core: open more fields up to StartTransientUnit via Varlink (#43516)

This is split out of #43058 but makes a ton of sense on its own.

src/shared/exec-util.c

@@ -519,6 +519,16 @@ ExecCommandFlags exec_command_flags_from_string(const char *s) {
         return 1U << idx;
 }
 
+char* exec_command_flags_to_exec_chars(ExecCommandFlags flags) {
+        /* Returns the prefix characters that encode the flags in unit file ExecStart= lines, i.e. the
+         * inverse of the prefix parsing in load-fragment.c */
+        return strjoin(FLAGS_SET(flags, EXEC_COMMAND_IGNORE_FAILURE)   ? "-" : "",
+                       FLAGS_SET(flags, EXEC_COMMAND_NO_ENV_EXPAND)    ? ":" : "",
+                       FLAGS_SET(flags, EXEC_COMMAND_FULLY_PRIVILEGED) ? "+" : "",
+                       FLAGS_SET(flags, EXEC_COMMAND_NO_SETUID)        ? "!" : "",
+                       FLAGS_SET(flags, EXEC_COMMAND_VIA_SHELL)        ? "|" : "");
+}
+
 int fexecve_or_execve(int executable_fd, const char *executable, char *const argv[], char *const envp[]) {
         /* Refuse invalid fds, regardless if fexecve() use is enabled or not */
         if (executable_fd < 0)

src/shared/exec-util.h

@@ -55,6 +55,7 @@ typedef enum ExecCommandFlags {
 
 int exec_command_flags_from_strv(char * const *ex_opts, ExecCommandFlags *ret);
 int exec_command_flags_to_strv(ExecCommandFlags flags, char ***ret);
+char* exec_command_flags_to_exec_chars(ExecCommandFlags flags);
 
 DECLARE_STRING_TABLE_LOOKUP(exec_command_flags, ExecCommandFlags);
 

src/shared/varlink-io.systemd.Unit.c

@@ -1569,11 +1569,11 @@ static SD_VARLINK_DEFINE_STRUCT_TYPE(
                 SD_VARLINK_FIELD_COMMENT("The parameter passed to the condition"),
                 SD_VARLINK_DEFINE_FIELD(parameter, SD_VARLINK_STRING, SD_VARLINK_NULLABLE));
 
-/* UnitContext is used both as input to StartTransient (subset settable at creation time: ID,
- * Description, CollectMode, Service, and the Exec subset {WorkingDirectory, Environment, SetCredential,
- * SetCredentialEncrypted}) and as output from List/StartTransient (full unit configuration). Fields
- * that are not settable at creation time are rejected with PropertyNotSupported when supplied as
- * input. */
+/* UnitContext is used both as input to StartTransient (subset settable at creation time: ID, Description,
+ * CollectMode, Service, Kill (currently KillMode and SendSIGHUP only), and the Exec subset listed in
+ * exec_properties[] in src/core/varlink-unit.c) and as output from List/StartTransient (full unit
+ * configuration). Fields that are not settable at creation time are rejected with PropertyNotSupported when
+ * supplied as input. */
 static SD_VARLINK_DEFINE_STRUCT_TYPE(
                 UnitContext,
                 SD_VARLINK_FIELD_COMMENT("The unit type"),