HPCC-31416 CDistributedSuperFile::querySubFile may return incorrect s…

…ubfile

Querying CDistributedSuperFile for a subfile that doesn't exist returned
a subfile instead of throwing an exception. By not throwing an error,
incorrect code that calls querySubFile may be appear to work.  This fix
modifies querySubFile so that it throws an exception when the subfile
number is not valid.

Signed-off-by: Shamser Ahmed <shamser.ahmed@lexisnexis.com>

dali/base/dadfs.cpp

@@ -6271,20 +6271,22 @@ class CDistributedSuperFile: public CDistributedFileBase<IDistributedSuperFile>
     virtual IDistributedFile &querySubFile(unsigned idx,bool sub) override
     {
         CriticalBlock block (sect);
-        if (sub) {
+        if (sub)
+        {
+            unsigned subfilen = idx;
             ForEachItemIn(i,subfiles) {
                 IDistributedFile &f=subfiles.item(i);
                 IDistributedSuperFile *super = f.querySuperFile();
                 if (super) {
                     unsigned ns = super->numSubFiles(true);
-                    if (ns>idx)
-                        return super->querySubFile(idx,true);
-                    idx -= ns;
+                    if (ns>subfilen)
+                        return super->querySubFile(subfilen,true);
+                    subfilen -= ns;
                 }
-                else if (idx--==0)
+                else if (subfilen--==0)
                     return f;
             }
-            // fall through to error
+            throw makeStringExceptionV(-1,"CDistributedSuperFile::querySubFile(%u) for superfile %s - subfile doesn't exist ", idx, logicalName.get());
         }
         return subfiles.item(idx);
     }