From 46f4967f6e28cbf52ea157e53c34391ce9a4cc72 Mon Sep 17 00:00:00 2001
From: Julian Smith <jules@op59.net>
Date: Sun, 23 Aug 2020 20:11:14 +0100
Subject: [PATCH] Allow use of old zlib-1.2.3 on OpenBSD.

As of 2020-08-01, OpenBSD's system zlib is 1.2.3 which doesn't
have gzoffset(). However we can get away with this by making
gzfilebuf::approxOffset() always return zero.
---
 CMakeLists.txt                     | 9 ++++++++-
 simgear/io/iostreams/gzfstream.cxx | 5 ++++-
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 57a1d89b..55005a45 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -278,7 +278,14 @@ else()
     endif()
 endif(SIMGEAR_HEADLESS)
 
-find_package(ZLIB 1.2.4 REQUIRED)
+if(${CMAKE_SYSTEM_NAME} MATCHES "OpenBSD")
+    # As of 2020-08-01, OpenBSD's system zlib is slightly old, but it's usable
+    # with a workaround in simgear/io/iostreams/gzfstream.cxx.
+    find_package(ZLIB 1.2.3 REQUIRED)
+else()
+    find_package(ZLIB 1.2.4 REQUIRED)
+endif()
+
 find_package(CURL REQUIRED)
 
 if (SYSTEM_EXPAT)
diff --git a/simgear/io/iostreams/gzfstream.cxx b/simgear/io/iostreams/gzfstream.cxx
index 582f389c..da30a724 100644
--- a/simgear/io/iostreams/gzfstream.cxx
+++ b/simgear/io/iostreams/gzfstream.cxx
@@ -185,6 +185,9 @@ gzfilebuf::setcompressionstrategy( int comp_strategy )
 
 z_off_t
 gzfilebuf::approxOffset() {
+    #ifdef __OpenBSD__
+    z_off_t res = 0;
+    #else
     z_off_t res = gzoffset(file);
 
     if (res == -1) {
@@ -201,7 +204,7 @@ gzfilebuf::approxOffset() {
         SG_LOG( SG_GENERAL, SG_ALERT, errMsg );
         throw sg_io_exception(errMsg);
     }
-
+    #endif
     return res;
 }