core: add "invocation ID" concept to service manager

This adds a new invocation ID concept to the service manager. The invocation ID
identifies each runtime cycle of a unit uniquely. A new randomized 128bit ID is
generated each time a unit moves from and inactive to an activating or active
state.

The primary usecase for this concept is to connect the runtime data PID 1
maintains about a service with the offline data the journal stores about it.
Previously we'd use the unit name plus start/stop times, which however is
highly racy since the journal will generally process log data after the service
already ended.

The "invocation ID" kinda matches the "boot ID" concept of the Linux kernel,
except that it applies to an individual unit instead of the whole system.

The invocation ID is passed to the activated processes as environment variable.
It is additionally stored as extended attribute on the cgroup of the unit. The
latter is used by journald to automatically retrieve it for each log logged
message and attach it to the log entry. The environment variable is very easily
accessible, even for unprivileged services. OTOH the extended attribute is only
accessible to privileged processes (this is because cgroupfs only supports the
"trusted." xattr namespace, not "user."). The environment variable may be
altered by services, the extended attribute may not be, hence is the better
choice for the journal.

Note that reading the invocation ID off the extended attribute from journald is
racy, similar to the way reading the unit name for a logging process is.

This patch adds APIs to read the invocation ID to sd-id128:
sd_id128_get_invocation() may be used in a similar fashion to
sd_id128_get_boot().

PID1's own logging is updated to always include the invocation ID when it logs
information about a unit.

A new bus call GetUnitByInvocationID() is added that allows retrieving a bus
path to a unit by its invocation ID. The bus path is built using the invocation
ID, thus providing a path for referring to a unit that is valid only for the
current runtime cycleof it.

Outlook for the future: should the kernel eventually allow passing of cgroup
information along AF_UNIX/SOCK_DGRAM messages via a unique cgroup id, then we
can alter the invocation ID to be generated as hash from that rather than
entirely randomly. This way we can derive the invocation race-freely from the
messages.

1 files changed, 9 insertions, 1 deletions

diff --git a/src/core/unit.h b/src/core/unit.h
index 3584c16d8c..a8dd3e602c 100644
--- a/src/core/unit.h
+++ b/src/core/unit.h
@@ -207,6 +207,10 @@ struct Unit {
         /* How to start OnFailure units */
         JobMode on_failure_job_mode;
 
+        /* The current invocation ID */
+        sd_id128_t invocation_id;
+        char invocation_id_string[SD_ID128_STRING_MAX]; /* useful when logging */
+
         /* Garbage collect us we nobody wants or requires us anymore */
         bool stop_when_unneeded;
 
@@ -546,6 +550,7 @@ bool unit_job_is_applicable(Unit *u, JobType j);
 int set_unit_path(const char *p);
 
 char *unit_dbus_path(Unit *u);
+char *unit_dbus_path_invocation_id(Unit *u);
 
 int unit_load_related_unit(Unit *u, const char *type, Unit **_found);
 
@@ -643,12 +648,15 @@ void unit_unref_uid_gid(Unit *u, bool destroy_now);
 
 void unit_notify_user_lookup(Unit *u, uid_t uid, gid_t gid);
 
+int unit_set_invocation_id(Unit *u, sd_id128_t id);
+int unit_acquire_invocation_id(Unit *u);
+
 /* Macros which append UNIT= or USER_UNIT= to the message */
 
 #define log_unit_full(unit, level, error, ...)                          \
         ({                                                              \
                 const Unit *_u = (unit);                                \
-                _u ? log_object_internal(level, error, __FILE__, __LINE__, __func__, _u->manager->unit_log_field, _u->id, ##__VA_ARGS__) : \
+                _u ? log_object_internal(level, error, __FILE__, __LINE__, __func__, _u->manager->unit_log_field, _u->id, _u->manager->invocation_log_field, _u->invocation_id_string, ##__VA_ARGS__) : \
                         log_internal(level, error, __FILE__, __LINE__, __func__, ##__VA_ARGS__); \
         })