Remove rabit dependency on public headers. (#6005)

include/xgboost/data.h

@@ -10,7 +10,6 @@
 #include <dmlc/base.h>
 #include <dmlc/data.h>
 #include <dmlc/serializer.h>
-#include <rabit/rabit.h>
 #include <xgboost/base.h>
 #include <xgboost/span.h>
 #include <xgboost/host_device_vector.h>
@@ -273,14 +272,7 @@ class SparsePage {
   inline Inst operator[](size_t i) const {
     const auto& data_vec = data.HostVector();
     const auto& offset_vec = offset.HostVector();
-    size_t size;
+    size_t size = offset_vec[i + 1] - offset_vec[i];
-    // in distributed mode, some partitions may not get any instance for a feature. Therefore
-    // we should set the size as zero
-    if (rabit::IsDistributed() && i + 1 >= offset_vec.size()) {
-      size = 0;
-    } else {
-      size = offset_vec[i + 1] - offset_vec[i];
-    }
     return {data_vec.data() + offset_vec[i],
             static_cast<Inst::index_type>(size)};
   }

include/xgboost/learner.h

@@ -9,7 +9,6 @@
 #define XGBOOST_LEARNER_H_
 
 #include <dmlc/any.h>
-#include <rabit/rabit.h>
 #include <xgboost/base.h>
 #include <xgboost/feature_map.h>
 #include <xgboost/predictor.h>
@@ -63,7 +62,7 @@ struct XGBAPIThreadLocalEntry {
  *
  *  \endcode
  */
-class Learner : public Model, public Configurable, public rabit::Serializable {
+class Learner : public Model, public Configurable, public dmlc::Serializable {
  public:
   /*! \brief virtual destructor */
   ~Learner() override;

tests/cpp/common/test_span.cc

@@ -519,5 +519,12 @@ TEST(Span, Empty) {
   }
 }
 
+TEST(SpanDeathTest, Empty) {
+  std::vector<float> data(1, 0);
+  ASSERT_TRUE(data.data());
+  Span<float> s{data.data(), Span<float>::index_type(0)};  // ok to define 0 size span.
+  EXPECT_DEATH(s[0], "\\[xgboost\\] Condition .* failed.\n");  // not ok to use it.
+}
+
 }  // namespace common
 }  // namespace xgboost