selftests/bpf: helpers: Add append_tid()

Some tests can't be run in parallel because they use same namespace
names or veth names.

Create an helper that appends the thread ID to a given string. 8
characters are used for it (7 digits + '\0')

Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250131-redirect-multi-v4-1-970b33678512@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/network_helpers.c

@@ -446,6 +446,23 @@ char *ping_command(int family)
 	return "ping";
 }
 
+int append_tid(char *str, size_t sz)
+{
+	size_t end;
+
+	if (!str)
+		return -1;
+
+	end = strlen(str);
+	if (end + 8 > sz)
+		return -1;
+
+	sprintf(&str[end], "%07d", gettid());
+	str[end + 7] = '\0';
+
+	return 0;
+}
+
 int remove_netns(const char *name)
 {
 	char *cmd;

tools/testing/selftests/bpf/network_helpers.h

@@ -98,6 +98,18 @@ int send_recv_data(int lfd, int fd, uint32_t total_bytes);
 int make_netns(const char *name);
 int remove_netns(const char *name);
 
+/**
+ * append_tid() - Append thread ID to the given string.
+ *
+ * @str: string to extend
+ * @sz: string's size
+ *
+ * 8 characters are used to append the thread ID (7 digits + '\0')
+ *
+ * Returns -1 on errors, 0 otherwise
+ */
+int append_tid(char *str, size_t sz);
+
 static __u16 csum_fold(__u32 csum)
 {
 	csum = (csum & 0xffff) + (csum >> 16);