platform: handle unset address timestamp as *now* in to_string()

When printing an address in nm_platform_ip4_address_to_string()
and nm_platform_ip6_address_to_string() treat an unset @timestamp
as counting from @now.

This is useful, if you just have the remaining lifetime at hand
and want to print an address. In general it is not a good idea to
leave the timestamp not anchored to an absolute @timestamp.

Signed-off-by: Thomas Haller <thaller@redhat.com>

src/platform/nm-platform.c

@@ -1436,6 +1436,7 @@ nm_platform_ip4_address_add (int ifindex,
 		addr.address = address;
 		addr.peer_address = peer_address;
 		addr.plen = plen;
+		addr.timestamp = 0; /* set it at zero, which to_string will treat as *now* */
 		addr.lifetime = lifetime;
 		addr.preferred = preferred;
 		if (label)
@@ -1470,6 +1471,7 @@ nm_platform_ip6_address_add (int ifindex,
 		addr.address = address;
 		addr.peer_address = peer_address;
 		addr.plen = plen;
+		addr.timestamp = 0; /* set it to zero, which to_string will treat as *now* */
 		addr.lifetime = lifetime;
 		addr.preferred = preferred;
 		addr.flags = flags;
@@ -1571,6 +1573,17 @@ _rebase_relative_time_on_now (guint32 timestamp, guint32 duration, guint32 now,
 	if (duration == NM_PLATFORM_LIFETIME_PERMANENT)
 		return NM_PLATFORM_LIFETIME_PERMANENT;
 
+	if (timestamp == 0) {
+		/* if the @timestamp is zero, assume it was just left unset and that the relative
+		 * @duration starts counting from @now. This is convenient to construct an address
+		 * and print it in nm_platform_ip4_address_to_string().
+		 *
+		 * In general it does not make sense to set the @duration without anchoring at
+		 * @timestamp because you don't know the absolute expiration time when looking
+		 * at the address at a later moment. */
+		timestamp = now;
+	}
+
 	/* For timestamp > now, just accept it and calculate the expected(?) result. */
 	t = (gint64) timestamp + (gint64) duration - (gint64) now;
 
@@ -1597,13 +1610,18 @@ _address_get_lifetime (const NMPlatformIPAddress *address, guint32 now, guint32
 		if (!lifetime)
 			return FALSE;
 		preferred = _rebase_relative_time_on_now (address->timestamp, address->preferred, now, padding);
-		if (preferred > lifetime) {
-			g_warn_if_reached ();
-			preferred = lifetime;
-		}
 
 		*out_lifetime = lifetime;
-		*out_preferred = preferred;
+		*out_preferred = MIN (preferred, lifetime);
+
+		/* Assert that non-permanent addresses have a (positive) @timestamp. _rebase_relative_time_on_now()
+		 * treats addresses with timestamp 0 as *now*. Addresses passed to _address_get_lifetime() always
+		 * should have a valid @timestamp, otherwise on every re-sync, their lifetime will be extended anew.
+		 */
+		g_return_val_if_fail (   address->timestamp != 0
+		                      || (   address->lifetime  == NM_PLATFORM_LIFETIME_PERMANENT
+		                          && address->preferred == NM_PLATFORM_LIFETIME_PERMANENT), TRUE);
+		g_return_val_if_fail (preferred <= lifetime, TRUE);
 	}
 	return TRUE;
 }

src/platform/nm-platform.h

@@ -167,11 +167,22 @@ typedef struct {
 	NMPlatformSource source; \
 	\
 	/* Timestamp in seconds in the reference system of nm_utils_get_monotonic_timestamp_*().
-	 * This value is mainly used to anchor the relative lifetime and preferred values.
-	 * For addresses originating from DHCP it might be set to nm_utils_get_monotonic_timestamp_s()
-	 * of when the lease was received. For addresses from platform/kernel it might be set to
-	 * when the address was last modified. For permanent addresses it is mostly set to 0.
-	 * For non-permanent addresses it should not be 0 (because 0 is not a valid timestamp).
+	 *
+	 * The rules are:
+	 * 1 @lifetime==0: @timestamp and @preferred is irrelevant (but mostly set to 0 too). Such addresses
+	 *   are permanent. This rule is so that unset addresses (calloc) are permanent by default.
+	 * 2 @lifetime==@preferred==NM_PLATFORM_LIFETIME_PERMANENT: @timestamp is irrelevant (but mostly
+	 *   set to 0). Such addresses are permanent.
+	 * 3 Non permanent addreses should (almost) always have @timestamp > 0. 0 is not a valid timestamp
+	 *   and never returned by nm_utils_get_monotonic_timestamp_s(). In this case @valid/@preferred
+	 *   is anchored at @timestamp.
+	 * 4 Non permanent addresses with @timestamp == 0 are implicitely anchored at *now*, thus the time
+	 *   moves as time goes by. This is usually not useful, except e.g. nm_platform_ip[46]_address_add().
+	 *
+	 * Non permanent addresses from DHCP/RA might have the @timestamp set to the moment of when the
+	 * lease was received. Addresses from kernel might have the @timestamp based on the last modification
+	 * time of the addresses. But don't rely on this behaviour, the @timestamp is only defined for anchoring
+	 * @lifetime and @preferred.
 	 */ \
 	guint32 timestamp; \
 	guint32 lifetime;   /* seconds since timestamp */ \