cgroup: fix various issues with compatibility with older bst spacetimes

cgroup.c

@@ -14,12 +14,14 @@
 #include "path.h"
 #include "util.h"
 
+extern const struct cgroup_driver_funcs cgroup_driver_none;
 extern const struct cgroup_driver_funcs cgroup_driver_native;
 #ifdef HAVE_SYSTEMD
 extern const struct cgroup_driver_funcs cgroup_driver_systemd;
 #endif
 
 static const struct cgroup_driver_funcs *cgroup_drivers[] = {
+	[CGROUP_DRIVER_NONE]   = &cgroup_driver_none,
 	[CGROUP_DRIVER_NATIVE] = &cgroup_driver_native,
 #ifdef HAVE_SYSTEMD
 	[CGROUP_DRIVER_SYSTEMD] = &cgroup_driver_systemd,
@@ -36,7 +38,11 @@ int cgroup_driver_init(enum cgroup_driver driver, bool fatal)
 		if (cgroup_detected_driver < 0 || cgroup_detected_driver >= lengthof(cgroup_drivers)) {
 			errx(1, "unknown cgroup driver ID %d", cgroup_detected_driver);
 		}
-		return cgroup_drivers[cgroup_detected_driver]->init(fatal);
+		int rc = cgroup_drivers[cgroup_detected_driver]->init(fatal);
+		if (rc < 0 && fatal) {
+			errx(1, "cgroup_driver_init: cgroup driver failed to initialize");
+		}
+		return rc;
 	}
 
 	static enum cgroup_driver attempts[] = {
@@ -233,3 +239,24 @@ void cgroup_enable_controllers(int cgroupfd)
 		err(1, "cgroup_enable_controllers: close cgroup.subtree_control");
 	}
 }
+
+static int cgroup_none_driver_init(bool fatal)
+{
+	return -1;
+}
+
+static bool cgroup_none_current_path(char *path)
+{
+	return false;
+}
+
+static int cgroup_none_join_cgroup(const char *parent, const char *name)
+{
+	return -1;
+}
+
+const struct cgroup_driver_funcs cgroup_driver_none = {
+	.init         = cgroup_none_driver_init,
+	.join_cgroup  = cgroup_none_join_cgroup,
+	.current_path = cgroup_none_current_path,
+};

cgroup.h

@@ -4,14 +4,15 @@
 # include <stdbool.h>
 
 struct cgroup_driver_funcs {
-    int (*init)(bool fatal);
-    int (*join_cgroup)(const char *parent, const char *name);
-    bool (*current_path)(char *out);
+	int (*init)(bool fatal);
+	int (*join_cgroup)(const char *parent, const char *name);
+	bool (*current_path)(char *out);
 };
 
 enum cgroup_driver {
-    CGROUP_DRIVER_NATIVE,
-    CGROUP_DRIVER_SYSTEMD,
+	CGROUP_DRIVER_NONE,
+	CGROUP_DRIVER_NATIVE,
+	CGROUP_DRIVER_SYSTEMD,
 };
 
 int cgroup_driver_init(enum cgroup_driver driver, bool fatal);

cgroup_native.c

@@ -15,9 +15,21 @@ static int cgroup_native_driver_init(bool fatal)
 	if (!cgroup_read_current(NULL)) {
 		return -1;
 	}
-	if (access("/sys/fs/cgroup/cgroup.controllers", F_OK) == -1) {
+
+	/* Attempting to open /sys/fs/cgroup/cgroup.procs privileged checks two
+	   things: first, that the cgroup hierarchy is v2 by checking that the
+	   file exists; and second, that the mounted cgroup hierarchy can be
+	   operated on, which might not be the case if bst was left in its
+	   original cgroup. */
+	make_capable(BST_CAP_DAC_OVERRIDE);
+	int fd = open("/sys/fs/cgroup/cgroup.procs", O_WRONLY, 0);
+	reset_capabilities();
+
+	if (fd == -1) {
 		return -1;
 	}
+	close(fd);
+
 	return 0;
 }
 

cgroup_systemd.c

@@ -224,8 +224,9 @@ static int cgroup_systemd_join_cgroup(const char *parent, const char *name)
 	ok = ok && dbus_message_iter_open_container(&iter, DBUS_TYPE_ARRAY, "(sv)", &props);
 
 	ok = ok && bus_message_append(&props, "(sv)(sv)(sv)(sv)",
-			"Description", "s", "/usr/bin/true",
+			"Description", "s", "bst",
 			"Delegate", "b", 1, /* Delegate all cgroup controllers to us */
+			"CollectMode", "s", "inactive-or-failed", /* Make sure failed invocations are garbage-collected */
 			"Slice", "s", parent,
 			"PIDs", "au", 1, (uint32_t)getpid());
 

main.c

@@ -663,7 +663,9 @@ int main(int argc, char *argv[], char *envp[])
 				break;
 
 			case OPTION_CGROUP_DRIVER:
-				if (strcmp(optarg, "native") == 0) {
+				if (strcmp(optarg, "none") == 0) {
+					opts.cgroup_driver = CGROUP_DRIVER_NONE;
+				} else if (strcmp(optarg, "native") == 0) {
 					opts.cgroup_driver = CGROUP_DRIVER_NATIVE;
 #ifdef HAVE_SYSTEMD
 				} else if (strcmp(optarg, "systemd") == 0) {

man/bst.1.scd

@@ -235,7 +235,7 @@ spacetime process.
 \--cgroup-driver <driver>
 	Specify the cgroup driver to use.
 
-	Valid values are _native_, or _systemd_.
+	Valid values are _native_, _systemd_, or _none_.
 
 	The _native_ driver manages and cleans up cgroups directly, with no
 	intermediary. It is appropriate to use in situations where nothing is
@@ -247,6 +247,10 @@ spacetime process.
 	without informing systemd on these systems causes bst to step on systemd's
 	toes, and vice-versa.
 
+	The _none_ driver disables cgroup support -- *bst* will leave the child
+	process in the parent control group. Using this driver is not recommended
+	for typical use, but has worth in testing or maintaining compatibility.
+
 	By default, *bst* will attempt to use the _systemd_ driver before falling
 	back to the _native_ driver.