ttools: fix upload matcher to cope with empty input tables

The CDS upload service currently fails with an error, at least under
some circumstances (selection=all) if there are no rows in the input
table.  So add logic which is able, configurably, to fail the match
with a well-behaved IOException in the case of zero input rows rather
than attempt the match.

Once the CDS service copes with empty upload tables this should
perhaps be switched back, since an empty output table may be of
some use (at least you can get the columns).  Or maybe not, you
probably don't want to keep the result.

topcat/src/main/uk/ac/starlink/topcat/join/UploadMatchPanel.java

@@ -435,9 +435,11 @@ private MatchWorker createMatchWorker() {
         JoinFixAction inFixAct = JoinFixAction.NO_ACTION;
         JoinFixAction cdsFixAct = fixSelector_.getJoinFixAction();
         boolean oneToOne = upMode.isOneToOne();
+        boolean uploadEmpty = CdsUploadMatcher.UPLOAD_EMPTY;
         BlockUploader blocker =
             new BlockUploader( umatcher, blocksize, maxrec, outName,
-                               inFixAct, cdsFixAct, serviceMode, oneToOne );
+                               inFixAct, cdsFixAct, serviceMode, oneToOne,
+                               uploadEmpty );
 
         /* Create and return the match worker. */
         return new MatchWorker( blocker, upMode, tcModel, qsFact, storage );

ttools/src/main/uk/ac/starlink/ttools/cone/BlockUploader.java

@@ -31,6 +31,7 @@ public class BlockUploader {
     private final JoinFixAction remoteFixAct_;
     private final ServiceFindMode serviceMode_;
     private final boolean oneToOne_;
+    private final boolean uploadEmpty_;
 
     private static final Logger logger_ =
         Logger.getLogger( "uk.ac.starlink.ttools.task" );
@@ -46,11 +47,15 @@ public class BlockUploader {
      * @param   remoteFixAct  name deduplication policy for remote table
      * @param   serviceMode   upload match mode
      * @param   oneToOne   true iff output rows match 1:1 with input rows
+     * @param   uploadEmpty  determines behaviour if there are no input rows:
+     *                       true means attempt the match anyway,
+     *                       false means throw an IOException
      */
     public BlockUploader( UploadMatcher umatcher, int blocksize, long maxrec,
                           String outName, JoinFixAction uploadFixAct,
                           JoinFixAction remoteFixAct,
-                          ServiceFindMode serviceMode, boolean oneToOne ) {
+                          ServiceFindMode serviceMode, boolean oneToOne,
+                          boolean uploadEmpty ) {
         umatcher_ = umatcher;
         blocksize_ = blocksize;
         maxrec_ = maxrec;
@@ -59,6 +64,7 @@ public BlockUploader( UploadMatcher umatcher, int blocksize, long maxrec,
         remoteFixAct_ = remoteFixAct;
         serviceMode_ = serviceMode;
         oneToOne_ = oneToOne;
+        uploadEmpty_ = uploadEmpty;
         if ( oneToOne_ && ! serviceMode.supportsOneToOne() ) {
             throw new IllegalArgumentException( "Mode " + serviceMode
                                               + " doesn't support 1:1" );
@@ -114,9 +120,13 @@ public StarTable runMatch( StarTable inTable, QuerySequenceFactory qsFact,
         while ( ! done && ( maxrec_ < 0 || totOut < maxrec_ ) ) {
             PreviewBlockSequence blockSeq =
                 new PreviewBlockSequence( coneSeq, blocksize_ );
-            if ( blockSeq.hasNext() ) {
-                BlockSink blockSink =
-                    new BlockSink( rawResultStore, iblock == 0 );
+            boolean isFirst = iblock == 0;
+            boolean hasNext = blockSeq.hasNext();
+            if ( isFirst && ! hasNext && ! uploadEmpty_ ) {
+                throw new IOException( "No candidate rows for upload match" );
+            }
+            if ( hasNext || isFirst ) {
+                BlockSink blockSink = new BlockSink( rawResultStore, isFirst );
                 long nRemain = maxrec_ >= 0 ? maxrec_ - totOut : -1;
                 boolean over =
                     umatcher_.streamRawResult( blockSeq, blockSink, rowMapper,
@@ -130,9 +140,7 @@ public StarTable runMatch( StarTable inTable, QuerySequenceFactory qsFact,
                 totOut += nOut;
                 iblock++;
             }
-            else {
-                done = true;
-            }
+            done = ! hasNext;
         };
         int nblock = iblock;
         coneSeq.close();

ttools/src/main/uk/ac/starlink/ttools/cone/CdsUploadMatcher.java

@@ -51,8 +51,13 @@ public class CdsUploadMatcher implements UploadMatcher {
     /** URL for the CDS Xmatch service. */
     public static final String XMATCH_URL =
         "http://cdsxmatch.u-strasbg.fr/xmatch/api/v1/sync";
+
+    /** Alias for Simbad flat view table. */
     public static final String SIMBAD_NAME = "simbad";
 
+    /** Whether it is safe/recommended to upload empty tables to match. */
+    public static final boolean UPLOAD_EMPTY = false;
+
     private static final String RA_NAME = "__UPLOAD_RA__";
     private static final String DEC_NAME = "__UPLOAD_DEC__";
     private static final String ID_NAME = "__UPLOAD_ID__";

ttools/src/main/uk/ac/starlink/ttools/task/CdsUploadSkyMatch.java

@@ -252,9 +252,11 @@ public TableProducer createProducer( Environment env )
         final TableProducer inProd = createInputProducer( env );
         final StoragePolicy storage =
             LineTableEnvironment.getStoragePolicy( env );
+        boolean uploadEmpty = CdsUploadMatcher.UPLOAD_EMPTY;
         final BlockUploader blocker =
             new BlockUploader( umatcher, blocksize, maxrec, tableName,
-                               inFixAct, cdsFixAct, serviceMode, oneToOne );
+                               inFixAct, cdsFixAct, serviceMode, oneToOne,
+                               uploadEmpty );
 
         /* Create and return an object which will produce the result. */
         return new TableProducer() {