Thread support is now required

Long ago we supported systems where there was no support for
threads, or where the threading library was broken. We shouldn't
have do that any more: on every OS that matters, threads exist, and
the OS supports running threads across multiple CPUs.

This resolves tickets 9495 and 12439.  It's a prerequisite to making
our workqueue code work better, since sensible workqueue
implementations don't split across multiple processes.

8 files changed, 21 insertions, 78 deletions

diff --git a/changes/threads-required b/changes/threads-required
new file mode 100644
index 000000000..a56cfe345
--- /dev/null
+++ b/changes/threads-required
@@ -0,0 +1,12 @@
+  o Removed features:
+    - Tor no longer supports systems without threading support.
+      When we began working on Tor, there were several systems that didn't
+      have threads, or where the thread support wasn't able to run the
+      threads of a single process on multiple CPUs. That no longer holds:
+      every system where Tor needs to run well now has threading support.
+      Resolves ticket 12439.
+
+  o Minor features:
+    - Threads are no longer disabled by default on Solaris; we believe that
+      the versions of Solaris with broken threading support are all obsolete
+      by now. Resolves ticket 9495.
diff --git a/configure.ac b/configure.ac
index 0d3758170..e858d0c0c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -107,26 +107,6 @@ AC_ARG_ENABLE(upnp,
         * ) AC_MSG_ERROR(bad value for --enable-upnp) ;;
       esac], [upnp=false])
 
-
-AC_ARG_ENABLE(threads,
-     AS_HELP_STRING(--disable-threads, disable multi-threading support))
-
-if test x$enable_threads = x; then
-   case $host in
-    *-*-solaris* )
-     # Don't try multithreading on solaris -- cpuworkers seem to lock.
-     AC_MSG_NOTICE([You are running Solaris; Sometimes threading makes
-cpu workers lock up here, so I will disable threads.])
-     enable_threads="no";;
-    *)
-     enable_threads="yes";;
-   esac
-fi
-
-if test "$enable_threads" = "yes"; then
-  AC_DEFINE(ENABLE_THREADS, 1, [Defined if we will try to use multithreading])
-fi
-
 case $host in
    *-*-solaris* )
      AC_DEFINE(_REENTRANT, 1, [Define on some platforms to activate x_r() functions in time.h])
@@ -326,10 +306,8 @@ if test "$LIBS" != "$saved_LIBS"; then
    have_rt=yes
 fi
 
-if test "$enable_threads" = "yes"; then
-  AC_SEARCH_LIBS(pthread_create, [pthread])
-  AC_SEARCH_LIBS(pthread_detach, [pthread])
-fi
+AC_SEARCH_LIBS(pthread_create, [pthread])
+AC_SEARCH_LIBS(pthread_detach, [pthread])
 
 dnl -------------------------------------------------------------------
 dnl Check for functions before libevent, since libevent-1.2 apparently
@@ -372,7 +350,7 @@ AC_CHECK_FUNCS(
 	_vscprintf
 )
 
-if test "$enable_threads" = "yes"; then
+if test "$bwin32" != true; then
   AC_CHECK_HEADERS(pthread.h)
   AC_CHECK_FUNCS(pthread_create)
 fi
diff --git a/src/common/compat.c b/src/common/compat.c
index 6fb9c210e..1760684cf 100644
--- a/src/common/compat.c
+++ b/src/common/compat.c
@@ -2722,7 +2722,7 @@ tor_gettimeofday(struct timeval *timeval)
   return;
 }
 
-#if defined(TOR_IS_MULTITHREADED) && !defined(_WIN32)
+#if !defined(_WIN32)
 /** Defined iff we need to add locks when defining fake versions of reentrant
  * versions of time-related functions. */
 #define TIME_FNS_NEED_LOCKS
@@ -2982,7 +2982,6 @@ tor_get_thread_id(void)
 }
 #endif
 
-#ifdef TOR_IS_MULTITHREADED
 /** Return a newly allocated, ready-for-use mutex. */
 tor_mutex_t *
 tor_mutex_new(void)
@@ -3000,7 +2999,6 @@ tor_mutex_free(tor_mutex_t *m)
   tor_mutex_uninit(m);
   tor_free(m);
 }
-#endif
 
 /* Conditions. */
 #ifdef USE_PTHREADS
diff --git a/src/common/compat.h b/src/common/compat.h
index 635e427f0..296042fa5 100644
--- a/src/common/compat.h
+++ b/src/common/compat.h
@@ -633,15 +633,12 @@ int get_total_system_memory(size_t *mem_out);
 int spawn_func(void (*func)(void *), void *data);
 void spawn_exit(void) ATTR_NORETURN;
 
-#if defined(ENABLE_THREADS) && defined(_WIN32)
+#if defined(_WIN32)
 #define USE_WIN32_THREADS
-#define TOR_IS_MULTITHREADED 1
-#elif (defined(ENABLE_THREADS) && defined(HAVE_PTHREAD_H) && \
-       defined(HAVE_PTHREAD_CREATE))
+#elif defined(HAVE_PTHREAD_H) && defined(HAVE_PTHREAD_CREATE)
 #define USE_PTHREADS
-#define TOR_IS_MULTITHREADED 1
 #else
-#undef TOR_IS_MULTITHREADED
+#error "No threading system was found"
 #endif
 
 int compute_num_cpus(void);
@@ -667,7 +664,6 @@ typedef struct tor_mutex_t {
 
 int tor_mlockall(void);
 
-#ifdef TOR_IS_MULTITHREADED
 tor_mutex_t *tor_mutex_new(void);
 void tor_mutex_init(tor_mutex_t *m);
 void tor_mutex_acquire(tor_mutex_t *m);
@@ -676,21 +672,10 @@ void tor_mutex_free(tor_mutex_t *m);
 void tor_mutex_uninit(tor_mutex_t *m);
 unsigned long tor_get_thread_id(void);
 void tor_threads_init(void);
-#else
-#define tor_mutex_new() ((tor_mutex_t*)tor_malloc(sizeof(int)))
-#define tor_mutex_init(m) STMT_NIL
-#define tor_mutex_acquire(m) STMT_VOID(m)
-#define tor_mutex_release(m) STMT_NIL
-#define tor_mutex_free(m) STMT_BEGIN tor_free(m); STMT_END
-#define tor_mutex_uninit(m) STMT_NIL
-#define tor_get_thread_id() (1UL)
-#define tor_threads_init() STMT_NIL
-#endif
 
 void set_main_thread(void);
 int in_main_thread(void);
 
-#ifdef TOR_IS_MULTITHREADED
 #if 0
 typedef struct tor_cond_t tor_cond_t;
 tor_cond_t *tor_cond_new(void);
@@ -699,7 +684,6 @@ int tor_cond_wait(tor_cond_t *cond, tor_mutex_t *mutex);
 void tor_cond_signal_one(tor_cond_t *cond);
 void tor_cond_signal_all(tor_cond_t *cond);
 #endif
-#endif
 
 /** Macros for MIN/MAX.  Never use these when the arguments could have
  * side-effects.
diff --git a/src/common/crypto.c b/src/common/crypto.c
index a247a87d4..16292455b 100644
--- a/src/common/crypto.c
+++ b/src/common/crypto.c
@@ -75,12 +75,10 @@
 /** Macro: is k a valid RSA private key? */
 #define PRIVATE_KEY_OK(k) ((k) && (k)->key && (k)->key->p)
 
-#ifdef TOR_IS_MULTITHREADED
 /** A number of preallocated mutexes for use by OpenSSL. */
 static tor_mutex_t **openssl_mutexes_ = NULL;
 /** How many mutexes have we allocated for use by OpenSSL? */
 static int n_openssl_mutexes_ = 0;
-#endif
 
 /** A public key, or a public/private key-pair. */
 struct crypto_pk_t
@@ -3090,8 +3088,6 @@ memwipe(void *mem, uint8_t byte, size_t sz)
   memset(mem, byte, sz);
 }
 
-#ifdef TOR_IS_MULTITHREADED
-
 #ifndef OPENSSL_THREADS
 #error OpenSSL has been built without thread support. Tor requires an \
  OpenSSL library with thread support enabled.
@@ -3178,13 +3174,6 @@ setup_openssl_threading(void)
   CRYPTO_set_dynlock_destroy_callback(openssl_dynlock_destroy_cb_);
   return 0;
 }
-#else
-static int
-setup_openssl_threading(void)
-{
-  return 0;
-}
-#endif
 
 /** Uninitialize the crypto library. Return 0 on success, -1 on failure.
  */
@@ -3208,7 +3197,7 @@ crypto_global_cleanup(void)
 
   CONF_modules_unload(1);
   CRYPTO_cleanup_all_ex_data();
-#ifdef TOR_IS_MULTITHREADED
+
   if (n_openssl_mutexes_) {
     int n = n_openssl_mutexes_;
     tor_mutex_t **ms = openssl_mutexes_;
@@ -3220,7 +3209,7 @@ crypto_global_cleanup(void)
     }
     tor_free(ms);
   }
-#endif
+
   tor_free(crypto_openssl_version_str);
   tor_free(crypto_openssl_header_version_str);
   return 0;
diff --git a/src/or/cpuworker.c b/src/or/cpuworker.c
index 61b2c29b3..62af8e2f5 100644
--- a/src/or/cpuworker.c
+++ b/src/or/cpuworker.c
@@ -414,12 +414,6 @@ cpuworker_main(void *data)
   cpuworker_reply_t rpl;
 
   fd = fdarray[1]; /* this side is ours */
-#ifndef TOR_IS_MULTITHREADED
-  tor_close_socket(fdarray[0]); /* this is the side of the socketpair the
-                                 * parent uses */
-  tor_free_all(1); /* so the child doesn't hold the parent's fd's open */
-  handle_signals(0); /* ignore interrupts from the keyboard, etc */
-#endif
   tor_free(data);
 
   setup_server_onion_keys(&onion_keys);
@@ -535,10 +529,6 @@ spawn_cpuworker(void)
     return -1;
   }
   log_debug(LD_OR,"just spawned a cpu worker.");
-#ifndef TOR_IS_MULTITHREADED
-  tor_close_socket(fdarray[1]); /* don't need the worker's side of the pipe */
-  tor_free(fdarray);
-#endif
 
   conn = connection_new(CONN_TYPE_CPUWORKER, AF_UNIX);
 
diff --git a/src/test/test_util.c b/src/test/test_util.c
index 151ec6912..86ff747ac 100644
--- a/src/test/test_util.c
+++ b/src/test/test_util.c
@@ -1383,12 +1383,6 @@ test_util_threads(void)
   tv.tv_sec=0;
   tv.tv_usec=100*1000;
 #endif
-#ifndef TOR_IS_MULTITHREADED
-  /* Skip this test if we aren't threading. We should be threading most
-   * everywhere by now. */
-  if (1)
-    return;
-#endif
   thread_test_mutex_ = tor_mutex_new();
   thread_test_start1_ = tor_mutex_new();
   thread_test_start2_ = tor_mutex_new();
diff --git a/src/win32/orconfig.h b/src/win32/orconfig.h
index d12d6676e..030341af3 100644
--- a/src/win32/orconfig.h
+++ b/src/win32/orconfig.h
@@ -14,8 +14,6 @@
 /* Define to 1 if you have the <ctype.h> header file. */
 #define HAVE_CTYPE_H
 
-#define ENABLE_THREADS
-
 /* Define to 1 if you have the <errno.h> header file. */
 #define HAVE_ERRNO_H