n-dhcp4: add an accessor for the file name in the lease

The name of the boot file can be either in option 67 or in a field of
the DHCP header. Add an accessor for the one from the DHCP header.

(cherry picked from commit 6a1349c5fb)

src/n-dhcp4/src/libndhcp4.sym

@@ -37,6 +37,7 @@ global:
         n_dhcp4_client_lease_get_siaddr;
         n_dhcp4_client_lease_get_lifetime;
         n_dhcp4_client_lease_get_server_identifier;
+        n_dhcp4_client_lease_get_file;
         n_dhcp4_client_lease_query;
         n_dhcp4_client_lease_select;
         n_dhcp4_client_lease_accept;

src/n-dhcp4/src/n-dhcp4-c-lease.c

@@ -269,6 +269,42 @@ _c_public_ int n_dhcp4_client_lease_get_server_identifier(NDhcp4ClientLease *lea
         return 0;
 }
 
+/**
+ * n_dhcp4_client_lease_get_file() - query the lease for the boot file name
+ * @lease:                      the lease to operate on
+ * @file:                       return argument for the file name
+ *
+ * Query the lease for the boot file name from the DHCP header. The file name
+ * is returned as a NULL-terminated string.
+ *
+ * Return: 0 on success,
+ *         N_DHCP4_E_UNSET if the lease does not contain a file name, or
+ *         N_DCHP4_E_INTERNAL if the file name is invalid.
+ */
+_c_public_ int n_dhcp4_client_lease_get_file(NDhcp4ClientLease *lease, const char **file) {
+        NDhcp4Message *message;
+
+        if (lease->message->options[N_DHCP4_OPTION_OVERLOAD].size > 0
+            && ((*lease->message->options[N_DHCP4_OPTION_OVERLOAD].value) & N_DHCP4_OVERLOAD_FILE)) {
+            /* The field is overloaded to contain other options */
+            return N_DHCP4_E_UNSET;
+        }
+
+        message = &lease->message->message;
+
+        if (message->file[0] == '\0')
+            return N_DHCP4_E_UNSET;
+
+        if (!memchr(message->file, '\0', sizeof(message->file))) {
+            /* The field is NULL-terminated (RFC 2131 section 2) */
+            return N_DHCP4_E_INTERNAL;
+        }
+
+        *file = (const char *) message->file;
+
+        return 0;
+}
+
 /**
  * n_dhcp4_client_lease_query() - query the lease for an option
  * @lease:                      the lease to operate on

src/n-dhcp4/src/n-dhcp4.h

@@ -172,6 +172,7 @@ void n_dhcp4_client_lease_get_basetime(NDhcp4ClientLease *lease, uint64_t *ns_ba
 void n_dhcp4_client_lease_get_lifetime(NDhcp4ClientLease *lease, uint64_t *ns_lifetimep);
 int n_dhcp4_client_lease_query(NDhcp4ClientLease *lease, uint8_t option, uint8_t **datap, size_t *n_datap);
 int n_dhcp4_client_lease_get_server_identifier(NDhcp4ClientLease *lease, struct in_addr *addr);
+int n_dhcp4_client_lease_get_file(NDhcp4ClientLease *lease, const char **file);
 
 int n_dhcp4_client_lease_select(NDhcp4ClientLease *lease);
 int n_dhcp4_client_lease_accept(NDhcp4ClientLease *lease);