Test with background_thread:true.

Add testing for background_thread:true, and condition a xallocx() -->
rallocx() escalation assertion to allow for spurious in-place rallocx()
following xallocx() failure.

scripts/gen_run_tests.py

@@ -28,6 +28,7 @@ possible_malloc_conf_opts = [
     'tcache:false',
     'dss:primary',
     'percpu_arena:percpu',
+    'background_thread:true',
 ]
 
 print 'set -e'
@@ -57,7 +58,8 @@ for cc, cxx in possible_compilers:
                 )
 
                 # Per CPU arenas are only supported on Linux.
-                linux_supported = ('percpu_arena:percpu' in malloc_conf_opts)
+                linux_supported = ('percpu_arena:percpu' in malloc_conf_opts \
+                  or 'background_thread:true' in malloc_conf_opts)
                 # Heap profiling and dss are not supported on OS X.
                 darwin_unsupported = ('--enable-prof' in config_opts or \
                   'dss:primary' in malloc_conf_opts)

scripts/gen_travis.py

@@ -49,6 +49,7 @@ malloc_conf_unusuals = [
     'tcache:false',
     'dss:primary',
     'percpu_arena:percpu',
+    'background_thread:true',
 ]
 
 all_unusuals = (
@@ -80,7 +81,8 @@ for unusual_combination in unusual_combinations_to_test:
         x for x in unusual_combination if x in malloc_conf_unusuals]
     # Filter out unsupported configurations on OS X.
     if os == 'osx' and ('dss:primary' in malloc_conf or \
-      'percpu_arena:percpu' in malloc_conf):
+      'percpu_arena:percpu' in malloc_conf or 'background_thread:true' \
+      in malloc_conf):
         continue
     if len(malloc_conf) > 0:
         configure_flags.append('--with-malloc-conf=' + ",".join(malloc_conf))