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, 7 insertions, 3 deletions

diff --git a/src/sysv-generator/sysv-generator.c b/src/sysv-generator/sysv-generator.c
index 042be97840..5075548507 100644
--- a/src/sysv-generator/sysv-generator.c
+++ b/src/sysv-generator/sysv-generator.c
@@ -742,6 +742,7 @@ static int fix_order(SysvStub *s, Hashmap *all_services) {
 
 static int enumerate_sysv(const LookupPaths *lp, Hashmap *all_services) {
         char **path;
+        int r;
 
         assert(lp);
         assert(all_services);
@@ -761,7 +762,6 @@ static int enumerate_sysv(const LookupPaths *lp, Hashmap *all_services) {
                         _cleanup_free_ char *fpath = NULL, *name = NULL;
                         _cleanup_(free_sysvstubp) SysvStub *service = NULL;
                         struct stat st;
-                        int r;
 
                         if (fstatat(dirfd(d), de->d_name, &st, 0) < 0) {
                                 log_warning_errno(errno, "stat() failed on %s/%s, ignoring: %m", *path, de->d_name);
@@ -781,8 +781,12 @@ static int enumerate_sysv(const LookupPaths *lp, Hashmap *all_services) {
                         if (hashmap_contains(all_services, name))
                                 continue;
 
-                        if (unit_file_lookup_state(UNIT_FILE_SYSTEM, NULL, lp, name) >= 0) {
-                                log_debug("Native unit for %s already exists, skipping", name);
+                        r = unit_file_lookup_state(UNIT_FILE_SYSTEM, NULL, lp, name, NULL);
+                        if (r < 0 && r != -ENOENT) {
+                                log_debug_errno(r, "Failed to detect whether %s exists, skipping: %m", name);
+                                continue;
+                        } else if (r >= 0) {
+                                log_debug("Native unit for %s already exists, skipping.", name);
                                 continue;
                         }