thread.c

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/*-------------------------------------------------------------------------
 *
 * thread.c
 *
 *        Prototypes and macros around system calls, used to help make
 *        threaded libraries reentrant and safe to use from threaded applications.
 *
 * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
 *
 * src/port/thread.c
 *
 *-------------------------------------------------------------------------
 */

#include "c.h"

#include <pwd.h>


/*
 *  Threading sometimes requires specially-named versions of functions
 *  that return data in static buffers, like strerror_r() instead of
 *  strerror().  Other operating systems use pthread_setspecific()
 *  and pthread_getspecific() internally to allow standard library
 *  functions to return static data to threaded applications. And some
 *  operating systems have neither.
 *
 *  Additional confusion exists because many operating systems that
 *  use pthread_setspecific/pthread_getspecific() also have *_r versions
 *  of standard library functions for compatibility with operating systems
 *  that require them.  However, internally, these *_r functions merely
 *  call the thread-safe standard library functions.
 *
 *  For example, BSD/OS 4.3 uses Bind 8.2.3 for getpwuid().  Internally,
 *  getpwuid() calls pthread_setspecific/pthread_getspecific() to return
 *  static data to the caller in a thread-safe manner.  However, BSD/OS
 *  also has getpwuid_r(), which merely calls getpwuid() and shifts
 *  around the arguments to match the getpwuid_r() function declaration.
 *  Therefore, while BSD/OS has getpwuid_r(), it isn't required.  It also
 *  doesn't have strerror_r(), so we can't fall back to only using *_r
 *  functions for threaded programs.
 *
 *  The current setup is to try threading in this order:
 *
 *      use *_r function names if they exit
 *          (*_THREADSAFE=yes)
 *      use non-*_r functions if they are thread-safe
 *
 *  One thread-safe solution for gethostbyname() might be to use getaddrinfo().
 *
 *  Run src/test/thread to test if your operating system has thread-safe
 *  non-*_r functions.
 */


/*
 * Wrapper around strerror and strerror_r to use the former if it is
 * available and also return a more useful value (the error string).
 */
char *
pqStrerror(int errnum, char *strerrbuf, size_t buflen)
{
#if defined(FRONTEND) && defined(ENABLE_THREAD_SAFETY) && defined(HAVE_STRERROR_R)
    /* reentrant strerror_r is available */
#ifdef STRERROR_R_INT
    /* SUSv3 version */
    if (strerror_r(errnum, strerrbuf, buflen) == 0)
        return strerrbuf;
    else
        return "Unknown error";
#else
    /* GNU libc */
    return strerror_r(errnum, strerrbuf, buflen);
#endif
#else
    /* no strerror_r() available, just use strerror */
    strlcpy(strerrbuf, strerror(errnum), buflen);

    return strerrbuf;
#endif
}

/*
 * Wrapper around getpwuid() or getpwuid_r() to mimic POSIX getpwuid_r()
 * behaviour, if that function is not available or required.
 *
 * Per POSIX, the possible cases are:
 * success: returns zero, *result is non-NULL
 * uid not found: returns zero, *result is NULL
 * error during lookup: returns an errno code, *result is NULL
 * (caller should *not* assume that the errno variable is set)
 */
#ifndef WIN32
int
pqGetpwuid(uid_t uid, struct passwd *resultbuf, char *buffer,
           size_t buflen, struct passwd **result)
{
#if defined(FRONTEND) && defined(ENABLE_THREAD_SAFETY) && defined(HAVE_GETPWUID_R)
    return getpwuid_r(uid, resultbuf, buffer, buflen, result);
#else
    /* no getpwuid_r() available, just use getpwuid() */
    errno = 0;
    *result = getpwuid(uid);
    /* paranoia: ensure we return zero on success */
    return (*result == NULL) ? errno : 0;
#endif
}
#endif

/*
 * Wrapper around gethostbyname() or gethostbyname_r() to mimic
 * POSIX gethostbyname_r() behaviour, if it is not available or required.
 * This function is called _only_ by our getaddinfo() portability function.
 */
#ifndef HAVE_GETADDRINFO
int
pqGethostbyname(const char *name,
                struct hostent *resultbuf,
                char *buffer, size_t buflen,
                struct hostent **result,
                int *herrno)
{
#if defined(FRONTEND) && defined(ENABLE_THREAD_SAFETY) && defined(HAVE_GETHOSTBYNAME_R)

    /*
     * broken (well early POSIX draft) gethostbyname_r() which returns 'struct
     * hostent *'
     */
    *result = gethostbyname_r(name, resultbuf, buffer, buflen, herrno);
    return (*result == NULL) ? -1 : 0;
#else

    /* no gethostbyname_r(), just use gethostbyname() */
    *result = gethostbyname(name);

    if (*result != NULL)
        *herrno = h_errno;

    if (*result != NULL)
        return 0;
    else
        return -1;
#endif
}

#endif