core: rework how transient unit files and property drop-ins work

With this change the logic for placing transient unit files and drop-ins
generated via "systemctl set-property" is reworked.

The latter are now placed in the newly introduced "control" unit file
directory. The fomer are now placed in the "transient" unit file directory.

Note that the properties originally set when a transient unit was created will
be written to and stay in the transient unit file directory, while later
changes are done via drop-ins.

This is preparation for a later "systemctl revert" addition, where existing
drop-ins are flushed out, but the original transient definition is restored.

4 files changed, 72 insertions, 18 deletions

diff --git a/src/core/scope.c b/src/core/scope.c
index 92a3aed331..7078d1f7e9 100644
--- a/src/core/scope.c
+++ b/src/core/scope.c
@@ -155,25 +155,26 @@ static int scope_load(Unit *u) {
         assert(u->load_state == UNIT_STUB);
 
         if (!u->transient && !MANAGER_IS_RELOADING(u->manager))
+                /* Refuse to load non-transient scope units, but allow them while reloading. */
                 return -ENOENT;
 
-        u->load_state = UNIT_LOADED;
-
-        r = unit_load_dropin(u);
+        r = unit_load_fragment_and_dropin_optional(u);
         if (r < 0)
                 return r;
 
-        r = unit_patch_contexts(u);
-        if (r < 0)
-                return r;
+        if (u->load_state == UNIT_LOADED) {
+                r = unit_patch_contexts(u);
+                if (r < 0)
+                        return r;
 
-        r = unit_set_default_slice(u);
-        if (r < 0)
-                return r;
+                r = unit_set_default_slice(u);
+                if (r < 0)
+                        return r;
 
-        r = scope_add_default_dependencies(s);
-        if (r < 0)
-                return r;
+                r = scope_add_default_dependencies(s);
+                if (r < 0)
+                        return r;
+        }
 
         return scope_verify(s);
 }
diff --git a/src/core/slice.c b/src/core/slice.c
index 667f61bde5..63a77c9bca 100644
--- a/src/core/slice.c
+++ b/src/core/slice.c
@@ -135,6 +135,7 @@ static int slice_load(Unit *u) {
         int r;
 
         assert(s);
+        assert(u->load_state == UNIT_STUB);
 
         r = unit_load_fragment_and_dropin_optional(u);
         if (r < 0)
diff --git a/src/core/unit.c b/src/core/unit.c
index ac29353299..c028f57f13 100644
--- a/src/core/unit.c
+++ b/src/core/unit.c
@@ -52,6 +52,7 @@
 #include "stdio-util.h"
 #include "string-util.h"
 #include "strv.h"
+#include "umask-util.h"
 #include "unit-name.h"
 #include "unit.h"
 #include "user-util.h"
@@ -492,6 +493,9 @@ void unit_free(Unit *u) {
 
         assert(u);
 
+        if (u->transient_file)
+                fclose(u->transient_file);
+
         if (!MANAGER_IS_RELOADING(u->manager))
                 unit_remove_transient(u);
 
@@ -1231,6 +1235,15 @@ int unit_load(Unit *u) {
         if (u->load_state != UNIT_STUB)
                 return 0;
 
+        if (u->transient_file) {
+                r = fflush_and_check(u->transient_file);
+                if (r < 0)
+                        goto fail;
+
+                fclose(u->transient_file);
+                u->transient_file = NULL;
+        }
+
         if (UNIT_VTABLE(u)->load) {
                 r = UNIT_VTABLE(u)->load(u);
                 if (r < 0)
@@ -3327,14 +3340,17 @@ ExecRuntime *unit_get_exec_runtime(Unit *u) {
 static const char* unit_drop_in_dir(Unit *u, UnitSetPropertiesMode mode) {
         assert(u);
 
+        if (!IN_SET(mode, UNIT_RUNTIME, UNIT_PERSISTENT))
+                return NULL;
+
         if (u->transient) /* Redirect drop-ins for transient units always into the transient directory. */
                 return u->manager->lookup_paths.transient;
 
         if (mode == UNIT_RUNTIME)
-                return u->manager->lookup_paths.runtime_config;
+                return u->manager->lookup_paths.runtime_control;
 
         if (mode == UNIT_PERSISTENT)
-                return u->manager->lookup_paths.persistent_config;
+                return u->manager->lookup_paths.persistent_control;
 
         return NULL;
 }
@@ -3346,6 +3362,13 @@ int unit_write_drop_in(Unit *u, UnitSetPropertiesMode mode, const char *name, co
 
         assert(u);
 
+        if (u->transient_file) {
+                /* When this is a transient unit file in creation, then let's not create a new drop-in but instead
+                 * write to the transient unit file. */
+                fputs(data, u->transient_file);
+                return 0;
+        }
+
         if (!IN_SET(mode, UNIT_PERSISTENT, UNIT_RUNTIME))
                 return 0;
 
@@ -3435,24 +3458,50 @@ int unit_write_drop_in_private_format(Unit *u, UnitSetPropertiesMode mode, const
 }
 
 int unit_make_transient(Unit *u) {
+        FILE *f;
+        char *path;
+
         assert(u);
 
         if (!UNIT_VTABLE(u)->can_transient)
                 return -EOPNOTSUPP;
 
-        u->load_state = UNIT_STUB;
-        u->load_error = 0;
-        u->transient = true;
+        path = strjoin(u->manager->lookup_paths.transient, "/", u->id, NULL);
+        if (!path)
+                return -ENOMEM;
+
+        /* Let's open the file we'll write the transient settings into. This file is kept open as long as we are
+         * creating the transient, and is closed in unit_load(), as soon as we start loading the file. */
+
+        RUN_WITH_UMASK(0022)
+                f = fopen(path, "we");
+        if (!f) {
+                free(path);
+                return -errno;
+        }
+
+        if (u->transient_file)
+                fclose(u->transient_file);
+        u->transient_file = f;
+
+        free(u->fragment_path);
+        u->fragment_path = path;
 
-        u->fragment_path = mfree(u->fragment_path);
         u->source_path = mfree(u->source_path);
         u->dropin_paths = strv_free(u->dropin_paths);
         u->fragment_mtime = u->source_mtime = u->dropin_mtime = 0;
 
+        u->load_state = UNIT_STUB;
+        u->load_error = 0;
+        u->transient = true;
+
         unit_add_to_dbus_queue(u);
         unit_add_to_gc_queue(u);
         unit_add_to_load_queue(u);
 
+        fputs("# This is a transient unit file, created programmatically via the systemd API. Do not edit.\n",
+              u->transient_file);
+
         return 0;
 }
 
diff --git a/src/core/unit.h b/src/core/unit.h
index 601e763ce2..cfdac852a5 100644
--- a/src/core/unit.h
+++ b/src/core/unit.h
@@ -95,6 +95,9 @@ struct Unit {
         usec_t source_mtime;
         usec_t dropin_mtime;
 
+        /* If this is a transient unit we are currently writing, this is where we are writing it to */
+        FILE *transient_file;
+
         /* If there is something to do with this unit, then this is the installed job for it */
         Job *job;