libsysprof: return NULL instance unless debuginfod works

If we are configured with support for debuginfod and it fails to initialize
or we are not configured to use debuginfod, always ensure g_object_new()
will return a NULL instance back.

This helps prevent against misconfigured instances so we do not need to
do so many checks in vfuncs as well as allowing us to have the GType be
available always even when not built with debuginfod.

src/libsysprof/sysprof-debuginfod-symbolizer.c

@@ -20,18 +20,22 @@
 
 #include "config.h"
 
-#include <elfutils/debuginfod.h>
 #include <errno.h>
-#include <stdatomic.h>
 
 #include <glib/gstdio.h>
 
 #include "sysprof-symbolizer-private.h"
 #include "sysprof-debuginfod-symbolizer.h"
-#include "sysprof-debuginfod-task-private.h"
 #include "sysprof-elf-loader-private.h"
 #include "sysprof-symbol-private.h"
 
+#if HAVE_DEBUGINFOD
+# include <elfutils/debuginfod.h>
+# include "sysprof-debuginfod-task-private.h"
+#else
+typedef struct _debuginfod_client debuginfod_client;
+#endif
+
 struct _SysprofDebuginfodSymbolizer
 {
   SysprofSymbolizer  parent_instance;
@@ -58,6 +62,7 @@ sysprof_debuginfod_symbolizer_symbolize (SysprofSymbolizer        *symbolizer,
                                          SysprofAddressContext     context,
                                          SysprofAddress            address)
 {
+#if HAVE_DEBUGINFOD
   SysprofDebuginfodSymbolizer *self = SYSPROF_DEBUGINFOD_SYMBOLIZER (symbolizer);
   g_autoptr(SysprofElf) elf = NULL;
   g_autofree char *name = NULL;
@@ -145,6 +150,9 @@ sysprof_debuginfod_symbolizer_symbolize (SysprofSymbolizer        *symbolizer,
                              SYSPROF_SYMBOL_KIND_USER);
 
   return sym;
+#else
+  return NULL;
+#endif
 }
 
 static void
@@ -177,19 +185,54 @@ sysprof_debuginfod_symbolizer_finalize (GObject *object)
 
   g_clear_pointer (&self->cache, g_hash_table_unref);
   g_clear_pointer (&self->failed, g_hash_table_unref);
+
+#if HAVE_DEBUGINFOD
   g_clear_pointer (&self->client, debuginfod_end);
+#endif
 
   g_weak_ref_clear (&self->loader_wr);
 
   G_OBJECT_CLASS (sysprof_debuginfod_symbolizer_parent_class)->finalize (object);
 }
 
+static GObject *
+sysprof_debuginfod_symbolizer_constructor (GType                  type,
+                                           guint                  n_construct_params,
+                                           GObjectConstructParam *construct_properties)
+{
+#if HAVE_DEBUGINFOD
+  debuginfod_client *client;
+  GObject *object;
+
+  g_assert (type == SYSPROF_TYPE_DEBUGINFOD_SYMBOLIZER);
+
+  /* Don't even allow creating a SysprofDebuginfodSymbolizer instance unless we
+   * can create a new debuginfod_client. This ensures that even if an application
+   * does `g_object_new(SYSPROF_TYPE_DEBUGINFOD_SYMBOLIZER, NULL)` they will get
+   * `NULL` back instead of a misconfigured instance.
+   */
+  if (!(client = debuginfod_begin ()))
+    return NULL;
+
+  object = G_OBJECT_CLASS (sysprof_debuginfod_symbolizer_parent_class)
+      ->constructor (type, n_construct_params, construct_properties);
+
+  SYSPROF_DEBUGINFOD_SYMBOLIZER (object)->client = client;
+
+  return object;
+#else
+  errno = ENOTSUP;
+  return NULL;
+#endif
+}
+
 static void
 sysprof_debuginfod_symbolizer_class_init (SysprofDebuginfodSymbolizerClass *klass)
 {
   GObjectClass *object_class = G_OBJECT_CLASS (klass);
   SysprofSymbolizerClass *symbolizer_class = SYSPROF_SYMBOLIZER_CLASS (klass);
 
+  object_class->constructor = sysprof_debuginfod_symbolizer_constructor;
   object_class->dispose = sysprof_debuginfod_symbolizer_dispose;
   object_class->finalize = sysprof_debuginfod_symbolizer_finalize;
 
@@ -201,17 +244,18 @@ static void
 sysprof_debuginfod_symbolizer_init (SysprofDebuginfodSymbolizer *self)
 {
   g_weak_ref_init (&self->loader_wr, NULL);
+
+  self->loader = sysprof_elf_loader_new ();
+  self->cache = g_hash_table_new_full (NULL, NULL, g_object_unref, NULL);
+  self->failed = g_hash_table_new_full (g_str_hash, g_str_equal, g_free, NULL);
 }
 
 SysprofSymbolizer *
 sysprof_debuginfod_symbolizer_new (GError **error)
 {
-  g_autoptr(SysprofDebuginfodSymbolizer) self = NULL;
+  SysprofSymbolizer *self;
 
-  self = g_object_new (SYSPROF_TYPE_DEBUGINFOD_SYMBOLIZER, NULL);
-  self->client = debuginfod_begin ();
-
-  if (self->client == NULL)
+  if (!(self = g_object_new (SYSPROF_TYPE_DEBUGINFOD_SYMBOLIZER, NULL)))
     {
       int errsv = errno;
       g_set_error_literal (error,
@@ -221,9 +265,5 @@ sysprof_debuginfod_symbolizer_new (GError **error)
       return NULL;
     }
 
-  self->loader = sysprof_elf_loader_new ();
-  self->cache = g_hash_table_new_full (NULL, NULL, g_object_unref, NULL);
-  self->failed = g_hash_table_new_full (g_str_hash, g_str_equal, g_free, NULL);
-
-  return SYSPROF_SYMBOLIZER (g_steal_pointer (&self));
+  return self;
 }