[services] kernel-switcher: when kernel_tag is not available, try to guess it

When no reverse dependencies are available, it's impossible to guess
the correct kernel tag. So, kernel_tag is None, which causes issue
when used in execve() (of course). So, handle the case trying to read
package configuration file containing the proper "uname -r" output required
to switch /usr/src/linux to the newly installed kernel using
"eselect kernel set".
Please note that this file (RELEASE_LEVEL) is quite new, and older
packages might have been shipped without it.

Also see bug #2227

services/kernel-switcher

@@ -59,6 +59,23 @@ def _setup_kernel_symlink(target_tag):
     if os.access(eselect_exec, os.X_OK):
         subprocess.call((eselect_exec, "kernel", "set", target_tag))
 
+def _guess_kernel_name(kernel_atom):
+    """
+    This method takes advantage of Entropy kernel package info files available
+    at /etc/kernels/<pkg-name>-<pkg-ver>/ directory.
+    This function tries to read uname -r from the RELEASE_LEVEL file.
+    """
+    namever = entropy.dep.remove_cat(kernel_atom)
+    kernel_meta_file = os.path.join("/etc/kernels", namever, "RELEASE_LEVEL")
+    if os.path.isfile(kernel_meta_file):
+        try:
+            with open(kernel_meta_file, "r") as km_f:
+                kernel_name = km_f.readline().strip()
+                if kernel_name:
+                    return kernel_name
+        except (OSError, IOError):
+            return None
+
 # removing and installing proprietary drivers might reset the selected
 # OpenGL implementation
 
@@ -117,6 +134,7 @@ def _switch_kernel(args):
             return 1
 
         kernel_atom = etp_client.open_repository(pkg_repo).retrieveAtom(pkg_id)
+        # this can be None !
         target_tag = _get_target_tag(etp_client, kernel_match)
 
         inst_repo = etp_client.installed_repository()
@@ -152,7 +170,14 @@ def _switch_kernel(args):
         )
         if (rc == 0) and (not etpUi['pretend']):
             _set_opengl_impl(opengl)
-            _setup_kernel_symlink(target_tag)
+            if target_tag:
+                # if target_tag is None, we are unable to set the symlink
+                _setup_kernel_symlink(target_tag)
+            else:
+                # try to guess, sigh, for now
+                guessed_kernel_name = _guess_kernel_name(kernel_atom)
+                if guessed_kernel_name:
+                    _setup_kernel_symlink(guessed_kernel_name)
             _show_kernel_warnings(kernel_atom)
         return rc