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

src/shared/install.h

@@ -25,13 +25,15 @@ typedef enum UnitFileScope UnitFileScope;
 typedef enum UnitFileState UnitFileState;
 typedef enum UnitFilePresetMode UnitFilePresetMode;
 typedef enum UnitFileChangeType UnitFileChangeType;
+typedef enum UnitFileType UnitFileType;
 typedef struct UnitFileChange UnitFileChange;
 typedef struct UnitFileList UnitFileList;
 typedef struct UnitFileInstallInfo UnitFileInstallInfo;
 
 #include "hashmap.h"
-#include "unit-name.h"
 #include "path-lookup.h"
+#include "strv.h"
+#include "unit-name.h"
 
 enum UnitFileScope {
         UNIT_FILE_SYSTEM,
@@ -51,7 +53,7 @@ enum UnitFileState {
         UNIT_FILE_STATIC,
         UNIT_FILE_DISABLED,
         UNIT_FILE_INDIRECT,
-        UNIT_FILE_INVALID,
+        UNIT_FILE_BAD,
         _UNIT_FILE_STATE_MAX,
         _UNIT_FILE_STATE_INVALID = -1
 };
@@ -82,6 +84,14 @@ struct UnitFileList {
         UnitFileState state;
 };
 
+enum UnitFileType {
+        UNIT_FILE_TYPE_REGULAR,
+        UNIT_FILE_TYPE_SYMLINK,
+        UNIT_FILE_TYPE_MASKED,
+        _UNIT_FILE_TYPE_MAX,
+        _UNIT_FILE_TYPE_INVALID = -1,
+};
+
 struct UnitFileInstallInfo {
         char *name;
         char *path;
@@ -93,8 +103,26 @@ struct UnitFileInstallInfo {
         char **also;
 
         char *default_instance;
+
+        UnitFileType type;
+
+        char *symlink_target;
 };
 
+static inline bool UNIT_FILE_INSTALL_INFO_HAS_RULES(UnitFileInstallInfo *i) {
+        assert(i);
+
+        return !strv_isempty(i->aliases) ||
+               !strv_isempty(i->wanted_by) ||
+               !strv_isempty(i->required_by);
+}
+
+static inline bool UNIT_FILE_INSTALL_INFO_HAS_ALSO(UnitFileInstallInfo *i) {
+        assert(i);
+
+        return !strv_isempty(i->also);
+}
+
 int unit_file_enable(UnitFileScope scope, bool runtime, const char *root_dir, char **files, bool force, UnitFileChange **changes, unsigned *n_changes);
 int unit_file_disable(UnitFileScope scope, bool runtime, const char *root_dir, char **files, UnitFileChange **changes, unsigned *n_changes);
 int unit_file_reenable(UnitFileScope scope, bool runtime, const char *root_dir, char **files, bool force, UnitFileChange **changes, unsigned *n_changes);
@@ -105,14 +133,14 @@ int unit_file_mask(UnitFileScope scope, bool runtime, const char *root_dir, char
 int unit_file_unmask(UnitFileScope scope, bool runtime, const char *root_dir, char **files, UnitFileChange **changes, unsigned *n_changes);
 int unit_file_set_default(UnitFileScope scope, const char *root_dir, const char *file, bool force, UnitFileChange **changes, unsigned *n_changes);
 int unit_file_get_default(UnitFileScope scope, const char *root_dir, char **name);
-int unit_file_add_dependency(UnitFileScope scope, bool runtime, const char *root_dir, char **files, char *target, UnitDependency dep, bool force, UnitFileChange **changes, unsigned *n_changes);
+int unit_file_add_dependency(UnitFileScope scope, bool runtime, const char *root_dir, char **files, const char *target, UnitDependency dep, bool force, UnitFileChange **changes, unsigned *n_changes);
 
-UnitFileState unit_file_lookup_state(UnitFileScope scope, const char *root_dir,const LookupPaths *paths, const char *name);
-UnitFileState unit_file_get_state(UnitFileScope scope, const char *root_dir, const char *filename);
+int unit_file_lookup_state(UnitFileScope scope, const char *root_dir,const LookupPaths *paths, const char *name, UnitFileState *ret);
+int unit_file_get_state(UnitFileScope scope, const char *root_dir, const char *filename, UnitFileState *ret);
 
 int unit_file_get_list(UnitFileScope scope, const char *root_dir, Hashmap *h);
+Hashmap* unit_file_list_free(Hashmap *h);
 
-void unit_file_list_free(Hashmap *h);
 int unit_file_changes_add(UnitFileChange **changes, unsigned *n_changes, UnitFileChangeType type, const char *path, const char *source);
 void unit_file_changes_free(UnitFileChange *changes, unsigned n_changes);