ENH: add UPstream interface for MPI/IO

- supports writing, but without overlapping (begin/end) semantics.

applications/test/parallel-file-write1/Test-parallel-file-write1.cxx

@@ -0,0 +1,260 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2025 Mark Olesen
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+Application
+    Test-parallel-file-write1
+
+Description
+    Simple test of writing with MPI/IO
+
+\*---------------------------------------------------------------------------*/
+
+#include "argList.H"
+#include "Time.H"
+#include "Switch.H"
+#include "UPstreamFile.H"
+#include "SpanStream.H"
+
+using namespace Foam;
+
+template<class IntType>
+void zeropadded(std::ostream& os, IntType val, char fillch = '0')
+{
+    // set fill char and width
+    os.setf(std::ios_base::right, std::ios_base::adjustfield);
+    fillch = os.fill(fillch);
+    os.width(std::numeric_limits<IntType>::digits10+1);
+    os  << val;
+    // restore fill char
+    os.fill(fillch);
+}
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+int main(int argc, char *argv[])
+{
+    argList::noCheckProcessorDirectories();
+    argList::addVerboseOption();
+    argList::addBoolOption("master-footer", "Write footer from master");
+
+    #include "setRootCase.H"
+
+    if (!UPstream::parRun())
+    {
+        Info<< "###############" << nl
+            << "Not running in parallel. Stopping now" << nl
+            << "###############" << endl;
+        return 1;
+    }
+
+    const bool optMasterFooter = args.found("master-footer");
+
+    Info<< nl << "Write master-footer: " << Switch::name(optMasterFooter)
+        << nl << nl;
+
+    Info<< "Create time (without controlDict)\n" << endl;
+
+    auto runTimePtr = Time::New();
+    auto& runTime = runTimePtr();
+
+    const auto myProc = UPstream::myProcNo();
+    const auto nProcs = UPstream::nProcs();
+
+    // Some content
+    OCharStream charset;
+    for (int i = 0; i < 10; ++i)
+    {
+        charset<< char('A' + i);
+    }
+
+    // Header/footer buffers - these can be separate or bundled into
+    // the first/last blocks
+
+    OCharStream header;
+    OCharStream footer;
+
+    // Content buffer
+    OCharStream os(IOstream::BINARY);
+
+    {
+        const auto v = charset.view();
+
+        os << nl;
+        os.beginBlock(word("rank" + Foam::name(myProc)));
+
+        for (int repeat = 0;  repeat <= myProc; ++repeat)
+        {
+            os << indent << word("entry" + Foam::name(repeat))
+                << ' ' << word("List<char>");
+            // os << nl;
+            os << ' ';
+            os << label(v.size());
+            os.write(v.data(), v.size());
+            // os << nl;
+            os.endEntry();
+        }
+
+        os.endBlock();
+    }
+
+    // Bundle the footer into the last block
+    if (!optMasterFooter && (myProc == nProcs-1))
+    {
+        IOobject::writeEndDivider(os);
+    }
+
+
+    // All content now exists - commit to disk
+    const std::string_view blockData(os.view());
+    const int64_t blockSize(blockData.size());
+
+    // Collect sizes
+    const List<int64_t> sizes
+    (
+        UPstream::allGatherValues(blockSize, UPstream::worldComm)
+    );
+
+
+    // Format header with size information
+    if (UPstream::master())
+    {
+        header
+            << "Simple MPI/IO test with " << nProcs << " ranks" << nl << nl;
+
+        ocharstream labelbuf;
+        labelbuf.reserve_exact(32);
+
+        // Position before writing a label
+        auto labelBegin = header.tellp();
+
+        header.beginBlock("meta");
+        {
+            header << indent << word("data.start") << ' ';
+
+            labelBegin = header.tellp();
+
+            // Add the start value (placeholder)
+            {
+                labelbuf.rewind();
+                zeropadded(labelbuf, label(0));
+                header.append(labelbuf.view());
+            }
+
+            header.endEntry();
+
+            header << indent << word("data.sizes") << nl;
+            sizes.writeList(header);  // flatOutput
+            header.endEntry();
+        }
+        header.endBlock();
+
+        header << nl;
+        IOobject::writeDivider(header);
+
+        // Now update with the correct size
+        {
+            labelbuf.rewind();
+            zeropadded(labelbuf, label(header.view().size()));
+            header.overwrite(labelBegin, labelbuf.view());
+        }
+
+        // Bundled the footer into the last block or from master?
+        if (optMasterFooter)
+        {
+            IOobject::writeEndDivider(footer);
+        }
+    }
+
+    // With additional header/footer
+    int64_t headerSize(header.view().size());
+    int64_t footerSize(footer.view().size());
+
+    Pstream::broadcast(headerSize);
+    if (optMasterFooter)
+    {
+        Pstream::broadcast(footerSize);
+    }
+
+
+    int64_t totalSize(headerSize);
+    for (int i = 0; i < myProc; ++i)
+    {
+        totalSize += sizes[i];
+    }
+
+    const int64_t blockOffset(totalSize);
+
+    for (int i = myProc; i < nProcs; ++i)
+    {
+        totalSize += sizes[i];
+    }
+    const int64_t footerOffset(totalSize);
+    totalSize += footerSize;
+
+
+    Pout<< "write size=" << label(blockSize)
+        << " at=" << label(blockOffset) << " total=" << label(totalSize) << nl;
+
+    {
+        UPstream::File file;
+
+        bool ok = file.open_write
+        (
+            UPstream::worldComm,
+            runTime.globalPath()/"mpiio-test1.txt",
+            IOstreamOption::ATOMIC
+        );
+
+        if (ok)
+        {
+            Info<< "writing: " << file.name() << nl;
+
+            if (UPstream::master())
+            {
+                // A no-op for empty buffer
+                ok = file.write_at(0, header.view());
+            }
+
+            ok = file.write_at_all(blockOffset, blockData);
+
+            if (UPstream::master())
+            {
+                // A no-op for empty buffer
+                ok = file.write_at(footerOffset, footer.view());
+            }
+        }
+
+        file.set_size(totalSize);
+        file.close();
+    }
+
+    Info<< "\nEnd\n" << endl;
+
+    return 0;
+}
+
+
+// ************************************************************************* //