[mle] add HasAcceptableParentCandidate() and refactor code (#7470)

This commit adds `Mle::HasAcceptableParentCandidate()` method
which is used to during attach process after sending an MLE Parent
Request message and waiting for the proper delay (i.e., when attach
timer fires) to determine whether or not we have found an acceptable
parent candidate.

This is a refactor of the existing code with no change in behavior
with goal of making the code easier to read (all the conditions that
are checked when deciding to accept a parent candidate).

src/core/thread/mle.cpp

@@ -1713,43 +1713,66 @@ void Mle::HandleAttachTimer(Timer &aTimer)
     aTimer.Get<Mle>().HandleAttachTimer();
 }
 
-void Mle::HandleAttachTimer(void)
+bool Mle::HasAcceptableParentCandidate(void) const
 {
-    uint32_t delay          = 0;
-    bool     shouldAnnounce = true;
+    bool    hasAcceptableParent = false;
+    uint8_t linkQuality;
 
-    if (mAttachState == kAttachStateParentRequestRouter || mAttachState == kAttachStateParentRequestReed ||
-        (mAttachState == kAttachStateAnnounce && !HasMoreChannelsToAnnouce()))
+    VerifyOrExit(mParentCandidate.IsStateParentResponse());
+
+    switch (mAttachState)
     {
-        uint8_t linkQuality;
+    case kAttachStateAnnounce:
+        VerifyOrExit(!HasMoreChannelsToAnnouce());
+        break;
 
-        linkQuality = mParentCandidate.GetLinkInfo().GetLinkQuality();
+    case kAttachStateParentRequestRouter:
+        // If we cannot find a parent with best link quality (3) when
+        // in `kAttachStateParentRequestRouter` state we will keep the
+        // candidate and forward to REED stage to potentially find a
+        // better parent.
+        linkQuality = OT_MIN(mParentCandidate.GetLinkInfo().GetLinkQuality(), mParentCandidate.GetLinkQualityOut());
+        VerifyOrExit(linkQuality == 3);
+        break;
 
-        if (linkQuality > mParentCandidate.GetLinkQualityOut())
-        {
-            linkQuality = mParentCandidate.GetLinkQualityOut();
-        }
+    case kAttachStateParentRequestReed:
+        break;
 
+    default:
+        ExitNow();
+    }
+
+    if (IsChild())
+    {
         // If already attached, accept the parent candidate if
         // we are trying to attach to a better partition or if a
         // Parent Response was also received from the current parent
         // to which the device is attached. This ensures that the
         // new parent candidate is compared with the current parent
         // and that it is indeed preferred over the current one.
-        // If we are in kAttachStateParentRequestRouter and cannot
-        // find a parent with best link quality(3), we will keep
-        // the candidate and forward to REED stage to find a better
-        // parent.
 
-        if ((linkQuality == 3 || mAttachState != kAttachStateParentRequestRouter) &&
-            mParentCandidate.IsStateParentResponse() &&
-            (!IsChild() || mReceivedResponseFromParent || mParentRequestMode == kAttachBetter) &&
-            SendChildIdRequest() == kErrorNone)
-        {
-            SetAttachState(kAttachStateChildIdRequest);
-            delay = kParentRequestReedTimeout;
-            ExitNow();
-        }
+        VerifyOrExit(mReceivedResponseFromParent || (mParentRequestMode == kAttachBetter));
+    }
+
+    hasAcceptableParent = true;
+
+exit:
+    return hasAcceptableParent;
+}
+
+void Mle::HandleAttachTimer(void)
+{
+    uint32_t delay          = 0;
+    bool     shouldAnnounce = true;
+
+    // First, check if we are waiting to receive parent responses and
+    // found an acceptable parent candidate.
+
+    if (HasAcceptableParentCandidate() && (SendChildIdRequest() == kErrorNone))
+    {
+        SetAttachState(kAttachStateChildIdRequest);
+        delay = kParentRequestReedTimeout;
+        ExitNow();
     }
 
     switch (mAttachState)

src/core/thread/mle.hpp

@@ -1838,6 +1838,7 @@ private:
     Error SendLinkMetricsManagementResponse(const Ip6::Address &aDestination, LinkMetrics::Status aStatus);
 #endif
     uint32_t Reattach(void);
+    bool     HasAcceptableParentCandidate(void) const;
 
     bool IsBetterParent(uint16_t               aRloc16,
                         uint8_t                aLinkQuality,