configure: check if &errno is thread-local for OpenSSL

    APR_ADDTO(MOD_LDFLAGS, [$ap_openssl_mod_ldflags])

    APR_ADDTO(MOD_CFLAGS, [$ap_openssl_mod_cflags])

  fi

  dnl On most platforms, the default multithreading logic in OpenSSL 1.0.x uses

  dnl a threadid that is based on the address of errno. We need to double-check

  dnl that &errno is, in fact, different for each thread before using that

  dnl default.

  AC_CACHE_CHECK([if OpenSSL can use &errno as a THREADID],

                 [ac_cv_openssl_use_errno_threadid], [

    ac_cv_openssl_use_errno_threadid=no

    save_CFLAGS=$CFLAGS

    save_LIBS=$LIBS

    CFLAGS=`$apr_config --cflags --cppflags --includes`

    LIBS=`$apr_config --link-ld`

    AC_RUN_IFELSE([

      AC_LANG_PROGRAM([[

          #include <stdlib.h>

          #include "apr_pools.h"

          #include "apr_thread_cond.h"

          #include "apr_thread_proc.h"

          #define NUM_THREADS 3

          struct thread_data {

              apr_thread_mutex_t *mutex;

              apr_thread_cond_t  *cv;

              int                *init_count;

              void               *errno_addr;

          };

          /**

           * Thread entry point. Waits for all the threads to be started, then

           * records the address of errno into the thread_data.

           */

          void * APR_THREAD_FUNC tmain(apr_thread_t *thread, void *data)

          {

              struct thread_data *tdata = data;

              /* The only point of this barrier is to make sure that all threads

               * are started before we record &errno, hopefully preventing any

               * false negatives in case a platform "recycles" threads. */

              apr_thread_mutex_lock(tdata->mutex);

              ++(*tdata->init_count);

              if (*tdata->init_count == NUM_THREADS) {

                  apr_thread_cond_broadcast(tdata->cv);

              } else {

                  while (*tdata->init_count != NUM_THREADS) {

                      apr_thread_cond_wait(tdata->cv, tdata->mutex);

                  }

              }

              apr_thread_mutex_unlock(tdata->mutex);

              tdata->errno_addr = &errno;

              return NULL;

          }

      ]], [[

          int ret = 0;

          apr_status_t status;

          int i;

          apr_pool_t         *pool;

          apr_thread_mutex_t *mutex;

          apr_thread_cond_t  *cv;

          int                init_count = 0;

          struct thread_data tdata[NUM_THREADS] = { 0 };

          apr_thread_t *threads[NUM_THREADS] = { 0 };

          if (apr_initialize() != APR_SUCCESS) {

              exit(1);

          }

          /* Set up the shared APR primitives. */

          if ((apr_pool_create(&pool, NULL) != APR_SUCCESS)

              || (apr_thread_mutex_create(&mutex, 0, pool) != APR_SUCCESS)

              || (apr_thread_cond_create(&cv, pool) != APR_SUCCESS)) {

              ret = 2;

              goto out;

          }

          /* Start every thread. */

          for (i = 0; i < NUM_THREADS; ++i) {

              tdata[i].mutex = mutex;

              tdata[i].cv = cv;

              tdata[i].init_count = &init_count;

              status = apr_thread_create(&threads[i], NULL, tmain, &tdata[i],

                                         pool);

              if (status != APR_SUCCESS) {

                  ret = 3;

                  goto out;

              }

          }

          /* Wait for them to finish (they'll record and exit after every one

           * has been started). */

          for (i = 0; i < NUM_THREADS; ++i) {

              apr_thread_join(&status, threads[i]);

              if (status != APR_SUCCESS) {

                  ret = 4;

                  goto out;

              }

          }

          /* Check that no addresses were duplicated. */

          if ((tdata[0].errno_addr == tdata[1].errno_addr)

              || (tdata[1].errno_addr == tdata[2].errno_addr)

              || (tdata[0].errno_addr == tdata[2].errno_addr)) {

              ret = 5;

          }

      out:

          apr_terminate();

          exit(ret);

      ]])

    ], [

      ac_cv_openssl_use_errno_threadid=yes

    ], [

      ac_cv_openssl_use_errno_threadid=no

    ], [

      dnl Assume the worst when cross-compiling; users can override via either

      dnl cachevars or the config header if necessary.

      ac_cv_openssl_use_errno_threadid=no

    ])

    CFLAGS=$save_CFLAGS

    LIBS=$save_LIBS

  ])

  if test "x$ac_cv_openssl_use_errno_threadid" = "xyes"; then

    AC_DEFINE(AP_OPENSSL_USE_ERRNO_THREADID, 1,

              [Define if OpenSSL can use an errno-based threadid callback on this platform])

  fi

])

dnl