[srp-server] allow service instance label with dot character (#9198)

This commit updates the `Srp::Server` class to correctly handle the
case where a service is registered with a dot character in its
service instance name. The first label in a service instance name is
intended as a user-friendly name and can contain dot characters
(it has fewer restrictions than other labels in a DNS name).

In particular, this commit contains the following changes:
- The `PtrRecords::ReadPtrName()` method is used in `Srp::Server` to
  read and validate the first label and the rest of the labels
  separately. This also validates the format of the parsed PTR
  record.
- The `Service::Description` class now remembers the instance label in
  addition to the full instance name. This allows the instance label
  to be easily retrieved.
- The `Dns::Name::ReadName()` method is updated to only verify that
  the labels after the first label do not contain any dot characters.
  This allows it to be used to read instance service names.
- The tests are updated to validate the behavior of the SRP server
  when the instance label contains a dot character.

tests/unit/test_dns.cpp

@@ -478,6 +478,7 @@ void TestDnsCompressedName(void)
     static const char kExpectedReadName1[] = "F.ISI.ARPA.";
     static const char kExpectedReadName2[] = "FOO.F.ISI.ARPA.";
     static const char kExpectedReadName3[] = "ISI.ARPA.";
+    static const char kExpectedReadName4[] = "Human.Readable.F.ISI.ARPA.";
 
     static const char kBadName[] = "bad.name";
 
@@ -745,10 +746,12 @@ void TestDnsCompressedName(void)
         VerifyOrQuit(labelLength == strlen(label), "Name::ReadLabel() returned incorrect label length");
     }
 
-    // `ReadName()` for name-4 should fails due to first label containing dot char.
+    // `ReadName()` for name-4 should still succeed since only the first label contains dot char
     offset = name4Offset;
-    VerifyOrQuit(Dns::Name::ReadName(*message, offset, name, sizeof(name)) == kErrorParse,
-                 "Name::ReadName() did not fail with invalid label");
+    SuccessOrQuit(Dns::Name::ReadName(*message, offset, name, sizeof(name)));
+    printf("Read name =\"%s\"\n", name);
+    VerifyOrQuit(strcmp(name, kExpectedReadName4) == 0, "Name::ReadName() did not return expected name");
+    VerifyOrQuit(offset == name4Offset + kName4EncodedSize, "Name::ParseName() returned incorrect offset");
 
     offset = name4Offset;
 
@@ -826,9 +829,9 @@ void TestHeaderAndResourceRecords(void)
     const char    kServiceLabels[]  = "_service._udp";
     const char    kServiceName[]    = "_service._udp.example.com.";
     const char    kInstance1Label[] = "inst1";
-    const char    kInstance2Label[] = "instance2";
+    const char    kInstance2Label[] = "instance.2"; // Instance label includes dot '.' character.
     const char    kInstance1Name[]  = "inst1._service._udp.example.com.";
-    const char    kInstance2Name[]  = "instance2._service._udp.example.com.";
+    const char    kInstance2Name[]  = "instance.2._service._udp.example.com.";
     const char    kHostName[]       = "host.example.com.";
     const uint8_t kTxtData[]        = {9, 'k', 'e', 'y', '=', 'v', 'a', 'l', 'u', 'e', 0};
     const char    kHostAddress[]    = "fd00::abcd:";
@@ -988,16 +991,18 @@ void TestHeaderAndResourceRecords(void)
 
     VerifyOrQuit(offset == answerSectionOffset, "answer section offset is incorrect");
 
-    for (const char *instanceName : kInstanceNames)
+    for (const char *instanceLabel : kInstanceLabels)
     {
         SuccessOrQuit(Dns::Name::CompareName(*message, offset, kServiceName));
         SuccessOrQuit(Dns::ResourceRecord::ReadRecord(*message, offset, ptrRecord));
         VerifyOrQuit(ptrRecord.GetTtl() == kTtl, "Read PTR is incorrect");
 
-        SuccessOrQuit(ptrRecord.ReadPtrName(*message, offset, name, sizeof(name)));
-        VerifyOrQuit(strcmp(name, instanceName) == 0, "Inst1 name is incorrect");
+        SuccessOrQuit(ptrRecord.ReadPtrName(*message, offset, label, sizeof(label), name, sizeof(name)));
+        VerifyOrQuit(strcmp(label, instanceLabel) == 0, "Inst label is incorrect");
+        VerifyOrQuit(strcmp(name, kServiceName) == 0);
 
-        printf("    \"%s\" PTR %u %d \"%s\"\n", kServiceName, ptrRecord.GetTtl(), ptrRecord.GetLength(), name);
+        printf("    \"%s\" PTR %u %d \"%s.%s\"\n", kServiceName, ptrRecord.GetTtl(), ptrRecord.GetLength(), label,
+               name);
     }
 
     VerifyOrQuit(offset == additionalSectionOffset, "offset is incorrect after answer section parse");

tests/unit/test_srp_server.cpp

@@ -287,7 +287,7 @@ static const char kHostName[] = "myhost";
 void PrepareService1(Srp::Client::Service &aService)
 {
     static const char          kServiceName[]   = "_srv._udp";
-    static const char          kInstanceLabel[] = "srv-instance";
+    static const char          kInstanceLabel[] = "srv.instance";
     static const char          kSub1[]          = "_sub1";
     static const char          kSub2[]          = "_V1234567";
     static const char          kSub3[]          = "_XYZWS";