install: follow unit file symlinks in /usr, but not /etc when looking for [Install] data

Some distributions use alias unit files via symlinks in /usr to cover
for legacy service names. With this change we'll allow "systemctl
enable" on such aliases.

Previously, our rule was that symlinks are user configuration that
"systemctl enable" + "systemctl disable" creates and removes, while unit
files is where the instructions to do so are store. As a result of the
rule we'd never read install information through symlinks, since that
would mix enablement state with installation instructions.

Now, the new rule is that only symlinks inside of /etc are
configuration. Unit files, and symlinks in /usr are now valid for
installation instructions.

This patch is quite a rework of the whole install logic, and makes the
following addional changes:

- Adds a complete test "test-instal-root" that tests the install logic
  pretty comprehensively.

- Never uses canonicalize_file_name(), because that's incompatible with
  operation relative to a specific root directory.

- unit_file_get_state() is reworked to return a proper error, and
  returns the state in a call-by-ref parameter. This cleans up confusion
  between the enum type and errno-like errors.

- The new logic puts a limit on how long to follow unit file symlinks:
  it will do so only for 64 steps at max.

- The InstallContext object's fields are renamed to will_process and
  has_processed (will_install and has_installed) since they are also
  used for deinstallation and all kinds of other operations.

- The root directory is always verified before use.

- install.c is reordered to place the exported functions together.

- Stricter rules are followed when traversing symlinks: the unit suffix
  must say identical, and it's not allowed to link between regular units
  and templated units.

- Various modernizations

- The "invalid" unit file state has been renamed to "bad", in order to
  avoid confusion between UNIT_FILE_INVALID and
  _UNIT_FILE_STATE_INVALID. Given that the state should normally not be
  seen and is not documented this should not be a problematic change.
  The new name is now documented however.

Fixes #1375, #1718, #1706

2 files changed, 15 insertions, 7 deletions

diff --git a/src/core/dbus-manager.c b/src/core/dbus-manager.c
index efc716913b..d3bcc795ae 100644
--- a/src/core/dbus-manager.c
+++ b/src/core/dbus-manager.c
@@ -1521,9 +1521,9 @@ static int method_get_unit_file_state(sd_bus_message *message, void *userdata, s
 
         scope = m->running_as == MANAGER_SYSTEM ? UNIT_FILE_SYSTEM : UNIT_FILE_USER;
 
-        state = unit_file_get_state(scope, NULL, name);
-        if (state < 0)
-                return state;
+        r = unit_file_get_state(scope, NULL, name, &state);
+        if (r < 0)
+                return r;
 
         return sd_bus_reply_method_return(message, "s", unit_file_state_to_string(state));
 }
diff --git a/src/core/unit.c b/src/core/unit.c
index ccdc5191e8..a67b9f282f 100644
--- a/src/core/unit.c
+++ b/src/core/unit.c
@@ -3148,12 +3148,19 @@ int unit_following_set(Unit *u, Set **s) {
 }
 
 UnitFileState unit_get_unit_file_state(Unit *u) {
+        int r;
+
         assert(u);
 
-        if (u->unit_file_state < 0 && u->fragment_path)
-                u->unit_file_state = unit_file_get_state(
+        if (u->unit_file_state < 0 && u->fragment_path) {
+                r = unit_file_get_state(
                                 u->manager->running_as == MANAGER_SYSTEM ? UNIT_FILE_SYSTEM : UNIT_FILE_USER,
-                                NULL, basename(u->fragment_path));
+                                NULL,
+                                basename(u->fragment_path),
+                                &u->unit_file_state);
+                if (r < 0)
+                        u->unit_file_state = UNIT_FILE_BAD;
+        }
 
         return u->unit_file_state;
 }
@@ -3164,7 +3171,8 @@ int unit_get_unit_file_preset(Unit *u) {
         if (u->unit_file_preset < 0 && u->fragment_path)
                 u->unit_file_preset = unit_file_query_preset(
                                 u->manager->running_as == MANAGER_SYSTEM ? UNIT_FILE_SYSTEM : UNIT_FILE_USER,
-                                NULL, basename(u->fragment_path));
+                                NULL,
+                                basename(u->fragment_path));
 
         return u->unit_file_preset;
 }