dlopen GnuTLS instead of linking directly

I'd like to enable our GnuTLS GHmac patchset in Fedora in order to
ensure it is receiving sufficient real-world testing, since we've
discovered several bugs thus far. Problem is Fedora has one requirement
that RHEL does not: it needs to build glib as a static lib. This is
needed by QEMU in Fedora for complicated technical reasons that I don't
understand. However, nothing in RHEL needs it. This means we failed to
notice that glib2-static is broken in RHEL, because there is no
gnutls-static! We could fix this by adding a gnutls-static package, but
that seems like overkill, and adding more static libraries where they're
not truly necessary is not the direction we want to move in anyway. So
instead, let's just dlopen GnuTLS to sidestep this problem entirely.

This would not be a good solution for upstream, but upstream has made
clear that this patchset is already non-upstreamable, so it will be fine
for our purposes.

glib/ghmac-gnutls.c

@@ -19,8 +19,8 @@
 
 #include "config.h"
 
+#include <dlfcn.h>
 #include <string.h>
-#include <gnutls/crypto.h>
 
 #include "ghmac.h"
 
@@ -31,13 +31,16 @@
 #include "gstrfuncs.h"
 #include "gchecksumprivate.h"
 #include "gtestutils.h"
+#include "gthread.h"
 #include "gtypes.h"
 #include "glibintl.h"
 
-#ifndef HAVE_GNUTLS
+#ifndef USE_GNUTLS
 #error "build configuration error"
 #endif
 
+typedef gpointer gnutls_hmac_hd_t;
+
 struct _GHmac
 {
   int ref_count;
@@ -46,15 +49,107 @@ struct _GHmac
   gchar *digest_str;
 };
 
+typedef enum
+{
+  GNUTLS_MAC_MD5 = 2,
+  GNUTLS_MAC_SHA1 = 3,
+  GNUTLS_MAC_SHA256 = 6,
+  GNUTLS_MAC_SHA384 = 7,
+  GNUTLS_MAC_SHA512 = 8,
+} gnutls_mac_algorithm_t;
+
+/* Why are we dlopening GnuTLS instead of linking to it directly? Because we
+ * want to be able to build GLib as a static library without depending on a
+ * static build of GnuTLS. QEMU depends on static linking with GLib, but Fedora
+ * does not ship a static build of GnuTLS, and this allows us to avoid changing
+ * that.
+ */
+static int              (*gnutls_hmac_init)   (gnutls_hmac_hd_t *dig, gnutls_mac_algorithm_t algorithm, const void *key, size_t keylen);
+static gnutls_hmac_hd_t (*gnutls_hmac_copy)   (gnutls_hmac_hd_t handle);
+static void             (*gnutls_hmac_deinit) (gnutls_hmac_hd_t handle, void *digest);
+static int              (*gnutls_hmac)        (gnutls_hmac_hd_t handle, const void *ptext, size_t ptext_len);
+static void             (*gnutls_hmac_output) (gnutls_hmac_hd_t handle, void *digest);
+static const char *     (*gnutls_strerror)    (int error);
+
+static gsize gnutls_initialize_attempted = 0;
+static gboolean gnutls_initialize_successful = FALSE;
+
+static void
+initialize_gnutls (void)
+{
+  gpointer libgnutls;
+
+  libgnutls = dlopen ("libgnutls.so.30", RTLD_LAZY | RTLD_GLOBAL);
+  if (!libgnutls)
+    {
+      g_warning ("Cannot use GHmac: failed to load libgnutls.so.30: %s", dlerror ());
+      return;
+    }
+
+  gnutls_hmac_init = dlsym (libgnutls, "gnutls_hmac_init");
+  if (!gnutls_hmac_init)
+    {
+      g_warning ("Cannot use GHmac: failed to load gnutls_hmac_init: %s", dlerror ());
+      return;
+    }
+
+  gnutls_hmac_copy = dlsym (libgnutls, "gnutls_hmac_copy");
+  if (!gnutls_hmac_copy)
+    {
+      g_warning ("Cannot use GHmac: failed to load gnutls_hmac_copy: %s", dlerror ());
+      return;
+    }
+
+  gnutls_hmac_deinit = dlsym (libgnutls, "gnutls_hmac_deinit");
+  if (!gnutls_hmac_deinit)
+    {
+      g_warning ("Cannot use GHmac: failed to load gnutls_hmac_deinit: %s", dlerror ());
+      return;
+    }
+
+  gnutls_hmac = dlsym (libgnutls, "gnutls_hmac");
+  if (!gnutls_hmac)
+    {
+      g_warning ("Cannot use GHmac: failed to load gnutls_hmac: %s", dlerror ());
+      return;
+    }
+
+  gnutls_hmac_output = dlsym (libgnutls, "gnutls_hmac_output");
+  if (!gnutls_hmac_output)
+    {
+      g_warning ("Cannot use GHmac: failed to load gnutls_hmac_output: %s", dlerror ());
+      return;
+    }
+
+  gnutls_strerror = dlsym (libgnutls, "gnutls_strerror");
+  if (!gnutls_strerror)
+    {
+      g_warning ("Cannot use GHmac: failed to load gnutls_strerror: %s", dlerror ());
+      return;
+    }
+
+  gnutls_initialize_successful = TRUE;
+}
+
 GHmac *
 g_hmac_new (GChecksumType  digest_type,
             const guchar  *key,
             gsize          key_len)
 {
   gnutls_mac_algorithm_t algo;
-  GHmac *hmac = g_new0 (GHmac, 1);
+  GHmac *hmac;
   int ret;
 
+  if (g_once_init_enter (&gnutls_initialize_attempted))
+    {
+      initialize_gnutls ();
+      g_once_init_leave (&gnutls_initialize_attempted, 1);
+    }
+
+  if (!gnutls_initialize_successful)
+    return NULL;
+
+  hmac = g_new0 (GHmac, 1);
   hmac->ref_count = 1;
   hmac->digest_type = digest_type;
 

glib/ghmac.c

@@ -35,7 +35,7 @@
 #include "gtypes.h"
 #include "glibintl.h"
 
-#ifdef HAVE_GNUTLS
+#ifdef USE_GNUTLS
 #error "build configuration error"
 #endif
 

glib/meson.build

@@ -428,7 +428,6 @@ libglib = library('glib-2.0',
   link_with: [charset_lib, gnulib_lib],
   dependencies : [
     gnulib_libm_dependency,
-    libgnutls_dep,
     libiconv,
     libintl_deps,
     libm,

meson.build

@@ -2286,11 +2286,9 @@ if host_system == 'linux'
   endif
 endif
 
-# gnutls is used optionally by ghmac
-libgnutls_dep = []
+# gnutls is used optionally by GHmac
 if get_option('gnutls')
-  libgnutls_dep = [dependency('gnutls', version : '>=3.6.9', required : true)]
-  glib_conf.set('HAVE_GNUTLS', 1)
+  glib_conf.set('USE_GNUTLS', 1)
 endif
 
 if host_system == 'windows'