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

1 files changed, 3 insertions, 3 deletions

diff --git a/src/systemctl/systemctl.c b/src/systemctl/systemctl.c
index 83ec37d19b..be98bc9671 100644
--- a/src/systemctl/systemctl.c
+++ b/src/systemctl/systemctl.c
@@ -1335,7 +1335,7 @@ static void output_unit_file_list(const UnitFileList *units, unsigned c) {
                            UNIT_FILE_MASKED,
                            UNIT_FILE_MASKED_RUNTIME,
                            UNIT_FILE_DISABLED,
-                           UNIT_FILE_INVALID)) {
+                           UNIT_FILE_BAD)) {
                         on  = ansi_highlight_red();
                         off = ansi_normal();
                 } else if (u->state == UNIT_FILE_ENABLED) {
@@ -5741,8 +5741,8 @@ static int unit_is_enabled(int argc, char *argv[], void *userdata) {
                 STRV_FOREACH(name, names) {
                         UnitFileState state;
 
-                        state = unit_file_get_state(arg_scope, arg_root, *name);
-                        if (state < 0)
+                        r = unit_file_get_state(arg_scope, arg_root, *name, &state);
+                        if (r < 0)
                                 return log_error_errno(state, "Failed to get unit file state for %s: %m", *name);
 
                         if (IN_SET(state,