[posix] allow OT process to run when infra netif gets lost (#9583)

In `ot::Posix::InfraNetif::GetFlags()`, the OT process would die if
the infra network interface is removed on the POSIX platform. On
Android platform, this is not the desired behavior because the system
server may tell the OT process to switch another infra network
interface. It's fine for OT process to keep running when the previous
infra network interface disappears because later system server will
call `otSysSetInfraNetif()` to specify the new infra network
interface.

src/posix/platform/infra_if.cpp

@@ -289,6 +289,7 @@ uint32_t InfraNetif::GetFlags(void) const
 {
     int          sock;
     struct ifreq ifReq;
+    uint32_t     flags = 0;
 
     OT_ASSERT(mInfraIfIndex != 0);
 
@@ -299,11 +300,20 @@ uint32_t InfraNetif::GetFlags(void) const
     static_assert(sizeof(ifReq.ifr_name) >= sizeof(mInfraIfName), "mInfraIfName is not of appropriate size.");
     strcpy(ifReq.ifr_name, mInfraIfName);
 
-    VerifyOrDie(ioctl(sock, SIOCGIFFLAGS, &ifReq) != -1, OT_EXIT_ERROR_ERRNO);
+    if (ioctl(sock, SIOCGIFFLAGS, &ifReq) == -1)
+    {
+#if OPENTHREAD_POSIX_CONFIG_EXIT_ON_INFRA_NETIF_LOST_ENABLE
+        otLogCritPlat("The infra link %s may be lost. Exiting.", mInfraIfName);
+        DieNow(OT_EXIT_ERROR_ERRNO);
+#endif
+        ExitNow();
+    }
+    flags = static_cast<uint32_t>(ifReq.ifr_flags);
 
+exit:
     close(sock);
 
-    return static_cast<uint32_t>(ifReq.ifr_flags);
+    return flags;
 }
 
 void InfraNetif::CountAddresses(otSysInfraNetIfAddressCounters &aAddressCounters) const

src/posix/platform/openthread-posix-config.h

@@ -441,4 +441,15 @@
 #ifdef OPENTHREAD_POSIX_CONFIG_RCP_TIME_SYNC_INTERVAL
 #error "OPENTHREAD_POSIX_CONFIG_RCP_TIME_SYNC_INTERVAL was replaced by OPENTHREAD_SPINEL_CONFIG_RCP_TIME_SYNC_INTERVAL"
 #endif
+
+/**
+ * @def OPENTHREAD_POSIX_CONFIG_EXIT_ON_INFRA_NETIF_LOST_ENABLE
+ *
+ * Define as 1 to let the process exit when the infra network interface is lost on the POSIX platform.
+ *
+ */
+#ifndef OPENTHREAD_POSIX_CONFIG_EXIT_ON_INFRA_NETIF_LOST_ENABLE
+#define OPENTHREAD_POSIX_CONFIG_EXIT_ON_INFRA_NETIF_LOST_ENABLE 1
+#endif
+
 #endif // OPENTHREAD_PLATFORM_CONFIG_H_