nixpkgs/pkgs/servers/foundationdb/patches/clang-libcxx.patch

commit 7ed4745a092a203f92fc37ab5894e92117db0c94
Author: Austin Seipp <aseipp@pobox.com>
Date:   Sat May 4 15:23:35 2019 -0500

    flow: fix a build failure with Clang/libcxx on Linux
    
    11bd7d7da introduced a hack on Linux to work around a missing symbol in
    libstdc++'s _pic library on Ubuntu. Unfortunately, this causes the build
    to fail when using Clang, as it doesn't believe this symbol is part of
    its headers in c++11 mode.
    
    Unfortunately there's no good way to distinguish libcxx from libstdc++
    with the preprocessor, so we merely gate it by only checking for clang,
    iff we are on Linux.
    
    With this change, Clang 8.x can build FoundationDB on Linux using libcxx
    as the standard C++ library.
    
    Signed-off-by: Austin Seipp <aseipp@pobox.com>

diff --git a/flow/Platform.cpp b/flow/Platform.cpp
index 3d3f1ac0..9f21dfd4 100644
--- a/flow/Platform.cpp
+++ b/flow/Platform.cpp
@@ -2841,13 +2841,26 @@ void setupSlowTaskProfiler() {
 #endif
 }
 
-#ifdef __linux__
+#if defined(__linux__) && !defined(__clang__)
 // There's no good place to put this, so it's here.
 // Ubuntu's packaging of libstdc++_pic offers different symbols than libstdc++.  Go figure.
 // Notably, it's missing a definition of std::istream::ignore(long), which causes compilation errors
 // in the bindings.  Thus, we provide weak versions of their definitions, so that if the
 // linked-against libstdc++ is missing their definitions, we'll be able to use the provided
 // ignore(long, int) version.
+//
+// Note that this hack is DISABLED when we use Clang. It is only needed when we statically link
+// to the _pic libraries, but only official FDB Linux binaries are built this way using GCC. If we
+// don't use the _pic libraries, then this hack is entirely unneeded -- likely the case when using
+// Clang on Linux.
+//
+// Doing this allows us to use LLVM's libc++ with Clang on Linux -- otherwise, providing
+// a weak symbol definition for an internal (non-public) class member fails (due to that member
+// being non-existant on libc++.) See upstream GitHub issue #1533 for more information.
+//
+// TODO FIXME: Obliterate this when the official build environment is upgraded beyond Ubuntu 14.04.
+// (This problem should be fixed in later LTS releases.)
+
 #include <istream>
 namespace std {
 typedef basic_istream<char, std::char_traits<char>> char_basic_istream;