From 371b1a80e3e603607df19e783aa0c003af18b070 Mon Sep 17 00:00:00 2001
From: Axel Kohlmeyer <akohlmey@gmail.com>
Date: Thu, 19 Nov 2020 18:58:21 -0500
Subject: [PATCH] add support for '-in none' for multi-partition runs from the
 library interface

---
 doc/src/Run_options.rst                 | 19 +++++++++++--------
 src/input.cpp                           |  7 +++++++
 src/lammps.cpp                          | 13 ++++++++-----
 unittest/c-library/test_library_mpi.cpp |  5 +----
 4 files changed, 27 insertions(+), 17 deletions(-)

diff --git a/doc/src/Run_options.rst b/doc/src/Run_options.rst
index f5435ec3a9..9c269e552f 100644
--- a/doc/src/Run_options.rst
+++ b/doc/src/Run_options.rst
@@ -62,15 +62,18 @@ used.
 
 **-in file**
 
-Specify a file to use as an input script.  This is an optional switch
-when running LAMMPS in one-partition mode.  If it is not specified,
-LAMMPS reads its script from standard input, typically from a script
-via I/O redirection; e.g. lmp_linux < in.run.  I/O redirection should
-also work in parallel, but if it does not (in the unlikely case that
-an MPI implementation does not support it), then use the -in flag.
+Specify a file to use as an input script.  This is an optional but
+recommended switch when running LAMMPS in one-partition mode.  If it
+is not specified, LAMMPS reads its script from standard input, typically
+from a script via I/O redirection; e.g. lmp_linux < in.run.
+With many MPI implementations I/O redirection also works in parallel,
+but using the -in flag will always work.
+
 Note that this is a required switch when running LAMMPS in
 multi-partition mode, since multiple processors cannot all read from
-stdin.
+stdin concurrently.  The file name may be "none" for starting
+multi-partition calculations without reading an initial input file
+from the library interface.
 
 ----------
 
@@ -296,7 +299,7 @@ command-line option.
 **-pscreen file**
 
 Specify the base name for the partition screen file, so partition N
-writes screen information to file.N. If file is none, then no
+writes screen information to file.N. If file is "none", then no
 partition screen files are created.  This overrides the filename
 specified in the -screen command-line option.  This option is useful
 when working with large numbers of partitions, allowing the partition
diff --git a/src/input.cpp b/src/input.cpp
index c907a16c9d..abdc3775ce 100644
--- a/src/input.cpp
+++ b/src/input.cpp
@@ -189,8 +189,15 @@ void Input::file()
     // if line ends in continuation char '&', concatenate next line
 
     if (me == 0) {
+
       m = 0;
       while (1) {
+
+        if (infile == nullptr) {
+          n = 0;
+          break;
+        }
+
         if (maxline-m < 2) reallocate(line,maxline,0);
 
         // end of file reached, so break
diff --git a/src/lammps.cpp b/src/lammps.cpp
index ff0df97105..102d2f18cf 100644
--- a/src/lammps.cpp
+++ b/src/lammps.cpp
@@ -456,6 +456,7 @@ LAMMPS::LAMMPS(int narg, char **arg, MPI_Comm communicator) :
 
     if (universe->me == 0) {
       if (inflag == 0) infile = stdin;
+      else if (strcmp(arg[inflag], "none") == 0) infile = stdin;
       else infile = fopen(arg[inflag],"r");
       if (infile == nullptr)
         error->one(FLERR,fmt::format("Cannot open input script {}: {}",
@@ -530,10 +531,12 @@ LAMMPS::LAMMPS(int narg, char **arg, MPI_Comm communicator) :
                                        str, utils::getsyserror()));
       }
 
-      infile = fopen(arg[inflag],"r");
-      if (infile == nullptr)
-        error->one(FLERR,fmt::format("Cannot open input script {}: {}",
-                                     arg[inflag], utils::getsyserror()));
+      if (strcmp(arg[inflag], "none") != 0) {
+        infile = fopen(arg[inflag],"r");
+        if (infile == nullptr)
+          error->one(FLERR,fmt::format("Cannot open input script {}: {}",
+                                       arg[inflag], utils::getsyserror()));
+      }
     }
 
     // screen and logfile messages for universe and world
@@ -1103,7 +1106,7 @@ void _noopt LAMMPS::help()
           "List of command line options supported by this LAMMPS executable:\n\n"
           "-echo none/screen/log/both  : echoing of input script (-e)\n"
           "-help                       : print this help message (-h)\n"
-          "-in filename                : read input from file, not stdin (-i)\n"
+          "-in none/filename           : read input from file or stdin (default) (-i)\n"
           "-kokkos on/off ...          : turn KOKKOS mode on or off (-k)\n"
           "-log none/filename          : where to send log output (-l)\n"
           "-mpicolor color             : which exe in a multi-exe mpirun cmd (-m)\n"
diff --git a/unittest/c-library/test_library_mpi.cpp b/unittest/c-library/test_library_mpi.cpp
index 029af72c84..7936ee45ad 100644
--- a/unittest/c-library/test_library_mpi.cpp
+++ b/unittest/c-library/test_library_mpi.cpp
@@ -156,11 +156,9 @@ TEST(MPI, multi_partition)
     lammps_mpi_init();
     MPI_Comm_size(MPI_COMM_WORLD, &nprocs);
     MPI_Comm_rank(MPI_COMM_WORLD, &me);
-    fp = fopen("in.mp_dummy", "w");
-    fclose(fp);
 
     const char *args[] = {"LAMMPS_test", "-log",   "none",    "-partition", "2x2",
-                          "-echo",       "screen", "-nocite", "-in",        "in.mp_dummy"};
+                          "-echo",       "screen", "-nocite", "-in",        "none"};
     char **argv        = (char **)args;
     int argc           = sizeof(args) / sizeof(char *);
     void *lmp          = lammps_open(argc, argv, MPI_COMM_WORLD, nullptr);
@@ -184,7 +182,6 @@ TEST(MPI, multi_partition)
     }
 
     lammps_close(lmp);
-    unlink("in.mp_dummy");
 };
 
 class MPITest : public ::testing::Test {