Add a test for reaper functionality

test/apps/Makefile

@@ -33,6 +33,7 @@ TEST_APPS := \
 	mongoose \
 	network \
 	pipe \
+	prctl \
 	pthread \
 	pty \
 	shm \

test/apps/prctl/Makefile

@@ -0,0 +1,5 @@
+# SPDX-License-Identifier: MPL-2.0
+
+include ../test_common.mk
+
+EXTRA_C_FLAGS := -static -lpthread

test/apps/prctl/subreaper.c

@@ -0,0 +1,101 @@
+// SPDX-License-Identifier: MPL-2.0
+
+#include <sys/prctl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/wait.h>
+#include <errno.h>
+
+void set_child_subreaper()
+{
+	if (prctl(PR_SET_CHILD_SUBREAPER, 1) == -1) {
+		perror("prctl: PR_SET_CHILD_SUBREAPER failed");
+		exit(EXIT_FAILURE);
+	}
+	printf("Process %d is now a child subreaper\n", getpid());
+}
+
+void get_child_subreaper()
+{
+	int subreaper;
+	if (prctl(PR_GET_CHILD_SUBREAPER, &subreaper) == -1) {
+		perror("prctl: PR_GET_CHILD_SUBREAPER failed");
+		exit(EXIT_FAILURE);
+	}
+	printf("Process %d has_child_subreaper: %d\n", getpid(), subreaper);
+}
+
+void print_process_info(const char *name)
+{
+	printf("%s: PID=%d, PPID=%d\n", name, getpid(), getppid());
+}
+
+void child_process()
+{
+	print_process_info("Child process");
+
+	pid_t grandchild_pid = fork();
+	if (grandchild_pid < 0) {
+		perror("fork failed");
+		exit(EXIT_FAILURE);
+	} else if (grandchild_pid == 0) {
+		print_process_info("Grandchild process");
+		sleep(2);
+		printf("Grandchild process %d exiting\n", getpid());
+		exit(EXIT_SUCCESS);
+	} else {
+		sleep(1);
+		printf("Child process %d exiting\n", getpid());
+		exit(EXIT_SUCCESS);
+	}
+}
+
+int main()
+{
+	// Set the current process as the subreaper.
+	set_child_subreaper();
+	get_child_subreaper();
+
+	pid_t child_pid = fork();
+	if (child_pid < 0) {
+		perror("fork failed");
+		exit(EXIT_FAILURE);
+	} else if (child_pid == 0) {
+		child_process();
+	} else {
+		print_process_info("Parent process");
+
+		// Wait for the son process to exit
+		waitpid(child_pid, NULL, 0);
+		printf("Parent process %d: child %d exited\n", getpid(),
+		       child_pid);
+
+		// Wait for the grandson process to exit
+		printf("Parent process %d waiting for grandchild\n", getpid());
+
+		int status;
+		pid_t waited_pid = wait(&status);
+		// The first time successfully waited for the grandchild process to exit.
+		if (waited_pid != -1) {
+			printf("Parent process %d: grandchild %d exited with status %d\n",
+			       getpid(), waited_pid, WEXITSTATUS(status));
+		} else {
+			exit(EXIT_FAILURE);
+		}
+
+		waited_pid = wait(&status);
+		// The second time there were no processes left to wait for.
+		if (waited_pid == -1) {
+			if (errno == ECHILD) {
+				printf("Parent process %d: no more children to wait for\n",
+				       getpid());
+			} else {
+				perror("wait failed");
+				exit(EXIT_FAILURE);
+			}
+		}
+	}
+
+	return 0;
+}