Change OstreeFetcher to be dirfd-relative

This is a noticeable cleanup, and fixes another big user of GFile* in
performance/security sensitive codepaths.

I'm specifically making this change because the static deltas code was
leaking temporary files, and cleaning that up nicely would be best if
we were fd relative.

src/libostree/ostree-metalink.c

@@ -21,6 +21,7 @@
 #include "config.h"
 
 #include "ostree-metalink.h"
+#include <gio/gfiledescriptorbased.h>
 
 #include "otutil.h"
 #include "libgsystem.h"
@@ -72,7 +73,7 @@ typedef struct
   char *verification_sha256;
   char *verification_sha512;
 
-  GFile *result;
+  char *result;
 
   char *last_metalink_error;
   guint current_url_index;
@@ -429,34 +430,46 @@ on_fetched_url (GObject              *src,
   GTask *task = user_data;
   OstreeMetalinkRequest *self = g_task_get_task_data (task);
   GError *local_error = NULL;
-  gs_unref_object GFile *result = NULL;
-  gs_unref_object GFileInfo *finfo = NULL;
+  struct stat stbuf;
+  int parent_dfd = _ostree_fetcher_get_dfd (self->metalink->fetcher);
+  gs_unref_object GInputStream *instream = NULL;
+  gs_free char *result = NULL;
+  GChecksum *checksum = NULL;
 
   result = _ostree_fetcher_request_uri_with_partial_finish ((OstreeFetcher*)src, res, &local_error);
   if (!result)
     goto out;
-  
-  finfo = g_file_query_info (result, OSTREE_GIO_FAST_QUERYINFO, 0,
-                             g_task_get_cancellable (task), &local_error);
-  if (!finfo)
-    goto out;
 
-  if (g_file_info_get_size (finfo) != self->size)
+  if (!ot_openat_read_stream (parent_dfd, result, FALSE,
+                              &instream, NULL, &local_error))
+    goto out;
+  
+  if (fstat (g_file_descriptor_based_get_fd ((GFileDescriptorBased*)instream), &stbuf) != 0)
+    {
+      gs_set_error_from_errno (&local_error, errno);
+      goto out;
+    }
+
+  if (stbuf.st_size != self->size)
     {
       g_set_error (&local_error, G_IO_ERROR, G_IO_ERROR_FAILED,
                    "Expected size is %" G_GUINT64_FORMAT " bytes but content is %" G_GUINT64_FORMAT " bytes",
-                   self->size, g_file_info_get_size (finfo));
+                   self->size, stbuf.st_size);
       goto out;
     }
   
   if (self->verification_sha512)
     {
-      gs_free char *actual = ot_checksum_file (result, G_CHECKSUM_SHA512,
-                                               g_task_get_cancellable (task),
-                                               &local_error);
-      
-      if (!actual)
+      const char *actual;
+
+      checksum = g_checksum_new (G_CHECKSUM_SHA512);
+
+      if (!ot_gio_splice_update_checksum (NULL, instream, checksum,
+                                          g_task_get_cancellable (task),
+                                          &local_error))
         goto out;
+      
+      actual = g_checksum_get_string (checksum);
 
       if (strcmp (self->verification_sha512, actual) != 0)
         {
@@ -466,16 +479,19 @@ on_fetched_url (GObject              *src,
           goto out;
         }
     }
-
-  if (self->verification_sha256)
+  else if (self->verification_sha256)
     {
-      gs_free char *actual = ot_checksum_file (result, G_CHECKSUM_SHA256,
-                                               g_task_get_cancellable (task),
-                                               &local_error);
-      
-      if (!actual)
+      const char *actual;
+
+      checksum = g_checksum_new (G_CHECKSUM_SHA256);
+
+      if (!ot_gio_splice_update_checksum (NULL, instream, checksum,
+                                          g_task_get_cancellable (task),
+                                          &local_error))
         goto out;
 
+      actual = g_checksum_get_string (checksum);
+
       if (strcmp (self->verification_sha256, actual) != 0)
         {
           g_set_error (&local_error, G_IO_ERROR, G_IO_ERROR_FAILED,
@@ -486,6 +502,8 @@ on_fetched_url (GObject              *src,
     }
 
  out:
+  if (checksum)
+    g_checksum_free (checksum);
   if (local_error)
     {
       g_free (self->last_metalink_error);
@@ -498,7 +516,8 @@ on_fetched_url (GObject              *src,
     }
   else
     {
-      self->result = g_object_ref (result);
+      self->result = result;
+      result = NULL; /* Transfer ownership */
       g_task_return_boolean (self->task, TRUE);
     }
 }
@@ -584,7 +603,7 @@ ostree_metalink_request_unref (gpointer data)
 {
   OstreeMetalinkRequest  *request = data;
   g_object_unref (request->metalink);
-  g_clear_object (&request->result);
+  g_free (request->result);
   g_free (request->last_metalink_error);
   g_ptr_array_unref (request->urls);
   g_free (request);
@@ -601,7 +620,7 @@ static const GMarkupParser metalink_parser = {
 typedef struct
 {
   SoupURI               **out_target_uri;
-  GFile                 **out_data;
+  char                  **out_data;
   gboolean              success;
   GError                **error;
   GMainLoop             *loop;
@@ -611,7 +630,7 @@ static gboolean
 ostree_metalink_request_finish (OstreeMetalink         *self,
                                 GAsyncResult           *result,
                                 SoupURI               **out_target_uri,
-                                GFile                 **out_data,
+                                char                  **out_data,
                                 GError                **error)
 {
   OstreeMetalinkRequest *request;
@@ -624,7 +643,7 @@ ostree_metalink_request_finish (OstreeMetalink         *self,
     {
       g_assert_cmpint (request->current_url_index, <, request->urls->len);
       *out_target_uri = request->urls->pdata[request->current_url_index];
-      *out_data = g_object_ref (request->result);
+      *out_data = g_strdup (request->result);
       return TRUE;
     }
   else
@@ -668,7 +687,7 @@ gboolean
 _ostree_metalink_request_sync (OstreeMetalink        *self,
                                GMainLoop             *loop,
                                SoupURI               **out_target_uri,
-                               GFile                 **out_data,
+                               char                  **out_data,
                                SoupURI               **fetching_sync_uri,
                                GCancellable          *cancellable,
                                GError                **error)