[3234] Changes after review:

 - ChangeLog updated
 - Current reconfiguration limitations described in User's Guide
 - Dhcp{4,6}ParserTest.reconfigureRemoveSubnet added
 - Subnet::getNextID -> generateNextID
 - Various comments improved

ChangeLog

@@ -1,5 +1,5 @@
 7XX.	[bug]		tomek
-	b10-dhcp4,b10-dhcp6: Both servers used to unnecessarily increase
+	b10-dhcp4, b10-dhcp6: Both servers used to unnecessarily increase
 	subnet-id values after reconfiguration. The subnet-ids are now reset
 	to 1 every time a server is reconfigured.
 	(Trac #3234, git abcd)

doc/guide/bind10-guide.xml

@@ -4476,6 +4476,21 @@ Dhcp4/subnet4	[]	list	(default)
       development and should be treated as <quote>not implemented
       yet</quote>, rather than actual limitations.</para>
       <itemizedlist>
+          <listitem> <!-- see tickets #3234, #3281 -->
+            <para>
+              On-line configuration has some limitations. Adding new subnets or
+              modifying existing ones work, as is removing the last subnet from
+              the list. However, removing non-last (e.g. removing subnet 1,2 or 3 if
+              there are 4 subnets configured) will cause issues. The problem is
+              caused by simplistic subnet-id assignment. The subnets are always
+              numbered, starting from 1. That subnet-id is then used in leases
+              that are stored in the lease database. Removing non-last subnet will
+              cause the configuration information to mismatch data in the lease
+              database. It is possible to manually update subnet-id fields in
+              MySQL database, but it is awkward and error prone process. A better
+              reconfiguration support is planned.
+            </para>
+          </listitem>
           <listitem>
           <para>
             On startup, the DHCPv4 server does not get the full configuration from
@@ -5389,6 +5404,22 @@ should include options from the isc option space:
       yet</quote>, rather than actual limitations.</para>
       <itemizedlist>
 
+          <listitem> <!-- see tickets #3234, #3281 -->
+            <para>
+              On-line configuration has some limitations. Adding new subnets or
+              modifying existing ones work, as is removing the last subnet from
+              the list. However, removing non-last (e.g. removing subnet 1,2 or 3 if
+              there are 4 subnets configured) will cause issues. The problem is
+              caused by simplistic subnet-id assignment. The subnets are always
+              numbered, starting from 1. That subnet-id is then used in leases
+              that are stored in the lease database. Removing non-last subnet will
+              cause the configuration information to mismatch data in the lease
+              database. It is possible to manually update subnet-id fields in
+              MySQL database, but it is awkward and error prone process. A better
+              reconfiguration support is planned.
+            </para>
+          </listitem>
+
         <listitem>
           <para>
             On startup, the DHCPv6 server does not get the full configuration from

src/bin/dhcp4/tests/config_parser_unittest.cc

@@ -475,6 +475,129 @@ TEST_F(Dhcp4ParserTest, multipleSubnets) {
     } while (++cnt < 10);
 }
 
+// Goal of this test is to verify that a previously configured subnet can be
+// deleted in subsequent reconfiguration.
+TEST_F(Dhcp4ParserTest, reconfigureRemoveSubnet) {
+    ConstElementPtr x;
+
+    // All four subnets
+    string config4 = "{ \"interfaces\": [ \"*\" ],"
+        "\"rebind-timer\": 2000, "
+        "\"renew-timer\": 1000, "
+        "\"subnet4\": [ { "
+        "    \"pool\": [ \"192.0.2.1 - 192.0.2.100\" ],"
+        "    \"subnet\": \"192.0.2.0/24\" "
+        " },"
+        " {"
+        "    \"pool\": [ \"192.0.3.101 - 192.0.3.150\" ],"
+        "    \"subnet\": \"192.0.3.0/24\" "
+        " },"
+        " {"
+        "    \"pool\": [ \"192.0.4.101 - 192.0.4.150\" ],"
+        "    \"subnet\": \"192.0.4.0/24\" "
+        " },"
+        " {"
+        "    \"pool\": [ \"192.0.5.101 - 192.0.5.150\" ],"
+        "    \"subnet\": \"192.0.5.0/24\" "
+        " } ],"
+        "\"valid-lifetime\": 4000 }";
+
+    // Three subnets (the last one removed)
+    string config_first3 = "{ \"interfaces\": [ \"*\" ],"
+        "\"rebind-timer\": 2000, "
+        "\"renew-timer\": 1000, "
+        "\"subnet4\": [ { "
+        "    \"pool\": [ \"192.0.2.1 - 192.0.2.100\" ],"
+        "    \"subnet\": \"192.0.2.0/24\" "
+        " },"
+        " {"
+        "    \"pool\": [ \"192.0.3.101 - 192.0.3.150\" ],"
+        "    \"subnet\": \"192.0.3.0/24\" "
+        " },"
+        " {"
+        "    \"pool\": [ \"192.0.4.101 - 192.0.4.150\" ],"
+        "    \"subnet\": \"192.0.4.0/24\" "
+        " } ],"
+        "\"valid-lifetime\": 4000 }";
+
+    // Second subnet removed
+    string config_second_removed = "{ \"interfaces\": [ \"*\" ],"
+        "\"rebind-timer\": 2000, "
+        "\"renew-timer\": 1000, "
+        "\"subnet4\": [ { "
+        "    \"pool\": [ \"192.0.2.1 - 192.0.2.100\" ],"
+        "    \"subnet\": \"192.0.2.0/24\" "
+        " },"
+        " {"
+        "    \"pool\": [ \"192.0.4.101 - 192.0.4.150\" ],"
+        "    \"subnet\": \"192.0.4.0/24\" "
+        " },"
+        " {"
+        "    \"pool\": [ \"192.0.5.101 - 192.0.5.150\" ],"
+        "    \"subnet\": \"192.0.5.0/24\" "
+        " } ],"
+        "\"valid-lifetime\": 4000 }";
+
+    // CASE 1: Configure 4 subnets, then reconfigure and remove the
+    // last one.
+
+    ElementPtr json = Element::fromJSON(config4);
+    EXPECT_NO_THROW(x = configureDhcp4Server(*srv_, json));
+    ASSERT_TRUE(x);
+    comment_ = parseAnswer(rcode_, x);
+    ASSERT_EQ(0, rcode_);
+
+    const Subnet4Collection* subnets = CfgMgr::instance().getSubnets4();
+    ASSERT_TRUE(subnets);
+    ASSERT_EQ(4, subnets->size()); // We expect 4 subnets
+
+    // Do the reconfiguration (the last subnet is removed)
+    json = Element::fromJSON(config_first3);
+    EXPECT_NO_THROW(x = configureDhcp4Server(*srv_, json));
+    ASSERT_TRUE(x);
+    comment_ = parseAnswer(rcode_, x);
+    ASSERT_EQ(0, rcode_);
+
+    subnets = CfgMgr::instance().getSubnets4();
+    ASSERT_TRUE(subnets);
+    ASSERT_EQ(3, subnets->size()); // We expect 3 subnets now (4th is removed)
+
+    // Check subnet-ids of each subnet (it should be monotonously increasing)
+    EXPECT_EQ(1, subnets->at(0)->getID());
+    EXPECT_EQ(2, subnets->at(1)->getID());
+    EXPECT_EQ(3, subnets->at(2)->getID());
+
+    /// CASE 2: Configure 4 subnets, then reconfigure and remove one
+    /// from in between (not first, not last)
+
+#if 0
+    /// @todo: Uncomment subnet removal test as part of #3281.
+    json = Element::fromJSON(config4);
+    EXPECT_NO_THROW(x = configureDhcp4Server(*srv_, json));
+    ASSERT_TRUE(x);
+    comment_ = parseAnswer(rcode_, x);
+    ASSERT_EQ(0, rcode_);
+
+    // Do reconfiguration
+    json = Element::fromJSON(config_second_removed);
+    EXPECT_NO_THROW(x = configureDhcp4Server(*srv_, json));
+    ASSERT_TRUE(x);
+    comment_ = parseAnswer(rcode_, x);
+    ASSERT_EQ(0, rcode_);
+
+    subnets = CfgMgr::instance().getSubnets4();
+    ASSERT_TRUE(subnets);
+    ASSERT_EQ(3, subnets->size()); // We expect 4 subnets
+
+    EXPECT_EQ(1, subnets->at(0)->getID());
+    // The second subnet (with subnet-id = 2) is no longer there
+    EXPECT_EQ(3, subnets->at(1)->getID());
+    EXPECT_EQ(4, subnets->at(2)->getID());
+#endif
+
+}
+
+/// @todo: implement subnet removal test as part of #3281.
 
 // Checks if the next-server defined as global parameter is taken into
 // consideration.

src/bin/dhcp6/tests/config_parser_unittest.cc

@@ -495,6 +495,131 @@ TEST_F(Dhcp6ParserTest, multipleSubnets) {
     } while (++cnt < 10);
 }
 
+// Goal of this test is to verify that a previously configured subnet can be
+// deleted in subsequent reconfiguration.
+TEST_F(Dhcp6ParserTest, reconfigureRemoveSubnet) {
+    ConstElementPtr x;
+
+    // All four subnets
+    string config4 = "{ \"interfaces\": [ \"*\" ],"
+        "\"preferred-lifetime\": 3000,"
+        "\"rebind-timer\": 2000, "
+        "\"renew-timer\": 1000, "
+        "\"subnet6\": [ { "
+        "    \"pool\": [ \"2001:db8:1::/80\" ],"
+        "    \"subnet\": \"2001:db8:1::/64\" "
+        " },"
+        " {"
+        "    \"pool\": [ \"2001:db8:2::/80\" ],"
+        "    \"subnet\": \"2001:db8:2::/64\" "
+        " },"
+        " {"
+        "    \"pool\": [ \"2001:db8:3::/80\" ],"
+        "    \"subnet\": \"2001:db8:3::/64\" "
+        " },"
+        " {"
+        "    \"pool\": [ \"2001:db8:4::/80\" ],"
+        "    \"subnet\": \"2001:db8:4::/64\" "
+        " } ],"
+        "\"valid-lifetime\": 4000 }";
+
+    // Three subnets (the last one removed)
+    string config_first3 = "{ \"interfaces\": [ \"*\" ],"
+        "\"preferred-lifetime\": 3000,"
+        "\"rebind-timer\": 2000, "
+        "\"renew-timer\": 1000, "
+        "\"subnet6\": [ { "
+        "    \"pool\": [ \"2001:db8:1::/80\" ],"
+        "    \"subnet\": \"2001:db8:1::/64\" "
+        " },"
+        " {"
+        "    \"pool\": [ \"2001:db8:2::/80\" ],"
+        "    \"subnet\": \"2001:db8:2::/64\" "
+        " },"
+        " {"
+        "    \"pool\": [ \"2001:db8:3::/80\" ],"
+        "    \"subnet\": \"2001:db8:3::/64\" "
+        " } ],"
+        "\"valid-lifetime\": 4000 }";
+
+    // Second subnet removed
+    string config_second_removed = "{ \"interfaces\": [ \"*\" ],"
+        "\"preferred-lifetime\": 3000,"
+        "\"rebind-timer\": 2000, "
+        "\"renew-timer\": 1000, "
+        "\"subnet6\": [ { "
+        "    \"pool\": [ \"2001:db8:1::/80\" ],"
+        "    \"subnet\": \"2001:db8:1::/64\" "
+        " },"
+        " {"
+        "    \"pool\": [ \"2001:db8:3::/80\" ],"
+        "    \"subnet\": \"2001:db8:3::/64\" "
+        " },"
+        " {"
+        "    \"pool\": [ \"2001:db8:4::/80\" ],"
+        "    \"subnet\": \"2001:db8:4::/64\" "
+        " } ],"
+        "\"valid-lifetime\": 4000 }";
+
+    // CASE 1: Configure 4 subnets, then reconfigure and remove the
+    // last one.
+
+    ElementPtr json = Element::fromJSON(config4);
+    EXPECT_NO_THROW(x = configureDhcp6Server(srv_, json));
+    ASSERT_TRUE(x);
+    comment_ = parseAnswer(rcode_, x);
+    ASSERT_EQ(0, rcode_);
+
+    const Subnet6Collection* subnets = CfgMgr::instance().getSubnets6();
+    ASSERT_TRUE(subnets);
+    ASSERT_EQ(4, subnets->size()); // We expect 4 subnets
+
+    // Do the reconfiguration (the last subnet is removed)
+    json = Element::fromJSON(config_first3);
+    EXPECT_NO_THROW(x = configureDhcp6Server(srv_, json));
+    ASSERT_TRUE(x);
+    comment_ = parseAnswer(rcode_, x);
+    ASSERT_EQ(0, rcode_);
+
+    subnets = CfgMgr::instance().getSubnets6();
+    ASSERT_TRUE(subnets);
+    ASSERT_EQ(3, subnets->size()); // We expect 3 subnets now (4th is removed)
+
+    EXPECT_EQ(1, subnets->at(0)->getID());
+    EXPECT_EQ(2, subnets->at(1)->getID());
+    EXPECT_EQ(3, subnets->at(2)->getID());
+
+    /// CASE 2: Configure 4 subnets, then reconfigure and remove one
+    /// from in between (not first, not last)
+
+#if 0
+    /// @todo: Uncomment subnet removal test as part of #3281.
+    json = Element::fromJSON(config4);
+    EXPECT_NO_THROW(x = configureDhcp6Server(srv_, json));
+    ASSERT_TRUE(x);
+    comment_ = parseAnswer(rcode_, x);
+    ASSERT_EQ(0, rcode_);
+
+    // Do reconfiguration
+    json = Element::fromJSON(config_second_removed);
+    EXPECT_NO_THROW(x = configureDhcp6Server(srv_, json));
+    ASSERT_TRUE(x);
+    comment_ = parseAnswer(rcode_, x);
+    ASSERT_EQ(0, rcode_);
+
+    subnets = CfgMgr::instance().getSubnets6();
+    ASSERT_TRUE(subnets);
+    ASSERT_EQ(3, subnets->size()); // We expect 4 subnets
+
+    EXPECT_EQ(1, subnets->at(0)->getID());
+    // The second subnet (with subnet-id = 2) is no longer there
+    EXPECT_EQ(3, subnets->at(1)->getID());
+    EXPECT_EQ(4, subnets->at(2)->getID());
+#endif
+}
+
+
+
 // This test checks if it is possible to override global values
 // on a per subnet basis.
 TEST_F(Dhcp6ParserTest, subnetLocal) {

src/lib/dhcpsrv/subnet.cc

@@ -31,7 +31,7 @@ Subnet::Subnet(const isc::asiolink::IOAddress& prefix, uint8_t len,
                const Triplet<uint32_t>& t1,
                const Triplet<uint32_t>& t2,
                const Triplet<uint32_t>& valid_lifetime)
-    :id_(getNextID()), prefix_(prefix), prefix_len_(len), t1_(t1),
+    :id_(generateNextID()), prefix_(prefix), prefix_len_(len), t1_(t1),
      t2_(t2), valid_(valid_lifetime),
      last_allocated_ia_(lastAddrInPrefix(prefix, len)),
      last_allocated_ta_(lastAddrInPrefix(prefix, len)),

src/lib/dhcpsrv/subnet.h

@@ -387,7 +387,12 @@ protected:
     /// @brief Protected constructor
     //
     /// By making the constructor protected, we make sure that noone will
-    /// ever instantiate that class. Pool4 and Pool6 should be used instead.
+    /// ever instantiate that class. Subnet4 and Subnet6 should be used instead.
+    ///
+    /// This constructor assigns a new subnet-id (see @ref generateNextID).
+    /// This subnet-id has unique value that is strictly monotonously increasing
+    /// for each subnet, until it is explicitly reset back to 1 during
+    /// reconfiguration process.
     Subnet(const isc::asiolink::IOAddress& prefix, uint8_t len,
            const Triplet<uint32_t>& t1,
            const Triplet<uint32_t>& t2,
@@ -409,8 +414,13 @@ protected:
 
     /// @brief returns the next unique Subnet-ID
     ///
+    /// This method generates and returns the next unique subnet-id.
+    /// It is a strictly monotonously increasing value (1,2,3,...) for
+    /// each new Subnet object created. It can be explicitly reset
+    /// back to 1 during reconfiguration (@ref resetSubnetID).
+    ///
     /// @return the next unique Subnet-ID
-    static SubnetID getNextID() {
+    static SubnetID generateNextID() {
         return (static_id_++);
     }
 
@@ -511,6 +521,8 @@ public:
 
     /// @brief Constructor with all parameters
     ///
+    /// This constructor calls Subnet::Subnet, where subnet-id is generated.
+    ///
     /// @param prefix Subnet4 prefix
     /// @param length prefix length
     /// @param t1 renewal timer (in seconds)
@@ -575,6 +587,8 @@ public:
 
     /// @brief Constructor with all parameters
     ///
+    /// This constructor calls Subnet::Subnet, where subnet-id is generated.
+    ///
     /// @param prefix Subnet6 prefix
     /// @param length prefix length
     /// @param t1 renewal timer (in seconds)