Better dynamic linking

Makefile

@@ -340,7 +340,7 @@ endif
 endif
 
 ifeq ($(SYSTEM),Linux)
-LIBS = rt m pthread
+LIBS = dl rt m pthread
 LDFLAGS += -pthread
 endif
 

src/core/support/env_linux.c

@@ -42,33 +42,23 @@
 
 #include "src/core/support/env.h"
 
+#include <dlfcn.h>
 #include <stdlib.h>
 
 #include <grpc/support/log.h>
 #include <grpc/support/string_util.h>
+#include <grpc/support/useful.h>
 
 #include "src/core/support/string.h"
 
-/* Declare weak symbols for versions of secure_getenv that *may* be
- * on a users machine. Older libc's call this __secure_getenv, even
- * older don't support the functionality.
- *
- * If a symbol is not present, these will be equal to NULL.
- */
-char *__attribute__((weak)) secure_getenv(const char *name);
-char *__attribute__((weak)) __secure_getenv(const char *name);
-
 char *gpr_getenv(const char *name) {
-  static char *(*getenv_func)(const char *) = secure_getenv;
+  typedef char *(*getenv_type)(const char *);
+  static getenv_type getenv_func = NULL;
   /* Check to see which getenv variant is supported (go from most
    * to least secure) */
-  if (getenv_func == NULL) {
-    getenv_func = __secure_getenv;
-    if (getenv_func == NULL) {
-      gpr_log(GPR_DEBUG,
-              "No secure_getenv. Please consider upgrading your libc.");
-      getenv_func = getenv;
-    }
+  const char *names[] = {"secure_getenv", "__secure_getenv", "getenv"};
+  for (size_t i = 0; getenv_func == NULL && i < GPR_ARRAY_SIZE(names); i++) {
+    getenv_func = (getenv_type)dlsym(RTLD_DEFAULT, names[i]);
   }
   char *result = getenv_func(name);
   return result == NULL ? result : gpr_strdup(result);

templates/Makefile.template

@@ -253,7 +253,7 @@
   endif
 
   ifeq ($(SYSTEM),Linux)
-  LIBS = rt m pthread
+  LIBS = dl rt m pthread
   LDFLAGS += -pthread
   endif