BUG#27018: Partial blob write inside blob clobbers data after the write.

When doing partial blob update with NdbBlob::writeData(), zero-padding
after the write was wrongly done, causing part of the old blob value
to be overwritten with zeros (or spaces for text field).

Fixed by only padding when needed (when writing at end of the blob).

ndb/src/ndbapi/NdbBlob.cpp

@@ -800,6 +800,8 @@ NdbBlob::writeDataPrivate(const char* buf, Uint32 bytes)
         DBUG_RETURN(-1);
       Uint32 n = thePartSize - off;
       if (n > len) {
+        /* If we are adding data at the end, fill rest of part. */
+        if (pos + len >= theLength)
           memset(thePartBuf.data + off + len, theFillChar, n - len);
         n = len;
       }

ndb/test/ndbapi/testBlobs.cpp

@@ -138,6 +138,7 @@ printusage()
     << "  2           readData / writeData" << endl
     << "bug tests (no blob test)" << endl
     << "  -bug 4088   ndb api hang with mixed ops on index table" << endl
+    << "  -bug 27018  middle partial part write clobbers rest of part" << endl
     << "  -bug nnnn   delete + write gives 626" << endl
     << "  -bug nnnn   acc crash on delete and long key" << endl
     ;
@@ -1806,6 +1807,56 @@ bugtest_4088()
   return 0;
 }
 
+static int
+bugtest_27018()
+{
+  DBG("bug test 27018 - middle partial part write clobbers rest of part");
+
+  // insert rows
+  calcTups(false);
+  CHK(insertPk(false) == 0);
+  // new trans
+  for (unsigned k= 0; k < g_opt.m_rows; k++)
+  {
+    Tup& tup= g_tups[k];
+
+    CHK((g_con= g_ndb->startTransaction()) != 0);
+    CHK((g_opr= g_con->getNdbOperation(g_opt.m_tname)) != 0);
+    CHK(g_opr->updateTuple() == 0);
+    CHK(g_opr->equal("PK1", tup.m_pk1) == 0);
+    if (g_opt.m_pk2len != 0)
+      CHK(g_opr->equal("PK2", tup.m_pk2) == 0);
+    CHK(getBlobHandles(g_opr) == 0);
+    CHK(g_con->execute(NoCommit) == 0);
+
+    /* Update one byte in random position. */
+    Uint32 offset= urandom(tup.m_blob1.m_len);
+    tup.m_blob1.m_buf[0]= 0xff ^ tup.m_blob1.m_val[offset];
+    CHK(g_bh1->setPos(offset) == 0);
+    CHK(g_bh1->writeData(&(tup.m_blob1.m_buf[0]), 1) == 0);
+    CHK(g_con->execute(Commit) == 0);
+    g_ndb->closeTransaction(g_con);
+
+    CHK((g_con= g_ndb->startTransaction()) != 0);
+    CHK((g_opr= g_con->getNdbOperation(g_opt.m_tname)) != 0);
+    CHK(g_opr->readTuple() == 0);
+    CHK(g_opr->equal("PK1", tup.m_pk1) == 0);
+    if (g_opt.m_pk2len != 0)
+      CHK(g_opr->equal("PK2", tup.m_pk2) == 0);
+    CHK(getBlobHandles(g_opr) == 0);
+
+    CHK(g_bh1->getValue(tup.m_blob1.m_buf, tup.m_blob1.m_len) == 0);
+    CHK(g_con->execute(Commit) == 0);
+    Uint64 len= ~0;
+    CHK(g_bh1->getLength(len) == 0 && len == tup.m_blob1.m_len);
+    tup.m_blob1.m_buf[offset]^= 0xff;
+    CHK(memcmp(tup.m_blob1.m_buf, tup.m_blob1.m_val, tup.m_blob1.m_len) == 0);
+    g_ndb->closeTransaction(g_con);
+  }
+
+  return 0;
+}
+
 static int
 bugtest_2222()
 {
@@ -1822,7 +1873,8 @@ static struct {
   int m_bug;
   int (*m_test)();
 } g_bugtest[] = {
-  { 4088, bugtest_4088 }
+  { 4088, bugtest_4088 },
+  { 27018, bugtest_27018 }
 };
 
 NDB_COMMAND(testOdbcDriver, "testBlobs", "testBlobs", "testBlobs", 65535)