Check PVA "put" result with plain PV.write

Since for the time being a read-only channel can only be detected by
performing a put and checking the result, have to await the result even
for a fire-and-forget type of write.
Using a configurable timeout to prevent GUI hangups.

Author: kasemir

core/pv/src/main/java/org/phoebus/pv/pva/PVA_PV.java

@@ -164,33 +164,38 @@ public VType get(final long timeout, final TimeUnit unit)
     @Override
     public void write(final Object new_value) throws Exception
     {
-        // With Channel Access, there are different protocol options
-        // for "put" vs. "put-callback".
-        // With PVA, it is currently unclear how to distinguish between
-        // these two. The PVA server might honor certain values in the
-        // "request", but that is not documented as part of the protocol.
-        // On one hand, we should 'get()' the result of the write to
-        // receive exceptions for read-only PVs.
-        // On the other hand, such a 'get()' could last a long time
-        // in case some detail in the 'request' caused the PVA server
-        // to perform a put-callback type of operation,
-        // and a GUI calling write() expect an immediate return.
-
         // Perform a disconnect check right now to alert caller
         // of clearly disconnected channel
         if (isDisconnected(read()))
             throw new IllegalStateException("Channel '" + getName() + "' is not connected");
 
         // The channel could still disconnect in the middle of the write,
-        // the channel may be read-only or experience other errors
-        // that we'll only see as log messages since we don't want to
-        // wait in 'get()' here...
-        channel.write(name_helper.getWriteRequest(), new_value);
+        // or experience other errors which we'll receive in the
+        // response checked below.
+
+        // Perform a plain "put", not "put-callback"
+        final Future<Void> response = channel.write(false, name_helper.getWriteRequest(), new_value);
+
+        // Compared to Channel Access, PVA currently offers no
+        // information about writable vs. read-only channels.
+        // A read-only channel will only inform us about the failed
+        // write in the put response, for which we need to await
+        // the return value from the PVA server.
+        // Waiting for the response from the server, however,
+        // can take a little time, enough to be noticeable in a GUI
+        // that directly calls 'write' from the UI thread.
+        // Still, there seems no alternative to waiting a little bit,
+        // then potentially timing out.
+        response.get(PVA_Preferences.epics_pva_write_reply_timeout_ms, TimeUnit.MILLISECONDS);
     }
 
     @Override
     public Future<?> asyncWrite(final Object new_value) throws Exception
     {
+        // Perform a put with completion,
+        // i.e., process target and block until processing completes,
+        // akin to a Channel Access put-callback.
+        // Return the Future that can be used to await completion
         return channel.write(true, name_helper.getWriteRequest(), new_value);
     }
 

core/pv/src/main/java/org/phoebus/pv/pva/PVA_Preferences.java

@@ -43,6 +43,8 @@ public class PVA_Preferences
 {
     private static final PVA_Preferences instance = new PVA_Preferences();
 
+    public static int epics_pva_write_reply_timeout_ms;
+
     /** Prevent direct instantiation */
     private PVA_Preferences()
     {
@@ -76,6 +78,8 @@ public void installPreferences() throws Exception
                            new Object[] { propname, PVA_PVFactory.class.getPackageName(), setting, value });
             }
         }
+
+        epics_pva_write_reply_timeout_ms = prefs.getInt("epics_pva_write_reply_timeout_ms");
     }
 
     /** @return Singleton instance */

core/pv/src/main/resources/pv_pva_preferences.properties

@@ -42,3 +42,11 @@ epics_pva_max_array_formatting
 
 # TCP buffer size for sending data
 epics_pva_send_buffer_size
+
+# Timeout used by plain "put" type of write
+# when checking success or failure.
+# Note this is not used with asyncWrite,
+# the "put-callback" which returns a Future
+# for awaiting the completion,
+# but only with the plain "put" that returns ASAP
+epics_pva_write_reply_timeout_ms=1000

core/pva/src/main/java/org/epics/pva/client/PVAChannel.java

@@ -241,7 +241,7 @@ public Future<PVAStructure> read(final String request)
      *  @param request Request for element to write, e.g. "field(value)"
      *  @param new_value New value: Number, String
      *  @throws Exception on error
-     *  @return {@link Future} for awaiting completion
+     *  @return {@link Future} for awaiting completion and getting Exception in case of error
      *  @deprecated Use {@link #write(boolean, String, Object)}
      */
     @Deprecated
@@ -271,7 +271,7 @@ public Future<Void> write(final String request, final Object new_value) throws E
     *  @param request Request for element to write, e.g. "field(value)"
     *  @param new_value New value: Number, String
     *  @throws Exception on error
-    *  @return {@link Future} for awaiting completion
+    *  @return {@link Future} for awaiting completion and getting Exception in case of error
     */
     public Future<Void> write(final boolean completion, final String request, final Object new_value) throws Exception
     {