Don't reset cachedSize to 0 in getSerializedSize

This avoids a race-condition when cachedSize is momentarily set to 0
for non-empty messages if multiple threads call getSerializedSize
(e.g. during serialization).

This is a retry of https://android-review.googlesource.com/#/c/88570/.
getSerializedSize() has been kept non-final so that messages generated
with a previous version of the compiler will not break.

Change-Id: I8d8154a10938cde579ae19c55eae55b1e70e0bda

1 files changed, 9 insertions, 0 deletions

diff --git a/java/README.txt b/java/README.txt
index 13865f6d..f958d147 100644
--- a/java/README.txt
+++ b/java/README.txt
@@ -437,6 +437,15 @@ and the runtime overhead. An overview of Nano features:
   MessageNano.
 - The 'bytes' type translates to the Java type byte[].
 
+The generated messages are not thread-safe for writes, but may be
+used simultaneously from multiple threads in a read-only manner.
+In other words, an appropriate synchronization mechanism (such as
+a ReadWriteLock) must be used to ensure that a message, its
+ancestors, and descendants are not accessed by any other threads
+while the message is being modified. Field reads, getter methods,
+toByteArray(...), writeTo(...), getCachedSize(), and
+getSerializedSize() are all considered read-only operations.
+
 IMPORTANT: If you have fields with defaults and opt out of accessors
 
 How fields with defaults are serialized has changed. Because we don't