[CLATJ#21] ClatCoordinator: start clatd

launch clatd with the given cli arguments.

Bug: 212345928
Test: flash and boot
- run "atest ClatCoordinatorTest" in a follow commit.
- manual test
  1. Connect to ipv6-only wifi.
  2. Try IPv4 traffic.
     $ ping 8.8.8.8

Change-Id: Ie57a7c7a9d3d77396e91fa9f94c02f1ad05487c1

service/src/com/android/server/connectivity/ClatCoordinator.java

@@ -170,12 +170,12 @@ public class ClatCoordinator {
         }
 
         /**
-         * Maybe start bpf.
+         * Start clatd.
          */
-        public int maybeStartBpf(@NonNull FileDescriptor tunfd, @NonNull FileDescriptor readsock6,
+        public int startClatd(@NonNull FileDescriptor tunfd, @NonNull FileDescriptor readsock6,
                 @NonNull FileDescriptor writesock6, @NonNull String iface, @NonNull String pfx96,
                 @NonNull String v4, @NonNull String v6) throws IOException {
-            return native_maybeStartBpf(tunfd, readsock6, writesock6, iface, pfx96, v4, v6);
+            return native_startClatd(tunfd, readsock6, writesock6, iface, pfx96, v4, v6);
         }
 
         /**
@@ -219,6 +219,9 @@ public class ClatCoordinator {
     public String clatStart(final String iface, final int netId,
             @NonNull final IpPrefix nat64Prefix)
             throws IOException {
+        if (mIface != null || mPid != INVALID_PID) {
+            throw new IOException("Clatd has started on " + mIface + " (pid " + mPid + ")");
+        }
         if (nat64Prefix.getPrefixLength() != 96) {
             throw new IOException("Prefix must be 96 bits long: " + nat64Prefix);
         }
@@ -327,20 +330,19 @@ public class ClatCoordinator {
             throw new IOException("configure packet socket failed: " + e);
         }
 
-        // [5] Maybe start bpf.
+        // [5] Start clatd.
         try {
-            mDeps.maybeStartBpf(tunFd.getFileDescriptor(), readSock6.getFileDescriptor(),
+            mPid = mDeps.startClatd(tunFd.getFileDescriptor(), readSock6.getFileDescriptor(),
                     writeSock6.getFileDescriptor(), iface, pfx96, v4, v6);
             mIface = iface;
             mNat64Prefix = pfx96;
             mXlatLocalAddress4 = v4;
             mXlatLocalAddress6 = v6;
         } catch (IOException e) {
-            throw new IOException("Error start bpf on " + iface + ": " + e);
+            throw new IOException("Error start clatd on " + iface + ": " + e);
         }
 
-        // TODO: start clatd and returns local xlat464 v6 address.
-        return null;
+        return v6;
     }
 
     /**
@@ -372,7 +374,7 @@ public class ClatCoordinator {
             int ifindex) throws IOException;
     private static native void native_configurePacketSocket(FileDescriptor sock, String v6,
             int ifindex) throws IOException;
-    private static native int native_maybeStartBpf(FileDescriptor tunfd, FileDescriptor readsock6,
+    private static native int native_startClatd(FileDescriptor tunfd, FileDescriptor readsock6,
             FileDescriptor writesock6, String iface, String pfx96, String v4, String v6)
             throws IOException;
     private static native void native_maybeStopBpf(String iface, String pfx96, String v4,