Disable config from file or envvar with build flag

This adds a new autoconf flag, --disable-user-config, which disables
reading the configuration from /etc/malloc.conf or the MALLOC_CONF
environment variable. This can be useful when integrating jemalloc in a
binary that internally handles all aspects of the configuration and
shouldn't be impacted by ambient change in the environment.

configure.ac

@@ -1401,6 +1401,23 @@ if test "x$enable_stats" = "x1" ; then
 fi
 AC_SUBST([enable_stats])
 
+dnl Disable reading configuration from file and environment variable
+AC_ARG_ENABLE([user_config],
+  [AS_HELP_STRING([--disable-user-config],
+  [Do not read malloc config from /etc/malloc.conf or MALLOC_CONF])],
+[if test "x$enable_user_config" = "xno" ; then
+  enable_user_config="0"
+else
+  enable_user_config="1"
+fi
+],
+[enable_user_config="1"]
+)
+if test "x$enable_user_config" = "x1" ; then
+  AC_DEFINE([JEMALLOC_CONFIG_ENV], [ ], [ ])
+  AC_DEFINE([JEMALLOC_CONFIG_FILE], [ ], [ ])
+fi
+
 dnl Do not enable smallocx by default.
 AC_ARG_ENABLE([experimental_smallocx],
   [AS_HELP_STRING([--enable-experimental-smallocx], [Enable experimental smallocx API])],
@@ -2962,6 +2979,7 @@ AC_MSG_RESULT([static libs        : ${enable_static}])
 AC_MSG_RESULT([autogen            : ${enable_autogen}])
 AC_MSG_RESULT([debug              : ${enable_debug}])
 AC_MSG_RESULT([stats              : ${enable_stats}])
+AC_MSG_RESULT([user_config        : ${enable_user_config}])
 AC_MSG_RESULT([experimental_smallocx : ${enable_experimental_smallocx}])
 AC_MSG_RESULT([prof               : ${enable_prof}])
 AC_MSG_RESULT([prof-libunwind     : ${enable_prof_libunwind}])

include/jemalloc/jemalloc_defs.h.in

@@ -46,6 +46,12 @@
  */
 #undef JEMALLOC_USE_CXX_THROW
 
+/*
+ * If undefined, disables reading configuration from environment variable or file
+ */
+#undef JEMALLOC_CONFIG_ENV
+#undef JEMALLOC_CONFIG_FILE
+
 #ifdef _MSC_VER
 #  ifdef _WIN64
 #    define LG_SIZEOF_PTR_WIN 3

src/jemalloc.c

@@ -985,44 +985,53 @@ obtain_malloc_conf(unsigned which_source, char readlink_buf[PATH_MAX + 1]) {
 		}
 		break;
 	case 2: {
+#ifndef JEMALLOC_CONFIG_FILE
+		ret = NULL;
+		break;
+#else
 		ssize_t linklen = 0;
-#ifndef _WIN32
+#  ifndef _WIN32
 		int saved_errno = errno;
 		const char *linkname =
-#  ifdef JEMALLOC_PREFIX
+#    ifdef JEMALLOC_PREFIX
 		    "/etc/"JEMALLOC_PREFIX"malloc.conf"
-#  else
+#    else
 		    "/etc/malloc.conf"
-#  endif
+#    endif
 		    ;
 
 		/*
 		 * Try to use the contents of the "/etc/malloc.conf" symbolic
 		 * link's name.
 		 */
-#ifndef JEMALLOC_READLINKAT
+#    ifndef JEMALLOC_READLINKAT
 		linklen = readlink(linkname, readlink_buf, PATH_MAX);
-#else
+#    else
 		linklen = readlinkat(AT_FDCWD, linkname, readlink_buf, PATH_MAX);
-#endif
+#    endif
 		if (linklen == -1) {
 			/* No configuration specified. */
 			linklen = 0;
 			/* Restore errno. */
 			set_errno(saved_errno);
 		}
-#endif
+#  endif
 		readlink_buf[linklen] = '\0';
 		ret = readlink_buf;
 		break;
-	} case 3: {
-		const char *envname =
-#ifdef JEMALLOC_PREFIX
-		    JEMALLOC_CPREFIX"MALLOC_CONF"
-#else
-		    "MALLOC_CONF"
 #endif
-		    ;
+	} case 3: {
+#ifndef JEMALLOC_CONFIG_ENV
+		ret = NULL;
+		break;
+#else
+		const char *envname =
+#  ifdef JEMALLOC_PREFIX
+			JEMALLOC_CPREFIX"MALLOC_CONF"
+#  else
+			"MALLOC_CONF"
+#  endif
+			;
 
 		if ((ret = jemalloc_getenv(envname)) != NULL) {
 			opt_malloc_conf_env_var = ret;
@@ -1031,6 +1040,7 @@ obtain_malloc_conf(unsigned which_source, char readlink_buf[PATH_MAX + 1]) {
 			ret = NULL;
 		}
 		break;
+#endif
 	} case 4: {
 		ret = je_malloc_conf_2_conf_harder;
 		break;