Detect broken stdatomic.h, and pretend that it isn't there at all

I hope that the debian clang maintainers will look at debian bug
903709 soon. But until they do, this should keep our users and our
CI happy on sid with clang.

Closes ticket 26779.

changes/bug26779

@@ -0,0 +1,4 @@
+  o Minor features (bug workaround):
+    - Compile correctly on systems that provide the C11 stdatomic.h header,
+      but where C11 atomic functions don't actually compile.
+      Closes ticket 26779; workaround for Debian issue 903709.

configure.ac

@@ -1557,6 +1557,26 @@ AC_CHECK_SIZEOF(socklen_t, , [AC_INCLUDES_DEFAULT()
 
 AC_CHECK_SIZEOF(cell_t)
 
+# Let's see if stdatomic works. (There are some debian clangs that screw it
+# up; see Tor bug #26779 and debian bug 903709.)
+AC_CACHE_CHECK([whether C11 stdatomic.h actually works],
+               tor_cv_stdatomic_works,
+[AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
+#include <stdatomic.h>
+struct x { atomic_size_t y; };
+void try_atomic_init(struct x *xx)
+{
+  atomic_init(&xx->y, 99);
+  atomic_fetch_add(&xx->y, 1);
+}
+]])], [tor_cv_stdatomic_works=yes], [tor_cv_stdatomic_works=no])])
+
+if test "$tor_cv_stdatomic_works" = "yes"; then
+   AC_DEFINE(STDATOMIC_WORKS, 1, [Set to 1 if we can compile a simple stdatomic example.])
+elif test "$ac_cv_header_stdatomic_h" = "yes"; then
+   AC_MSG_WARN([Your compiler provides the stdatomic.h header, but it doesn't seem to work.  I'll pretend it isn't there. If you are using Clang on Debian, maybe this is because of https://bugs.debian.org/903709 ])
+fi
+
 # Now make sure that NULL can be represented as zero bytes.
 AC_CACHE_CHECK([whether memset(0) sets pointers to NULL], tor_cv_null_is_zero,
 [AC_RUN_IFELSE([AC_LANG_SOURCE(

src/common/compat_threads.c

@@ -352,7 +352,7 @@ alert_sockets_close(alert_sockets_t *socks)
   socks->read_fd = socks->write_fd = -1;
 }
 
-#ifndef HAVE_STDATOMIC_H
+#ifndef HAVE_WORKING_STDATOMIC
 /** Initialize a new atomic counter with the value 0 */
 void
 atomic_counter_init(atomic_counter_t *counter)
@@ -403,5 +403,4 @@ atomic_counter_exchange(atomic_counter_t *counter, size_t newval)
   tor_mutex_release(&counter->mutex);
   return oldval;
 }
-#endif /* !defined(HAVE_STDATOMIC_H) */
-
+#endif /* !defined(HAVE_WORKING_STDATOMIC) */

src/common/compat_threads.h

@@ -14,7 +14,11 @@
 #include <pthread.h>
 #endif
 
-#ifdef HAVE_STDATOMIC_H
+#if defined(HAVE_STDATOMIC_H) && defined(STDATOMIC_WORKS)
+#define HAVE_WORKING_STDATOMIC
+#endif
+
+#ifdef HAVE_WORKING_STDATOMIC
 #include <stdatomic.h>
 #endif
 
@@ -156,18 +160,18 @@ void tor_threadlocal_set(tor_threadlocal_t *threadlocal, void *value);
 /**
  * Atomic counter type; holds a size_t value.
  */
-#ifdef HAVE_STDATOMIC_H
+#ifdef HAVE_WORKING_STDATOMIC
 typedef struct atomic_counter_t {
   atomic_size_t val;
 } atomic_counter_t;
 #define ATOMIC_LINKAGE static
-#else /* !(defined(HAVE_STDATOMIC_H)) */
+#else /* !(defined(HAVE_WORKING_STDATOMIC)) */
 typedef struct atomic_counter_t {
   tor_mutex_t mutex;
   size_t val;
 } atomic_counter_t;
 #define ATOMIC_LINKAGE
-#endif /* defined(HAVE_STDATOMIC_H) */
+#endif /* defined(HAVE_WORKING_STDATOMIC) */
 
 ATOMIC_LINKAGE void atomic_counter_init(atomic_counter_t *counter);
 ATOMIC_LINKAGE void atomic_counter_destroy(atomic_counter_t *counter);
@@ -178,7 +182,7 @@ ATOMIC_LINKAGE size_t atomic_counter_exchange(atomic_counter_t *counter,
                                               size_t newval);
 #undef ATOMIC_LINKAGE
 
-#ifdef HAVE_STDATOMIC_H
+#ifdef HAVE_WORKING_STDATOMIC
 /** Initialize a new atomic counter with the value 0 */
 static inline void
 atomic_counter_init(atomic_counter_t *counter)
@@ -216,8 +220,7 @@ atomic_counter_exchange(atomic_counter_t *counter, size_t newval)
   return atomic_exchange(&counter->val, newval);
 }
 
-#else /* !(defined(HAVE_STDATOMIC_H)) */
-#endif /* defined(HAVE_STDATOMIC_H) */
+#else /* !(defined(HAVE_WORKING_STDATOMIC)) */
+#endif /* defined(HAVE_WORKING_STDATOMIC) */
 
 #endif /* !defined(TOR_COMPAT_THREADS_H) */
-