ncmpio: keep zero-length collective requests in the numrecs sync

src/drivers/ncmpio/ncmpio_getput.m4

@@ -122,7 +122,7 @@ put_varm(NC               *ncp,
     MPI_Offset nelems=0, bnelems=0, nbytes=0;
     MPI_Datatype itype, imaptype;
 
-    if (varp == NULL) { /* zero-sized request */
+    if (varp == NULL || fIsSet(reqMode, NC_REQ_ZERO)) { /* zero-sized request */
         itype = MPI_BYTE;
         el_size = 0;
         bnelems = 0;
@@ -345,7 +345,7 @@ get_varm(NC               *ncp,
     MPI_Offset nelems=0, bnelems=0, nbytes=0;
     MPI_Datatype itype, imaptype=MPI_DATATYPE_NULL;
 
-    if (varp == NULL) { /* zero-sized request */
+    if (varp == NULL || fIsSet(reqMode, NC_REQ_ZERO)) { /* zero-sized request */
         itype = MPI_BYTE;
         el_size = 0;
         bnelems = 0;
@@ -509,14 +509,16 @@ ncmpio_$1_var(void             *ncdp,
 
 {
     NC     *ncp=(NC*)ncdp;
-    NC_var *varp;
+    NC_var *varp=NULL;
 
     /* Check if this is a true zero-sized request. Note NC_REQ_ZERO is added to
-     * reqMode only when an error is detected at the dispatcher level.
+     * reqMode only when an error is detected at the dispatcher level, in which
+     * case varid may not be valid and must not be used to look up varp.
      */
     if (!fIsSet(reqMode, NC_REQ_ZERO)) {
         int i;
-        for (i=0; i<ncp->vars.value[varid]->ndims; i++)
+        varp = ncp->vars.value[varid];
+        for (i=0; i<varp->ndims; i++)
             if (count[i] == 0) {
                 reqMode |= NC_REQ_ZERO;
                 break;
@@ -529,8 +531,17 @@ ncmpio_$1_var(void             *ncdp,
         /* In case some processes in an aggregation group have nothing to
          * write, they still need to participate the communication part of the
          * intra-node aggregation operation.
+         *
+         * varp is handed down whenever it is known to be valid, because
+         * put_varm() consults it to decide whether this is a record variable.
+         * A zero-length collective request to a record variable must still
+         * take part in the MPI_Allreduce() and ncmpio_write_numrecs() that
+         * synchronise numrecs: the processes that do have data call them
+         * unconditionally, so a process skipping them leaves the communicator
+         * out of step. varp stays NULL only when NC_REQ_ZERO came from a
+         * dispatcher-level error, where varid cannot be trusted.
          */
-        return $1_varm(ncp, NULL, NULL, NULL, NULL, imap, NULL, 0,
+        return $1_varm(ncp, varp, NULL, NULL, NULL, imap, NULL, 0,
                        buftype, reqMode);
     }
 

test/testcases/Makefile.am

@@ -104,6 +104,7 @@ check_PROGRAMS = file_create_open \
                  put_all_kinds \
                  redef1 \
                  iput_all_kinds \
+                 tst_zero_len_recvar \
                  tst_version
 
 M4_SRCS      = put_all_kinds.m4 \

test/testcases/tst_zero_len_recvar.c

@@ -0,0 +1,147 @@
+/******************************************************************************
+ *
+ *  Copyright (C) 2026, Northwestern University and Argonne National Laboratory
+ *  See COPYRIGHT notice in top-level directory.
+ *
+ *****************************************************************************/
+/* $Id$ */
+
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * This program tests a collective write to a record variable in which one of
+ * the processes has nothing to contribute, i.e. it passes count == 0. Every
+ * process still calls the collective API, so the call is legal.
+ *
+ * Such a request must not opt out of the collective synchronization of numrecs
+ * that follows the I/O. If it does, the processes that do have data to write
+ * are left calling an MPI_Allreduce() alone and the communicator goes out of
+ * step, which shows up as a wrong error code, as corruption of a later
+ * collective call, or as a hang.
+ *
+ * Fixed-size variables take no part in the numrecs synchronization and are
+ * therefore not affected; the record variable is the case worth guarding.
+ *
+ *    To compile:
+ *        mpicc -O2 tst_zero_len_recvar.c -o tst_zero_len_recvar -lpnetcdf
+ *
+ *    % mpiexec -n 4 ./tst_zero_len_recvar testfile.nc
+ *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <libgen.h> /* basename() */
+#include <pnetcdf.h>
+
+#include <testutils.h>
+
+#define NDBL 4  /* number of doubles written per process */
+
+static
+int test_io(const char *out_path,
+            const char *in_path, /* ignored */
+            int         format,
+            int         coll_io,
+            MPI_Info    info)
+{
+    int i, err, nerrs=0, ncid, dimid[2], varid, rank, nprocs, is_empty;
+    MPI_Offset start[2], count[2], numrecs;
+    double buf[NDBL], rbuf[NDBL];
+
+    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+    MPI_Comm_size(MPI_COMM_WORLD, &nprocs);
+
+    /* the last process contributes nothing; with a single process there is
+     * nobody left to write, so keep its request non-empty
+     */
+    is_empty = (nprocs > 1 && rank == nprocs - 1);
+
+    for (i=0; i<NDBL; i++) buf[i] = (double)(rank * NDBL + i);
+
+    err = ncmpi_set_default_format(format, NULL); CHECK_ERR
+
+    err = ncmpi_create(MPI_COMM_WORLD, out_path, NC_CLOBBER, info, &ncid); CHECK_ERR
+    err = ncmpi_def_dim(ncid, "time",  NC_UNLIMITED,   &dimid[0]); CHECK_ERR
+    err = ncmpi_def_dim(ncid, "nelem", nprocs * NDBL,  &dimid[1]); CHECK_ERR
+    err = ncmpi_def_var(ncid, "v", NC_DOUBLE, 2, dimid, &varid); CHECK_ERR
+    err = ncmpi_set_fill(ncid, NC_NOFILL, NULL); CHECK_ERR
+    err = ncmpi_enddef(ncid); CHECK_ERR
+
+    /* all processes write into record 0 */
+    start[0] = 0;
+    count[0] = 1;
+    start[1] = (MPI_Offset)rank * NDBL;
+    count[1] = NDBL;
+    if (is_empty) { start[1] = 0; count[1] = 0; }
+
+    if (coll_io) {
+        err = ncmpi_put_vara_double_all(ncid, varid, start, count, buf);
+        CHECK_ERR
+    }
+    else {
+        err = ncmpi_begin_indep_data(ncid); CHECK_ERR
+        err = ncmpi_put_vara_double(ncid, varid, start, count, buf); CHECK_ERR
+        err = ncmpi_end_indep_data(ncid); CHECK_ERR
+    }
+
+    /* one record must have been created, and all processes must agree */
+    err = ncmpi_inq_dimlen(ncid, dimid[0], &numrecs); CHECK_ERR
+    if (numrecs != 1) {
+        printf("Error at %s:%d: expecting numrecs 1 but got %lld\n",
+               __FILE__, __LINE__, numrecs);
+        nerrs++;
+    }
+
+    err = ncmpi_close(ncid); CHECK_ERR
+
+    /* read back and check the contents written by the non-empty processes */
+    err = ncmpi_open(MPI_COMM_WORLD, out_path, NC_NOWRITE, info, &ncid); CHECK_ERR
+    err = ncmpi_inq_varid(ncid, "v", &varid); CHECK_ERR
+
+    if (!is_empty) {
+        for (i=0; i<NDBL; i++) rbuf[i] = -1.0;
+        err = ncmpi_get_vara_double_all(ncid, varid, start, count, rbuf); CHECK_ERR
+        for (i=0; i<NDBL; i++) {
+            if (rbuf[i] != buf[i]) {
+                printf("Error at %s:%d: expecting v[%lld] %g but got %g\n",
+                       __FILE__, __LINE__, start[1]+i, buf[i], rbuf[i]);
+                nerrs++;
+                break;
+            }
+        }
+    }
+    else {
+        /* still a collective call, with nothing to read */
+        err = ncmpi_get_vara_double_all(ncid, varid, start, count, rbuf); CHECK_ERR
+    }
+
+    err = ncmpi_close(ncid); CHECK_ERR
+
+    return nerrs;
+}
+
+int main(int argc, char **argv) {
+
+    int err;
+    int formats[] = {NC_FORMAT_CLASSIC, NC_FORMAT_64BIT_OFFSET,
+                     NC_FORMAT_64BIT_DATA};
+    loop_opts opt;
+
+    MPI_Init(&argc, &argv);
+
+    opt.num_fmts = sizeof(formats) / sizeof(int);
+    opt.formats  = formats;
+    opt.ina      = 2;    /* enable and disable intra-node aggregation */
+    opt.drv      = 2;    /* test GIO and MPI-IO driver */
+    opt.ibuf     = 0;    /* test default hint nc_ibuf_size only */
+    opt.bb       = 2;    /* enable and disable burst-buffering feature */
+    opt.mod      = 2;    /* test both collective and independent data mode */
+    opt.hdr_diff = true; /* run ncmpidiff for file header */
+    opt.var_diff = false;/* skip ncmpidiff for variables */
+
+    err = tst_main(argc, argv, "zero-length request to a record variable", opt,
+                   test_io);
+
+    MPI_Finalize();
+    return err;
+}