utils: Add flatpak_copy_bytes

This copies data from one fd to another.

common/flatpak-utils.c

@@ -1984,6 +1984,44 @@ flatpak_openat_noatime (int            dfd,
     }
 }
 
+#define COPY_BUFFER_SIZE (16*1024)
+gboolean
+flatpak_copy_bytes (int fdf,
+                    int fdt,
+                    GError **error)
+{
+  char buf[COPY_BUFFER_SIZE];
+  int r;
+
+  g_return_val_if_fail (fdf >= 0, FALSE);
+  g_return_val_if_fail (fdt >= 0, FALSE);
+
+  for (;;)
+    {
+      size_t m = COPY_BUFFER_SIZE;
+      ssize_t n;
+
+      n = read (fdf, buf, m);
+      if (n < 0)
+        {
+          glnx_set_error_from_errno (error);
+          return FALSE;
+        }
+      if (n == 0) /* EOF */
+        break;
+
+      r = glnx_loop_write (fdt, buf, (size_t) n);
+      if (r < 0)
+        {
+          errno = r;
+          glnx_set_error_from_errno (error);
+          return FALSE;
+        }
+    }
+
+  return TRUE;
+}
+
 gboolean
 flatpak_cp_a (GFile         *src,
               GFile         *dest,

common/flatpak-utils.h

@@ -362,6 +362,10 @@ gboolean flatpak_openat_noatime (int            dfd,
                                  GCancellable  *cancellable,
                                  GError       **error);
 
+gboolean flatpak_copy_bytes (int fdf,
+                             int fdt,
+                             GError **error);
+
 typedef enum {
   FLATPAK_CP_FLAGS_NONE = 0,
   FLATPAK_CP_FLAGS_MERGE = 1<<0,