From 9ecf9141da64954e5288bb4e7b2332569f364b65 Mon Sep 17 00:00:00 2001
From: Paul Bauer <paul.bauer.q@gmail.com>
Date: Wed, 20 Jan 2021 06:10:40 +0000
Subject: [PATCH] Fix TPR handling for older versions.

Reading in a newer TPR file in a pre-2021 version would fail hard with
symtab errors due to us forgetting to increment the tpr generation when
adding the VSite1 type to ftupd.

This fixes the omission an makes the tpr generation an enum similar to
the tpr version.

Fixes #3867
---
 src/gromacs/fileio/tpxio.cpp | 27 +++++++++++++++++++++------
 src/gromacs/fileio/tpxio.h   |  5 ++++-
 2 files changed, 25 insertions(+), 7 deletions(-)

diff --git a/src/gromacs/fileio/tpxio.cpp b/src/gromacs/fileio/tpxio.cpp
index 9ea2337f86..2d87034417 100644
--- a/src/gromacs/fileio/tpxio.cpp
+++ b/src/gromacs/fileio/tpxio.cpp
@@ -4,7 +4,7 @@
  * Copyright (c) 1991-2000, University of Groningen, The Netherlands.
  * Copyright (c) 2001-2004, The GROMACS development team.
  * Copyright (c) 2013,2014,2015,2016,2017 by the GROMACS development team.
- * Copyright (c) 2018,2019,2020, by the GROMACS development team, led by
+ * Copyright (c) 2018,2019,2020,2021, by the GROMACS development team, led by
  * Mark Abraham, David van der Spoel, Berk Hess, and Erik Lindahl,
  * and including many others, as listed in the AUTHORS file in the
  * top-level source directory and at http://www.gromacs.org.
@@ -151,8 +151,13 @@ enum tpxv
 static const int tpx_version = tpxv_Count - 1;
 
 
-/* This number should only be increased when you edit the TOPOLOGY section
- * or the HEADER of the tpx format.
+/*! \brief
+ * Enum keeping track of incompatible changes for older TPR versions.
+ *
+ * The enum should be updated with a new field when editing the TOPOLOGY
+ * or HEADER of the tpx format. In particular, updating ftupd or
+ * changing the fields of TprHeaderVersion often trigger such needs.
+ *
  * This way we can maintain forward compatibility too for all analysis tools
  * and/or external programs that only need to know the atom/residue names,
  * charges, and bond connectivity.
@@ -163,10 +168,17 @@ static const int tpx_version = tpxv_Count - 1;
  *
  * In particular, it must be increased when adding new elements to
  * ftupd, so that old code can read new .tpr files.
- *
- * Updated for added field that contains the number of bytes of the tpr body, excluding the header.
  */
-static const int tpx_generation = 27;
+enum class TpxGeneration : int
+{
+    Initial = 26, //! First version is 26
+    AddSizeField, //! TPR header modified for writing as a block.
+    AddVSite1,    //! ftupd changed to include VSite1 type.
+    Count         //! Number of entries.
+};
+
+//! Value of Current TPR generation.
+static const int tpx_generation = static_cast<int>(TpxGeneration::Count) - 1;
 
 /* This number should be the most recent backwards incompatible version
  * I.e., if this number is 9, we cannot read tpx version 9 with this code.
@@ -194,6 +206,9 @@ typedef struct
  * obsolete t_interaction_function types. Any data read from such
  * fields is discarded. Their names have _NOLONGERUSED appended to
  * them to make things clear.
+ *
+ * When adding to or making breaking changes to reading this struct,
+ * update TpxGeneration.
  */
 static const t_ftupd ftupd[] = {
     { 70, F_RESTRBONDS },
diff --git a/src/gromacs/fileio/tpxio.h b/src/gromacs/fileio/tpxio.h
index 5fc0bea2bc..08229c62a4 100644
--- a/src/gromacs/fileio/tpxio.h
+++ b/src/gromacs/fileio/tpxio.h
@@ -4,7 +4,7 @@
  * Copyright (c) 1991-2000, University of Groningen, The Netherlands.
  * Copyright (c) 2001-2004, The GROMACS development team.
  * Copyright (c) 2013,2014,2015,2016,2017 by the GROMACS development team.
- * Copyright (c) 2018,2019,2020, by the GROMACS development team, led by
+ * Copyright (c) 2018,2019,2020,2021, by the GROMACS development team, led by
  * Mark Abraham, David van der Spoel, Berk Hess, and Erik Lindahl,
  * and including many others, as listed in the AUTHORS file in the
  * top-level source directory and at http://www.gromacs.org.
@@ -63,6 +63,9 @@ class ArrayRef;
  * \brief
  * First part of the TPR file structure containing information about
  * the general aspect of the system.
+ *
+ * When adding to or making breaking changes to reading this struct,
+ * update TpxGeneration.
  */
 struct TpxFileHeader
 {
-- 
2.22.0