bres: Use mdup to duplicate file descriptors for the child.

[mLib] / bres.c

/* -*-c-*-
 *
 * $Id: bres.c,v 1.7 2004/04/08 01:36:11 mdw Exp $
 *
 * Background reverse name resolution
 *
 * (c) 1999 Straylight/Edgeware
 */

/*----- Licensing notice --------------------------------------------------*
 *
 * This file is part of the mLib utilities library.
 *
 * mLib is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Library General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or (at your option) any later version.
 *
 * mLib 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 Library General Public License for more details.
 *
 * You should have received a copy of the GNU Library General Public
 * License along with mLib; if not, write to the Free
 * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
 * MA 02111-1307, USA.
 */

/*----- Header files ------------------------------------------------------*/

#include "config.h"

#include <errno.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <sys/types.h>
#include <sys/time.h>
#include <unistd.h>
#include <sys/wait.h>

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>

#include "alloc.h"
#include "bres.h"
#include "mdup.h"
#include "report.h"
#include "sel.h"

/*----- Magic numbers -----------------------------------------------------*/

#define BRES_MAX 15			/* Maximum number of resolvers */
#define BRES_IDLE 60			/* Lifetime of an idle resolver */

/*----- Static variables --------------------------------------------------*/

#ifndef BRES_STANDALONE

static bres_server servers[BRES_MAX];	/* Statically allocated servers */

#define FREE ((bres_server *)&freelist)
static struct { bres_server *next, *prev; } freelist = { FREE, FREE };

#define QUEUE ((bres_client *)&queue)
static struct { bres_client *next, *prev; } queue = { QUEUE, QUEUE };

static sel_state *sel;

static const char *server = 0;

#endif

/*----- Background resolver protocol --------------------------------------*/

/* --- Requests and responses --- *
 *
 * There are two types of requests: name and addr, corresponding to the
 * standard @gethostbyname@ and @gethostbyaddr@ calls.  There are two types
 * of responses too: a positive response consists of an encoded equivalent of
 * a @struct hostent@ structure containing the requested information; a
 * negative response consists of an @h_errno@ value explaining the problem.
 */

#define BRES_BYNAME 0			/* Request: resolve given name */
#define BRES_BYADDR 1			/* Request: resolve given address */

#define BRES_HOSTENT 0			/* Response: resolved ok */
#define BRES_ERROR 1			/* Response: resolution failed */

/* --- Encodings --- *
 *
 * A string is encoded as a @size_t@ length followed by the actual data.  The
 * null terminator is not transmitted.
 *
 * Addresses for resolution are transmitted as raw @struct in_addr@
 * structures.
 *
 * A @hostent@ structure is transmitted as a header containing fixed-size
 * information, followed by the official name, an array of aliases, and an
 * array of addresses.  The number of items in the arrays is specified in the
 * header.
 *
 * The implementation assumes that a complete request or reply is always
 * sent.  Undesirable blocking will occur if this is not the case.  Both ends
 * are assumed to trust each other.  A protocol failure results in the child
 * in question being terminated.
 */

typedef struct hostskel {
  size_t nalias;
  int addrtype;
  size_t addrsz;
  size_t naddr;
} hostskel;

/* --- @doread@, @dowrite@ --- *
 *
 * Arguments:	@int fd@ = file descriptor
 *		@void *buf@ = buffer for data
 *		@size_t sz@ = size of data
 *
 * Returns:	Zero if successful, nonzero otherwise.
 *
 * Use:		Reads or writes a chunk of data.  @EINTR@ errors are retried;
 *		incomplete reads and writes are continued from where they
 *		left off.  End-of-file is considered an I/O error.
 */

static int doread(int fd, void *buf, size_t sz)
{
  char *p = buf;
  while (sz) {
    int r = read(fd, p, sz);
    if (r < 0) {
      if (errno == EINTR)
	continue;
      return (-1);
    } else if (r == 0) {
      errno = EIO;
      return (-1);
    }
    sz -= r;
    p += r;
  }
  return (0);
}

static int dowrite(int fd, const void *buf, size_t sz)
{
  const char *p = buf;
  while (sz) {
    int r = write(fd, p, sz);
    if (r < 0) {
      if (errno == EINTR)
	continue;
      return (-1);
    } else if (r == 0) {
      errno = EIO;
      return (-1);
    }
    sz -= r;
    p += r;
  }
  return (0);
}

/* --- @getstring@ --- *
 *
 * Arguments:	@int fd@ = file descriptor to read
 *
 * Returns:	String in heap-allocated block, or a null pointer.
 *
 * Use:		Decodes a string.
 */

static char *getstring(int fd)
{
  size_t sz;
  char *p;

  if (doread(fd, &sz, sizeof(sz)) || (p = malloc(sz + 1)) == 0)
    return (0);
  if (doread(fd, p, sz)) {
    free(p);
    return (0);
  }
  p[sz] = 0;
  return (p);
}

/* --- @putstring@ --- *
 *
 * Arguments:	@int fd@ = file descriptor to write on
 *		@const char *p@ = pointer to string to write
 *
 * Returns:	Zero if successful.
 *
 * Use:		Encodes a string.
 */

static int putstring(int fd, const char *p)
{
  size_t sz = strlen(p);
  if (dowrite(fd, &sz, sizeof(sz)) || dowrite(fd, p, sz))
    return (-1);
  return (0);
}

/* --- @gethost@ --- *
 *
 * Arguments:	@int fd@ = file descriptor to read
 *
 * Returns:	Pointer to heap-allocated @struct hostent@, or null.
 *
 * Use:		Decodes a host structure.  The resulting structure is all in
 *		one big heap block.
 */

#ifndef BRES_STANDALONE

static struct hostent *gethost(int fd)
{
  hostskel hsk;
  struct hostent *h;
  char *name;
  char **alias = 0;

  /* --- Read the skeleton structure --- */

  if (doread(fd, &hsk, sizeof(hsk)))
    goto tidy_0;

  /* --- Read the hostname and alias strings --- *
   *
   * Count the length of the strings as we go.
   */

  {
    size_t sz =
      sizeof(struct hostent) +
      hsk.naddr * hsk.addrsz +
      (hsk.naddr + hsk.nalias + 2) * sizeof(char *);

    /* --- Read the primary host name --- */

    if ((name = getstring(fd)) == 0)
      goto tidy_0;
    sz += strlen(name) + 1;

    /* --- Read in the alias names --- */

    if (hsk.nalias) {
      int i;
      if ((alias = malloc(hsk.nalias * sizeof(char *))) == 0)
	goto tidy_1;
      for (i = 0; i < hsk.nalias; i++)
	alias[i] = 0;
      for (i = 0; i < hsk.nalias; i++) {
	if ((alias[i] = getstring(fd)) == 0)
	  goto tidy_2;
	sz += strlen(alias[i]) + 1;
      }
    }

    /* --- Allocate the output structure --- */

    if ((h = malloc(sz)) == 0)
      goto tidy_2;
  }

  /* --- Fill in the base structure --- */

  h->h_addrtype = hsk.addrtype;
  h->h_length = hsk.addrsz;

  /* --- Start putting everything else in --- */

  {
    char **p = (char **)(h + 1);
    char *a = (char *)(p + hsk.nalias + hsk.naddr + 2);
    int i;

    /* --- Start with the address table --- */

    h->h_addr_list = p;
    if (doread(fd, a, hsk.naddr * hsk.addrsz))
      goto tidy_2;
    for (i = 0; i < hsk.naddr; i++) {
      *p++ = a;
      a += hsk.addrsz;
    }
    *p++ = 0;

    /* --- Finally copy the strings over --- */

#define PUT(_p) do {							\
  size_t _len = strlen(_p) + 1;						\
  memcpy(a, (_p), _len);						\
  a += _len;								\
} while (0)

    h->h_name = a;
    PUT(name);
    free(name);
    h->h_aliases = p;
    for (i = 0; i < hsk.nalias; i++) {
      *p++ = a;
      PUT(alias[i]);
      free(alias[i]);
    }
    *p++ = 0;
    free(alias);

#undef PUT
  }

  return (h);

  /* --- Tidy up after various types of failure --- */

tidy_2:
  {
    int i;
    for (i = 0; i < hsk.nalias && alias[i]; i++)
      free(alias[i]);
    free(alias);
  }
tidy_1:
  free(name);
tidy_0:
  return (0);
}

#endif

/* --- @puthost@ --- *
 *
 * Arguments:	@int fd@ = file descriptor
 *		@struct hostent *h@ = pointer to host structure
 *
 * Returns:	Zero if successful.
 *
 * Use:		Encodes a host structure.
 */

static int puthost(int fd, struct hostent *h)
{
  hostskel hsk;
  int i;

  /* --- Fill in and send the skeleton structure --- */

  for (i = 0; h->h_aliases[i]; i++)
    ;
  hsk.nalias = i;
  for (i = 0; h->h_addr_list[i]; i++)
    ;
  hsk.naddr = i;
  hsk.addrtype = h->h_addrtype;
  hsk.addrsz = h->h_length;
  if (dowrite(fd, &hsk, sizeof(hsk)))
    return (-1);

  /* --- Send the name and alias strings --- */

  if (putstring(fd, h->h_name))
    return (-1);
  for (i = 0; h->h_aliases[i]; i++) {
    if (putstring(fd, h->h_aliases[i]))
      return (-1);
  }

  /* --- Send the address data --- */

  for (i = 0; h->h_addr_list[i]; i++) {
    if (dowrite(fd, h->h_addr_list[i], hsk.addrsz))
      return (-1);
  }

  /* --- OK, done --- */

  return (0);
}

/*----- Resolver server ---------------------------------------------------*/

/* --- @child@ --- *
 *
 * Arguments:	@int rfd@ = output file descriptor for resolved hostnames
 *		@int cfd@ = input file descriptor for raw addresses
 *
 * Returns:	Never.
 *
 * Use:		Asynchronous name resolving process.
 */

static void child(int rfd, int cfd)
{
  /* --- Close other file descriptors --- */

  {
    int i;
#if defined(_SC_OPEN_MAX)
    int maxfd = sysconf(_SC_OPEN_MAX);
#elif defined(OPEN_MAX)
    int maxfd = OPEN_MAX;
#else
    int maxfd = -1;
#endif

    if (maxfd < 0)
      maxfd = 256; /* Fingers crossed... */
    for (i = 0; i < maxfd; i++) {
      if (i != rfd && i != cfd && i != 1)
	close(i);
    }
  }

  signal(SIGTERM, SIG_DFL);
  signal(SIGHUP, SIG_DFL);
  signal(SIGQUIT, SIG_DFL);
  signal(SIGALRM, SIG_DFL);
  signal(SIGINT, SIG_DFL);

  /* --- Main request/response loop --- */

  for (;;) {
    int req, resp;
    struct hostent *h;

    /* --- Read the request --- */

    if (doread(cfd, &req, sizeof(req)))
      goto lose;

    /* --- Process it into a host structure --- */

    switch (req) {

      /* --- Normal forward lookup --- */

      case BRES_BYNAME: {
	char *name = getstring(cfd);
	if (!name)
	  goto lose;
	h = gethostbyname(name);
	free(name);
      }	break;

      /* --- Reverse lookup --- */

      case BRES_BYADDR: {
	struct in_addr addr;
	char *p;
	if (doread(cfd, &addr, sizeof(addr)))
	  goto lose;
	if ((h = gethostbyaddr((char *)&addr, sizeof(addr), AF_INET)) == 0)
	  goto fail;

	/* --- Do a forward lookup to confirm --- */

	{
	  size_t sz = strlen(h->h_name) + 1;
	  if ((p = malloc(sz)) == 0)
	    goto fail;
	  memcpy(p, h->h_name, sz);
	}

	h = gethostbyname(p);
	free(p);
	if (!h)
	  goto fail;
	p = 0;
	if (h) {
	  char **pp;
	  for (pp = h->h_addr_list; *pp; pp++) {
	    struct in_addr a;
	    memcpy(&a, *pp, sizeof(a));
	    if (a.s_addr == addr.s_addr) {
	      p = h->h_name;
	      break;
	    }
	  }
	}
	if (!p) {
	  h = 0;
	  h_errno = NO_RECOVERY;
	}
      fail:;
      }	break;

      /* --- Unknown request -- may have lost sync --- */

      default:
	goto lose;
    }

    /* --- Transmit the response --- */

    if (h) {
      resp = BRES_HOSTENT;
      if (dowrite(rfd, &resp, sizeof(resp)) || puthost(rfd, h))
	goto lose;
    } else {
      resp = BRES_ERROR;
      if (dowrite(rfd, &resp, sizeof(resp)) ||
	  dowrite(rfd, &h_errno, sizeof(h_errno)))
	goto lose;
    }
  }

lose:
  _exit(1);
}

/* --- @main@ --- *
 *
 * Arguments:	@int argc@ = number of command line arguments
 *		@char *argv[]@ = array of arguments
 *
 * Returns:	Runs until killed or an error occurs.
 *
 * Use:		A name resolver server process for mLib programs which need
 *		this sort of thing.
 */

#ifdef BRES_STANDALONE

int main(int argc, char *argv[])
{
  if (isatty(STDIN_FILENO)) {
    char *p = strrchr(argv[0], '/');
    if (p)
      p++;
    else
      p = argv[0];
    fprintf(stderr,
	    "%s: don't run this program unless you know what you're doing.\n",
	    p);
    exit(1);
  }
  child(STDOUT_FILENO, STDIN_FILENO);
  return (1);
}

#endif

/*----- Main code ---------------------------------------------------------*/

#ifndef BRES_STANDALONE

/* --- @zap@ --- *
 *
 * Arguments:	@bres_server *rs@ = pointer to server block
 *
 * Returns:	---
 *
 * Use:		Kills a server process, reaps the losing child and makes
 *		things generally clean again.
 */

static void zap(bres_server *rs)
{
  /* --- Close the pipes, kill the child, and reap it --- */

  if (rs->kid != -1) {
    close(rs->fd);
    close(rs->f.fd);
    kill(rs->kid, SIGTERM);
    waitpid(rs->kid, 0, 0);
    rs->kid = -1;
  }

  /* --- Move the server to the back of the list --- */

  rs->next->prev = rs->prev;
  rs->prev->next = rs->next;
  rs->next = FREE;
  rs->prev = FREE->prev;
  FREE->prev->next = rs;
  FREE->prev = rs;
}

/* --- @bres_abort@ --- *
 *
 * Arguments:	@bres_client *rc@ = pointer to client block
 *
 * Returns:	---
 *
 * Use:		Removes a queued job.
 */

void bres_abort(bres_client *rc)
{
  if (rc->q == BRES_BYNAME)
    xfree(rc->u.name);
  if (rc->rs) {
    sel_rmfile(&rc->rs->f);
    zap(rc->rs);
    rc->rs = 0;
  } else {
    rc->next->prev = rc->prev;
    rc->prev->next = rc->next;
  }
}

/* --- @idle@ --- *
 *
 * Arguments:	@struct timeval *tv@ = pointer to the current time
 *		@void *vp@ = pointer to a server block
 *
 * Returns:	---
 *
 * Use:		Kills off a child which has been idle for too long.
 */

static void idle(struct timeval *tv, void *vp)
{
  bres_server *rs = vp;
  zap(rs);
}

/* --- @answer@ --- *
 *
 * Arguments:	@int fd@ = file descriptor which is ready
 *		@unsigned mode@ = what it's doing now
 *		@void *vp@ = pointer to server block
 *
 * Returns:	---
 *
 * Use:		Retrieves an answer from a name resolver process.
 */

static void attach(bres_client */*rc*/);

static void answer(int fd, unsigned mode, void *vp)
{
  bres_server *rs = vp;
  bres_client *rc = rs->rc;
  struct hostent *h = 0;
  int resp;
  int fail = 1;

  /* --- Report the result to my client --- */

  sel_rmfile(&rs->f);
  h_errno = -1;
  if (doread(fd, &resp, sizeof(resp)) == 0) {
    switch (resp) {
      case BRES_ERROR:
	doread(fd, &h_errno, sizeof(h_errno));
	fail = 0;
	break;
      case BRES_HOSTENT:
	h = gethost(fd);
	fail = 0;
	break;
    }
  }
  if (rc) {
    rc->func(h, rc->p);
    if (rc->q == BRES_BYNAME)
      xfree(rc->u.name);
  }
  if (h)
    free(h);
  if (fail)
    zap(rs);
  if (!rc)
    return;

  /* --- Wrap up the various structures --- */

  rs->rc = 0;
  rc->rs = 0;
  rs->next = FREE->next;
  rs->prev = FREE;
  FREE->next->prev = rs;
  FREE->next = rs;

  /* --- Tie a timer onto the server block --- */

  {
    struct timeval tv;

    gettimeofday(&tv, 0);
    tv.tv_sec += BRES_IDLE;
    sel_addtimer(sel, &rs->t, &tv, idle, rs);
  }

  /* --- If there are any clients waiting, attach one --- */

  if (QUEUE->next != QUEUE) {
    rc = QUEUE->next;
    QUEUE->next = rc->next;
    rc->next->prev = QUEUE;
    attach(rc);
  }
}

/* --- @start@ --- *
 *
 * Arguments:	@bres_server *rs@ = pointer to a server block
 *
 * Returns:	Zero if OK, nonzero if something failed.
 *
 * Use:		Starts up a child resolver process.
 */

static int start(bres_server *rs)
{
  int rfd[2], cfd[2];
  pid_t kid;
  mdup_fd md[2];

  /* --- Make the pipes --- */

  if (pipe(rfd))
    goto fail_0;
  if (pipe(cfd))
    goto fail_1;

  /* --- Start up the child process --- */

  if ((kid = fork()) < 0)
    goto fail_2;
  if (kid == 0) {
    close(cfd[1]);
    close(rfd[0]);

    if (server) {
      md[0].cur = cfd[0]; md[0].want = STDIN_FILENO;
      md[1].cur = rfd[1]; md[1].want = STDOUT_FILENO;
      if (mdup(md, 2) ||  execlp(server, server, (char *)0))
	child(STDOUT_FILENO, STDIN_FILENO);
    } else
      child(rfd[1], cfd[0]);
    _exit(1);
  }

  /* --- Fix up everything in the server block --- */

  close(cfd[0]);
  close(rfd[1]);
  rs->fd = cfd[1];
  sel_initfile(sel, &rs->f, rfd[0], SEL_READ, answer, rs);
  rs->kid = kid;
  return (0);

  /* --- Fix up after errors --- */

fail_2:
  close(cfd[0]);
  close(cfd[1]);
fail_1:
  close(rfd[0]);
  close(rfd[1]);
fail_0:
  return (-1);
}

/* --- @attach@ --- *
 *
 * Arguments:	@bres_client *rc@ = pointer to a client block
 *
 * Returns:	---
 *
 * Use:		Attaches a client to a spare server (which is assumed to
 *		exist).
 */

static void attach(bres_client *rc)
{
  bres_server *rs;
  int lose = 0;

  /* --- Fix up the server ready for the job --- *
   *
   * If the server has a process, remove its timer.  Otherwise, fork off a
   * new resolver process.  This is also where I go if I find that the child
   * resolver process has lost while I wasn't looking.  Only one attempt at
   * forking is performed.
   */

again:
  rs = FREE->next;
  if (rs->kid != -1)
    sel_rmtimer(&rs->t);
  else {
    if (lose || start(rs))
      goto lost;
    lose = 1;
  }

  /* --- Submit the job to the resolver --- */

  {
    struct sigaction sa, osa;
    int e;

    /* --- Ignore @SIGPIPE@ for now --- *
     *
     * This way I can trap @EPIPE@ and reap a losing child, if there was one.
     */

    sa.sa_handler = SIG_IGN;
    sa.sa_flags = 0;
    sigemptyset(&sa.sa_mask);
    sigaction(SIGPIPE, &sa, &osa);

    /* --- Write the new job to the child --- */

    e = 0;
    if (dowrite(rs->fd, &rc->q, sizeof(rc->q)))
      e = errno;
    else switch (rc->q) {
      case BRES_BYADDR:
	if (dowrite(rs->fd, &rc->u.addr, sizeof(rc->u.addr)))
	  e = errno;
	break;
      case BRES_BYNAME:
	if (putstring(rs->fd, rc->u.name))
	  e = errno;
	break;
    }
    sigaction(SIGPIPE, &osa, 0);

    /* --- Sort out various errors --- *
     *
     * This was once more complicated, handling @EPIPE@ separately from other
     * errors.  Now everything's handled the same way.
     */

    if (e) {
      zap(rs);
      goto again;
    }
  }

  /* --- Fiddle with lists so that everything's OK --- */

  sel_addfile(&rs->f);
  rs->next->prev = FREE;
  FREE->next = rs->next;
  rs->next = rs->prev = rs;
  rs->rc = rc;
  rc->rs = rs;
  return;

lost:
  rc->func(0, rc->p);
  if (rc->q == BRES_BYNAME)
    xfree(rc->u.name);
}

/* --- @resolve@ --- *
 *
 * Arguments:	@bres_client *rc@ = pointer to filled-in client block
 *
 * Returns:	---
 *
 * Use:		Dispatcher for incoming resolution jobs.
 */

static void resolve(bres_client *rc)
{
  /* --- If there's a free server, plug it in --- */

  rc->rs = 0;
  if (FREE->next == FREE) {
    rc->next = QUEUE;
    rc->prev = QUEUE->prev;
    QUEUE->prev->next = rc;
    QUEUE->prev = rc;
  } else
    attach(rc);
}

/* --- @bres_byaddr@ --- *
 *
 * Arguments:	@bres_client *rc@ = pointer to client block
 *		@struct in_addr addr@ = address to resolve
 *		@void (*func)(struct hostent *h, void *p)@ = handler function
 *		@void *p@ = argument for handler function
 *
 * Returns:	---
 *
 * Use:		Adds an address lookup job to the queue.  The job will be
 *		processed when there's a spare resolver process to deal with
 *		it.
 */

void bres_byaddr(bres_client *rc, struct in_addr addr,
		 void (*func)(struct hostent */*h*/, void */*p*/),
		 void *p)
{
  rc->q = BRES_BYADDR;
  rc->u.addr = addr;
  rc->func = func;
  rc->p = p;
  resolve(rc);
}

/* --- @bres_byname@ --- *
 *
 * Arguments:	@bres_client *rc@ = pointer to client block
 *		@const char *name@ = name to resolve
 *		@void (*func)(struct hostent *h, void *p)@ = handler function
 *		@void *p@ = argument for handler function
 *
 * Returns:	---
 *
 * Use:		Adds a name lookup job to the queue.  The job will be
 *		processed when there's a spare resolver process to deal with
 *		it.
 */

void bres_byname(bres_client *rc, const char *name,
		 void (*func)(struct hostent */*h*/, void */*p*/),
		 void *p)
{
  rc->q = BRES_BYNAME;
  rc->u.name = xstrdup(name);
  rc->func = func;
  rc->p = p;
  resolve(rc);
}

/* --- @bres_exec@ --- *
 *
 * Arguments:	@const char *file@ = file containing server code or null
 *
 * Returns:	---
 *
 * Use:		Makes `bres' use a standalone server rather than copies of
 *		the current process.  This can reduce memory consumption for
 *		large processes, at the expense of startup time (which
 *		shouldn't be too bad anyway, because of the resolver design).
 *		If the filename is null, a default set up at install time is
 *		used.  It's probably a good idea to leave it alone.
 */

void bres_exec(const char *file)
{
  if (file)
    server = file;
  else
    server = BRES_SERVER;
}

/* --- @bres_init@ --- *
 *
 * Arguments:	@sel_state *s@ = pointer to select multiplexor
 *
 * Returns:	---
 *
 * Use:		Initializes the background resolver for use.
 */

void bres_init(sel_state *s)
{
  int i;

  sel = s;
  for (i = 0; i < BRES_MAX; i++) {
    servers[i].next = FREE;
    servers[i].prev = FREE->prev;
    servers[i].kid = -1;
    servers[i].rc = 0;
    FREE->prev->next = &servers[i];
    FREE->prev = &servers[i];
  }
}

#endif

/*----- That's all, folks -------------------------------------------------*/