H

server-tools/instance-manager/thread_repository.cc

+/* Copyright (C) 2000 MySQL AB & MySQL Finland AB & TCX DataKonsult AB
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
+
+#ifdef __GNUC__
+#pragma implementation 
+#endif
+
+#include "thread_repository.h"
+#include <assert.h>
+#include <signal.h>
+#include "log.h"
+
+
+/* Kick-off signal handler */
+  
+enum { THREAD_KICK_OFF_SIGNAL= SIGUSR2 };
+
+static void handle_signal(int __attribute__((unused)) sig_no)
+{
+}
+
+
+/*
+  TODO: think about moving signal information (now it's shutdown_in_progress)
+  to Thread_info. It will reduce contention and allow signal deliverence to
+  a particular thread, not to the whole worker crew 
+*/
+
+Thread_repository::Thread_repository() :
+  shutdown_in_progress(false)
+{
+  pthread_mutex_init(&LOCK_thread_repository, 0);
+  pthread_cond_init(&COND_thread_repository_is_empty, 0);
+
+  /* head is used by-value to simplify nodes inserting */
+  head.next= head.prev= &head;
+}
+
+
+Thread_repository::~Thread_repository()
+{
+  /* Check that no one uses the repository. */
+  pthread_mutex_lock(&LOCK_thread_repository);
+
+  /* All threads must unregister */
+  DBUG_ASSERT(head.next == &head);
+
+  pthread_mutex_unlock(&LOCK_thread_repository);
+  pthread_cond_destroy(&COND_thread_repository_is_empty);
+  pthread_mutex_destroy(&LOCK_thread_repository);
+}
+
+
+/*
+  
+  Set signal handler for kick-off thread, and insert a thread info to the
+  repository. New node is appended to the end of the list; head.prev always
+  points to the last node.
+*/
+
+void Thread_repository::register_thread(Thread_info *info)
+{
+  struct sigaction sa;
+  sa.sa_handler= handle_signal;
+  sa.sa_flags= 0;
+  sigemptyset(&sa.sa_mask);
+  sigaction(THREAD_KICK_OFF_SIGNAL, &sa, 0);
+
+  info->current_cond= 0;
+
+  pthread_mutex_lock(&LOCK_thread_repository);
+  info->next= &head;
+  info->prev= head.prev;
+  head.prev->next= info;
+  head.prev= info;
+  pthread_mutex_unlock(&LOCK_thread_repository);
+}
+
+
+/*
+  Unregister a thread from the repository and free Thread_info structure.
+  Every registered thread must unregister. Unregistering should be the last 
+  thing a thread is doing, otherwise it could have no time to finalize.
+*/
+
+void Thread_repository::unregister_thread(Thread_info *info)
+{
+  pthread_mutex_lock(&LOCK_thread_repository);
+  info->prev->next= info->next;
+  info->next->prev= info->prev;
+  if (head.next == &head)
+    pthread_cond_signal(&COND_thread_repository_is_empty);
+  pthread_mutex_unlock(&LOCK_thread_repository);
+}
+
+
+/*
+  Check whether shutdown is in progress, and if yes, return immidiately.
+  Else set info->current_cond and call pthread_cond_wait. When
+  pthread_cond_wait returns, unregister current cond and check the shutdown
+  status again.
+  RETURN VALUE
+    return value from pthread_cond_wait
+*/
+
+int Thread_repository::cond_wait(Thread_info *info, pthread_cond_t *cond,
+                                  pthread_mutex_t *mutex, bool *is_shutdown)
+{
+  pthread_mutex_lock(&LOCK_thread_repository);
+  *is_shutdown= shutdown_in_progress;
+  if (*is_shutdown)
+  {
+    pthread_mutex_unlock(&LOCK_thread_repository);
+    return 0;
+  }
+  info->current_cond= cond;
+  pthread_mutex_unlock(&LOCK_thread_repository);
+  /* sic: race condition here, cond can be signaled in deliver_shutdown */
+  int rc= pthread_cond_wait(cond, mutex);
+  pthread_mutex_lock(&LOCK_thread_repository);
+  info->current_cond= 0;
+  *is_shutdown= shutdown_in_progress;
+  pthread_mutex_unlock(&LOCK_thread_repository);
+  return rc;
+}
+
+
+/*
+  Deliver shutdown message to the workers crew.
+  As it's impossible to avoid all race conditions, we signal latecomers
+  again.
+*/
+
+void Thread_repository::deliver_shutdown()
+{
+  struct timespec shutdown_time;
+  set_timespec(shutdown_time, 1);
+
+  pthread_mutex_lock(&LOCK_thread_repository);
+  shutdown_in_progress= true;
+  for (Thread_info *info= head.next; info != &head; info= info->next)
+  {
+    pthread_kill(info->thread_id, THREAD_KICK_OFF_SIGNAL);
+    /*
+      sic: race condition here, the thread may not yet fall into
+      pthread_cond_wait.
+    */
+    if (info->current_cond)
+      pthread_cond_signal(info->current_cond);
+  }
+  while (pthread_cond_timedwait(&COND_thread_repository_is_empty,
+                                &LOCK_thread_repository,
+                                &shutdown_time) != ETIMEDOUT &&
+         head.next != &head)
+    ;
+  /*
+    If previous signals did not reach some threads, they must be sleeping 
+    in pthread_cond_wait or a blocking syscall. Wake them up: 
+    every thread shall check signal variables after each syscall/cond_wait,
+    so this time everybody should be informed (presumably each worker can
+    get CPU during shutdown_time.)
+  */
+  for (Thread_info *info= head.next; info != &head; info= info->next)
+  {
+    pthread_kill(info->thread_id, THREAD_KICK_OFF_SIGNAL);
+    if (info->current_cond)
+      pthread_cond_signal(info->current_cond);
+  }
+  pthread_mutex_unlock(&LOCK_thread_repository);
+}
+

server-tools/instance-manager/thread_repository.h

+#ifndef INCLUDES_MYSQL_INSTANCE_MANAGER_THREAD_REPOSITORY_HH
+#define INCLUDES_MYSQL_INSTANCE_MANAGER_THREAD_REPOSITORY_HH
+/* Copyright (C) 2000 MySQL AB & MySQL Finland AB & TCX DataKonsult AB
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
+
+/*
+  A multi-threaded application shall nicely work with signals.
+
+  This means it shall, first of all, shut down nicely on ``quit'' signals:
+  stop all running threads, cleanup and exit.
+
+  Note, that a thread can't be shut down nicely if it doesn't want to be.
+  That's why to perform clean shutdown, all threads consituting a process
+  must observe certain rules. Here we use the rules, described in Butenhof
+  book 'Programming with POSIX threads', namely:
+  - all user signals are handled in 'signal thread' in synchronous manner
+  (by means of sigwait). To guarantee that the signal thread is the only who
+  can receive user signals, all threads block them, and signal thread is 
+  the only who calls sigwait() with an apporpriate sigmask. 
+  To propogate a signal to the workers the signal thread sets
+  a variable, corresponding to the signal. Additionally the signal thread
+  sends each worker an internal signal (by means of pthread_kill) to kick it
+  out from possible blocking syscall, and possibly pthread_cond_signal if 
+  some thread is blocked in pthread_cond_[timed]wait.
+  - a worker handles only internal 'kick' signal (the handler does nothing).
+  In case when a syscall returns 'EINTR' the worker checks all
+  signal-related variables and behaves accordingly.
+  Also these variables shall be checked from time to time in long
+  CPU-bounded operations, and before/after pthread_cond_wait. (It's supposed
+  that a worker thread either waits in a syscall/conditional variable, or
+  computes something.)
+  - to guarantee signal deliverence, there should be some kind of feedback,
+  e. g. all workers shall account in the signal thread Thread Repository and
+  unregister from it on exit.
+  
+  Configuration reload (on SIGHUP) and thread timeouts/alarms can be handled
+  in manner, similar to ``quit'' signals.
+*/
+
+#ifdef __GNUC__
+#pragma interface
+#endif
+
+#include <my_global.h>
+#include <my_pthread.h>
+
+
+/*
+  Thread_info - repository entry for each worker thread
+  All entries comprise double-linked list like:
+     0 -- entry -- entry -- entry - 0
+  Double-linked list is used to unregister threads easy.
+*/
+
+class Thread_info
+{
+  pthread_cond_t *current_cond;
+  Thread_info *prev, *next;
+  pthread_t thread_id;
+  Thread_info() {}
+  friend class Thread_repository;
+public:
+  Thread_info(pthread_t thread_id_arg) : thread_id(thread_id_arg) {}
+};
+
+
+/*
+  Thread_repository - contains handles for each worker thread to deliver
+  signal information to workers.
+*/
+
+class Thread_repository
+{
+public:
+  Thread_repository();
+  ~Thread_repository();
+
+  void register_thread(Thread_info *info);
+  void unregister_thread(Thread_info *info);
+  void deliver_shutdown();
+  inline bool is_shutdown();
+  int cond_wait(Thread_info *info, pthread_cond_t *cond,
+                 pthread_mutex_t *mutex, bool *is_shutdown);
+private:
+  Thread_info head;
+  bool shutdown_in_progress;
+  pthread_mutex_t LOCK_thread_repository;
+  pthread_cond_t COND_thread_repository_is_empty;
+};
+
+
+inline bool Thread_repository::is_shutdown()
+{
+  pthread_mutex_lock(&LOCK_thread_repository);
+  bool res= shutdown_in_progress;
+  pthread_mutex_unlock(&LOCK_thread_repository);
+  return res;
+}
+
+#endif // INCLUDES_MYSQL_INSTANCE_MANAGER_THREAD_REPOSITORY_HH