Add Parcel::readExceptionCode() and Parcel::writeNoException()

Add native Parcel methods analogous to the Java versions.

Currently, these don't do much, but upcoming StrictMode work changes
the RPC calling conventions in some cases, so it's important that
everybody uses these consistently, rather than having a lot of code
trying to parse RPC responses out of Parcels themselves.

As a summary, the current convention that Java Binder services use is
to prepend the reply Parcel with an int32 signaling the exception
status:

     0: no exception
     -1: Security exception
     -2: Bad Parcelable
     -3: ...
     -4: ...
     -5: ...

... followed by Parceled String if the exception code is non-zero.

With an upcoming change, it'll be the case that a response Parcel can,
non-exceptionally return rich data in the header, and also return data
to the caller.  The important thing to note in this new case is that
the first int32 in the reply parcel *will not be zero*, so anybody
manually checking for it with reply.readInt32() will get false
negative failures.

Short summary: If you're calling into a Java service and manually
checking the exception status with reply.readInt32(), change it to
reply.readExceptionCode().

Change-Id: I23f9a0e53a8cfbbd9759242cfde16723641afe04

include/binder/Parcel.h

@@ -103,6 +103,11 @@ public:
     
     status_t            writeObject(const flat_binder_object& val, bool nullMetaData);
 
+    // Like Parcel.java's writeNoException().  Just writes a zero int32.
+    // Currently the native implementation doesn't do any of the StrictMode
+    // stack gathering and serialization that the Java implementation does.
+    status_t            writeNoException();
+
     void                remove(size_t start, size_t amt);
     
     status_t            read(void* outData, size_t len) const;
@@ -125,7 +130,14 @@ public:
     sp<IBinder>         readStrongBinder() const;
     wp<IBinder>         readWeakBinder() const;
     status_t            read(Flattenable& val) const;
-    
+
+    // Like Parcel.java's readExceptionCode().  Reads the first int32
+    // off of a Parcel's header, returning 0 or the negative error
+    // code on exceptions, but also deals with skipping over rich
+    // response headers.  Callers should use this to read & parse the
+    // response headers rather than doing it by hand.
+    int32_t             readExceptionCode() const;
+
     // Retrieve native_handle from the parcel. This returns a copy of the
     // parcel's native_handle (the caller takes ownership). The caller
     // must free the native_handle with native_handle_close() and 

libs/binder/IPermissionController.cpp

@@ -36,7 +36,7 @@ public:
         : BpInterface<IPermissionController>(impl)
     {
     }
-        
+
     virtual bool checkPermission(const String16& permission, int32_t pid, int32_t uid)
     {
         Parcel data, reply;
@@ -46,7 +46,7 @@ public:
         data.writeInt32(uid);
         remote()->transact(CHECK_PERMISSION_TRANSACTION, data, &reply);
         // fail on exception
-        if (reply.readInt32() != 0) return 0;
+        if (reply.readExceptionCode() != 0) return 0;
         return reply.readInt32() != 0;
     }
 };
@@ -66,8 +66,7 @@ status_t BnPermissionController::onTransact(
             int32_t pid = data.readInt32();
             int32_t uid = data.readInt32();
             bool res = checkPermission(permission, pid, uid);
-            // write exception
-            reply->writeInt32(0);
+            reply->writeNoException();
             reply->writeInt32(res ? 1 : 0);
             return NO_ERROR;
         } break;
@@ -77,4 +76,3 @@ status_t BnPermissionController::onTransact(
 }
 
 }; // namespace android
-

libs/binder/IServiceManager.cpp

@@ -158,7 +158,7 @@ public:
         data.writeString16(name);
         data.writeStrongBinder(service);
         status_t err = remote()->transact(ADD_SERVICE_TRANSACTION, data, &reply);
-        return err == NO_ERROR ? reply.readInt32() : err;
+        return err == NO_ERROR ? reply.readExceptionCode() : err;
     }
 
     virtual Vector<String16> listServices()

libs/binder/Parcel.cpp

@@ -754,6 +754,11 @@ restart_write:
     goto restart_write;
 }
 
+status_t Parcel::writeNoException()
+{
+    return writeInt32(0);
+}
+
 void Parcel::remove(size_t start, size_t amt)
 {
     LOG_ALWAYS_FATAL("Parcel::remove() not yet implemented!");
@@ -942,6 +947,12 @@ wp<IBinder> Parcel::readWeakBinder() const
     return val;
 }
 
+int32_t Parcel::readExceptionCode() const
+{
+  int32_t exception_code = readAligned<int32_t>();
+  // TODO: skip over the response header here, once that's in.
+  return exception_code;
+}
 
 native_handle* Parcel::readNativeHandle() const
 {