[enc] Throw error when DMatrix is empty.

include/xgboost/collective/result.h

@@ -159,5 +159,6 @@ template <typename Fn>
   return fn();
 }
 
-void SafeColl(Result const& rc);
+void SafeColl(Result const& rc, char const* file = __builtin_FILE(),
+              std::int32_t line = __builtin_LINE());
 }  // namespace xgboost::collective

python-package/xgboost/dask/__init__.py

@@ -55,7 +55,7 @@
 import logging
 from collections import defaultdict
 from contextlib import contextmanager
-from functools import cache, partial, update_wrapper
+from functools import partial, update_wrapper
 from threading import Thread
 from typing import (
     Any,
@@ -85,8 +85,6 @@
 from dask import dataframe as dd
 from dask.delayed import Delayed
 from distributed import Future
-from packaging.version import Version
-from packaging.version import parse as parse_version
 
 from .. import collective, config
 from .._data_utils import Categories
@@ -124,7 +122,7 @@
 from ..tracker import RabitTracker
 from ..training import train as worker_train
 from .data import _get_dmatrices, no_group_split
-from .utils import get_address_from_user, get_n_threads
+from .utils import _DASK_2024_12_1, _DASK_2025_3_0, get_address_from_user, get_n_threads
 
 _DaskCollection: TypeAlias = Union[da.Array, dd.DataFrame, dd.Series]
 _DataT: TypeAlias = Union[da.Array, dd.DataFrame]  # do not use series as predictor
@@ -174,21 +172,6 @@
 LOGGER = logging.getLogger("[xgboost.dask]")
 
 
-@cache
-def _DASK_VERSION() -> Version:
-    return parse_version(dask.__version__)
-
-
-@cache
-def _DASK_2024_12_1() -> bool:
-    return _DASK_VERSION() >= parse_version("2024.12.1")
-
-
-@cache
-def _DASK_2025_3_0() -> bool:
-    return _DASK_VERSION() >= parse_version("2025.3.0")
-
-
 def _try_start_tracker(
     n_workers: int,
     addrs: List[Union[Optional[str], Optional[Tuple[str, int]]]],

python-package/xgboost/dask/utils.py

@@ -1,10 +1,15 @@
+# pylint: disable=invalid-name
 """Utilities for the XGBoost Dask interface."""
 
 import logging
 import warnings
+from functools import cache as fcache
 from typing import Any, Dict, Optional, Tuple
 
+import dask
 import distributed
+from packaging.version import Version
+from packaging.version import parse as parse_version
 
 from ..collective import Config
 
@@ -97,3 +102,18 @@ def get_address_from_user(
         port = coll_cfg.tracker_port
 
     return host_ip, port
+
+
+@fcache
+def _DASK_VERSION() -> Version:
+    return parse_version(dask.__version__)
+
+
+@fcache
+def _DASK_2024_12_1() -> bool:
+    return _DASK_VERSION() >= parse_version("2024.12.1")
+
+
+@fcache
+def _DASK_2025_3_0() -> bool:
+    return _DASK_VERSION() >= parse_version("2025.3.0")

python-package/xgboost/testing/dask.py

@@ -1,12 +1,13 @@
+# pylint: disable=invalid-name
 """Tests for dask shared by different test modules."""
 
-from typing import Any, List, Literal, Tuple, cast
+from typing import Any, List, Literal, Tuple, Type, cast
 
 import numpy as np
 import pandas as pd
 from dask import array as da
 from dask import dataframe as dd
-from distributed import Client, get_worker, wait
+from distributed import Client, get_worker
 from packaging.version import parse as parse_version
 from sklearn.datasets import make_classification
 
@@ -17,7 +18,8 @@
 
 from .. import dask as dxgb
 from .._typing import EvalsLog
-from ..dask import _DASK_VERSION, _get_rabit_args
+from ..dask import _get_rabit_args
+from ..dask.utils import _DASK_VERSION
 from .data import make_batches
 from .data import make_categorical as make_cat_local
 from .ordinal import make_recoded
@@ -325,61 +327,77 @@ def pack(**kwargs: Any) -> dd.DataFrame:
 # pylint: disable=too-many-locals
 def run_recode(client: Client, device: Device) -> None:
     """Run re-coding test with the Dask interface."""
-    enc, reenc, y, _, _ = make_recoded(device, n_features=96)
-    workers = get_client_workers(client)
-    denc, dreenc, dy = (
-        dd.from_pandas(enc, npartitions=8).persist(workers=workers),
-        dd.from_pandas(reenc, npartitions=8).persist(workers=workers),
-        da.from_array(y, chunks=(y.shape[0] // 8,)).persist(workers=workers),
-    )
 
-    wait([denc, dreenc, dy])
+    def create_dmatrix(
+        DMatrixT: Type[dxgb.DaskDMatrix], *args: Any, **kwargs: Any
+    ) -> dxgb.DaskDMatrix:
+        if DMatrixT is dxgb.DaskQuantileDMatrix:
+            ref = kwargs.pop("ref", None)
+            return DMatrixT(*args, ref=ref, **kwargs)
 
-    if device == "cuda":
-        denc = denc.to_backend("cudf")
-        dreenc = dreenc.to_backend("cudf")
-        dy = dy.to_backend("cupy")
+        kwargs.pop("ref", None)
+        return DMatrixT(*args, **kwargs)
 
-    Xy = dxgb.DaskQuantileDMatrix(client, denc, dy, enable_categorical=True)
-    Xy_valid = dxgb.DaskQuantileDMatrix(
-        client, dreenc, dy, enable_categorical=True, ref=Xy
-    )
-    # Base model
-    results = dxgb.train(client, {"device": device}, Xy, evals=[(Xy_valid, "Valid")])
+    def run(DMatrixT: Type[dxgb.DaskDMatrix]) -> None:
+        enc, reenc, y, _, _ = make_recoded(device, n_features=96)
+        to = get_client_workers(client)
 
-    # Training continuation
-    Xy = dxgb.DaskQuantileDMatrix(client, denc, dy, enable_categorical=True)
-    Xy_valid = dxgb.DaskQuantileDMatrix(
-        client, dreenc, dy, enable_categorical=True, ref=Xy
-    )
-    results_1 = dxgb.train(
-        client,
-        {"device": device},
-        Xy,
-        evals=[(Xy_valid, "Valid")],
-        xgb_model=results["booster"],
-    )
+        denc, dreenc, dy = (
+            dd.from_pandas(enc, npartitions=8).persist(workers=to),
+            dd.from_pandas(reenc, npartitions=8).persist(workers=to),
+            da.from_array(y, chunks=(y.shape[0] // 8,)).persist(workers=to),
+        )
 
-    # Reversed training continuation
-    Xy = dxgb.DaskQuantileDMatrix(client, dreenc, dy, enable_categorical=True)
-    Xy_valid = dxgb.DaskQuantileDMatrix(
-        client, denc, dy, enable_categorical=True, ref=Xy
-    )
-    results_2 = dxgb.train(
-        client,
-        {"device": device},
-        Xy,
-        evals=[(Xy_valid, "Valid")],
-        xgb_model=results["booster"],
-    )
-    np.testing.assert_allclose(
-        results_1["history"]["Valid"]["rmse"], results_2["history"]["Valid"]["rmse"]
-    )
+        if device == "cuda":
+            denc = denc.to_backend("cudf")
+            dreenc = dreenc.to_backend("cudf")
+            dy = dy.to_backend("cupy")
+
+        Xy = create_dmatrix(DMatrixT, client, denc, dy, enable_categorical=True)
+        Xy_valid = create_dmatrix(
+            DMatrixT, client, dreenc, dy, enable_categorical=True, ref=Xy
+        )
+        # Base model
+        results = dxgb.train(
+            client, {"device": device}, Xy, evals=[(Xy_valid, "Valid")]
+        )
+
+        # Training continuation
+        Xy = create_dmatrix(DMatrixT, client, denc, dy, enable_categorical=True)
+        Xy_valid = create_dmatrix(
+            DMatrixT, client, dreenc, dy, enable_categorical=True, ref=Xy
+        )
+        results_1 = dxgb.train(
+            client,
+            {"device": device},
+            Xy,
+            evals=[(Xy_valid, "Valid")],
+            xgb_model=results["booster"],
+        )
+
+        # Reversed training continuation
+        Xy = create_dmatrix(DMatrixT, client, dreenc, dy, enable_categorical=True)
+        Xy_valid = create_dmatrix(
+            DMatrixT, client, denc, dy, enable_categorical=True, ref=Xy
+        )
+        results_2 = dxgb.train(
+            client,
+            {"device": device},
+            Xy,
+            evals=[(Xy_valid, "Valid")],
+            xgb_model=results["booster"],
+        )
+        np.testing.assert_allclose(
+            results_1["history"]["Valid"]["rmse"], results_2["history"]["Valid"]["rmse"]
+        )
+
+        predt_0 = dxgb.inplace_predict(client, results, denc).compute()
+        predt_1 = dxgb.inplace_predict(client, results, dreenc).compute()
+        assert_allclose(device, predt_0, predt_1)
 
-    predt_0 = dxgb.inplace_predict(client, results, denc).compute()
-    predt_1 = dxgb.inplace_predict(client, results, dreenc).compute()
-    assert_allclose(device, predt_0, predt_1)
+        predt_0 = dxgb.predict(client, results, Xy).compute()
+        predt_1 = dxgb.predict(client, results, Xy_valid).compute()
+        assert_allclose(device, predt_0, predt_1)
 
-    predt_0 = dxgb.predict(client, results, Xy).compute()
-    predt_1 = dxgb.predict(client, results, Xy_valid).compute()
-    assert_allclose(device, predt_0, predt_1)
+    for DMatrixT in [dxgb.DaskDMatrix, dxgb.DaskQuantileDMatrix]:
+        run(DMatrixT)

src/collective/result.cc

@@ -1,5 +1,5 @@
 /**
- *  Copyright 2024, XGBoost Contributors
+ *  Copyright 2024-2025, XGBoost Contributors
  */
 #include "xgboost/collective/result.h"
 
@@ -65,17 +65,27 @@ void ResultImpl::Concat(std::unique_ptr<ResultImpl> rhs) {
 std::string MakeMsg(std::string&& msg, char const* file, std::int32_t line) {
   dmlc::DateLogger logger;
   if (file && line != -1) {
-    auto name = std::filesystem::path{ file }.filename();
+    auto name = std::filesystem::path{file}.filename();
     return "[" + name.string() + ":" + std::to_string(line) + "|" + logger.HumanDate() +
            "]: " + std::forward<std::string>(msg);
   }
   return std::string{"["} + logger.HumanDate() + "]" + std::forward<std::string>(msg);  // NOLINT
 }
 }  // namespace detail
 
-void SafeColl(Result const& rc) {
-  if (!rc.OK()) {
-    LOG(FATAL) << rc.Report();
+void SafeColl(Result const& rc, char const* file, std::int32_t line) {
+  if (rc.OK()) {
+    return;
   }
+  if (file && line != -1) {
+    dmlc::DateLogger logger;
+    auto name = std::filesystem::path{file}.filename();
+    LOG(FATAL) << ("[" + name.string() + ":" + std::to_string(line) + "|" + logger.HumanDate() +
+                   "]:\n")
+               << rc.Report();
+    // Return just in case if this function is deep in ctypes callbacks.
+    return;
+  }
+  LOG(FATAL) << rc.Report();
 }
 }  // namespace xgboost::collective

src/data/cat_container.cc

@@ -9,9 +9,11 @@
 #include <utility>    // for move
 #include <vector>     // for vector
 
-#include "../common/error_msg.h"  // for NoFloatCat
-#include "../encoder/types.h"     // for Overloaded
-#include "xgboost/json.h"         // for Json
+#include "../collective/allreduce.h"         // for Allreduce
+#include "../collective/communicator-inl.h"  // for GetRank, GetWorldSize
+#include "../common/error_msg.h"             // for NoFloatCat
+#include "../encoder/types.h"                // for Overloaded
+#include "xgboost/json.h"                    // for Json
 
 namespace xgboost {
 CatContainer::CatContainer(enc::HostColumnsView const& df, bool is_ref) : CatContainer{} {
@@ -293,4 +295,22 @@ void CatContainer::Sort(Context const* ctx) {
   enc::SortNames(enc::Policy<EncErrorPolicy>{}, view, this->sorted_idx_.HostSpan());
 }
 #endif  // !defined(XGBOOST_USE_CUDA)
+
+void SyncCategories(Context const* ctx, CatContainer* cats, bool is_empty) {
+  CHECK(cats);
+  if (!collective::IsDistributed()) {
+    return;
+  }
+
+  auto rank = collective::GetRank();
+  std::vector<std::int32_t> workers(collective::GetWorldSize(), 0);
+  workers[rank] = is_empty;
+  collective::SafeColl(collective::Allreduce(ctx, &workers, collective::Op::kSum));
+  if (cats->HasCategorical() &&
+      std::any_of(workers.cbegin(), workers.cend(), [](auto v) { return v == 1; })) {
+    LOG(FATAL)
+        << "A worker cannot have empty input when a dataframe with categorical features is used. "
+           "XGBoost cannot infer the categories if the input is empty.";
+  }
+}
 }  // namespace xgboost

src/data/cat_container.h

@@ -162,7 +162,7 @@ class CatContainer {
    *        this method returns True.
    */
   [[nodiscard]] bool Empty() const;
-  [[nodiscard]] bool NeedRecode() const { return !this->Empty() && !this->is_ref_; }
+  [[nodiscard]] bool NeedRecode() const { return this->HasCategorical() && !this->is_ref_; }
 
   [[nodiscard]] std::size_t NumFeatures() const;
   /**
@@ -263,6 +263,8 @@ struct NoOpAccessor {
   [[nodiscard]] XGBOOST_DEVICE float operator()(Entry const& e) const { return e.fvalue; }
 };
 
+void SyncCategories(Context const* ctx, CatContainer* cats, bool is_empty);
+
 namespace cpu_impl {
 inline auto MakeCatAccessor(Context const* ctx, enc::HostColumnsView const& new_enc,
                             CatContainer const* orig_cats) {

src/data/extmem_quantile_dmatrix.cc

@@ -49,6 +49,8 @@ ExtMemQuantileDMatrix::ExtMemQuantileDMatrix(DataIterHandle iter_handle, DMatrix
   }
   this->batch_ = p;
   this->fmat_ctx_ = ctx;
+
+  SyncCategories(&ctx, info_.Cats(), info_.num_row_ == 0);
 }
 
 ExtMemQuantileDMatrix::~ExtMemQuantileDMatrix() {

src/data/iterative_dmatrix.cc

@@ -9,10 +9,11 @@
 #include <utility>    // for move
 #include <vector>     // for vector
 
-#include "../common/categorical.h"  // common::IsCat
+#include "../common/categorical.h"  // for IsCat
 #include "../common/hist_util.h"    // for HistogramCuts
 #include "../tree/param.h"          // FIXME(jiamingy): Find a better way to share this parameter.
 #include "batch_utils.h"            // for RegenGHist
+#include "cat_container.h"          // for SyncCategories
 #include "gradient_index.h"         // for GHistIndexMatrix
 #include "proxy_dmatrix.h"          // for DataIterProxy, DispatchAny
 #include "quantile_dmatrix.h"       // for GetCutsFromRef
@@ -50,6 +51,8 @@ IterativeDMatrix::IterativeDMatrix(DataIterHandle iter_handle, DMatrixHandle pro
   this->fmat_ctx_ = ctx;
   this->batch_ = p;
 
+  SyncCategories(&ctx, info_.Cats(), info_.num_row_ == 0);
+
   LOG(INFO) << "Finished constructing the `IterativeDMatrix`: (" << this->Info().num_row_ << ", "
             << this->Info().num_col_ << ", " << this->info_.num_nonzero_ << ").";
 }

src/data/simple_dmatrix.cc

@@ -341,6 +341,8 @@ SimpleDMatrix::SimpleDMatrix(AdapterT* adapter, float missing, int nthread,
   }
   info_.num_nonzero_ = data_vec.size();
 
+  SyncCategories(&ctx, info_.Cats(), info_.num_row_ == 0);
+
   // Sort the index for row partitioners used by variuos tree methods.
   if (!sparse_page_->IsIndicesSorted(ctx.Threads())) {
     sparse_page_->SortIndices(ctx.Threads());

src/data/sparse_page_dmatrix.cc

@@ -83,8 +83,9 @@ SparsePageDMatrix::SparsePageDMatrix(DataIterHandle iter_handle, DMatrixHandle p
   iter.Reset();
 
   ext_info_.SetInfo(&ctx, true, &this->info_);
-
   fmat_ctx_ = ctx;
+
+  SyncCategories(&ctx, info_.Cats(), info_.num_row_ == 0);
 }
 
 SparsePageDMatrix::~SparsePageDMatrix() {