Emscripten 1.37.16 to 1.37.36 (#37291)

* trying to build emscriptenPackages not all fail

* reading the console.log it turns out python executable is not in place and that is why emconfigure didnt work

* backup commit

* much more targets are compiling now

* added common revisioning

* revision bump to 1.37.36 (not tested)

* fixed xmllint

* forcing unit testing, will implement the tests after i get home

* json_c test working

* added tests

* tiny fixes

* added documentation

8 files changed, 340 insertions, 38 deletions

diff --git a/doc/default.nix b/doc/default.nix
index 60c613878c7..ec458634a42 100644
--- a/doc/default.nix
+++ b/doc/default.nix
@@ -81,6 +81,10 @@ pkgs.stdenv.mkDerivation {
       inputFile = ./languages-frameworks/vim.md;
       outputFile = "./languages-frameworks/vim.xml";
     }
+  + toDocbook {
+      inputFile = ./languages-frameworks/emscripten.md;
+      outputFile = "./languages-frameworks/emscripten.xml";
+    }
   + ''
     echo ${lib.nixpkgsVersion} > .version
 
diff --git a/doc/languages-frameworks/emscripten.md b/doc/languages-frameworks/emscripten.md
new file mode 100644
index 00000000000..24c49ec1409
--- /dev/null
+++ b/doc/languages-frameworks/emscripten.md
@@ -0,0 +1,185 @@
+# User's Guide to Emscripten in Nixpkgs
+
+[Emscripten](https://github.com/kripken/emscripten): An LLVM-to-JavaScript Compiler
+
+This section of the manual covers how to use `emscripten` in nixpkgs.
+
+Minimal requirements:
+
+* nix
+* nixpkgs
+
+Modes of use of `emscripten`:
+
+* **Imperative usage** (on the command line):
+
+   If you want to work with `emcc`, `emconfigure` and `emmake` as you are used to from Ubuntu and similar distributions you can use these commands:
+
+    * `nix-env -i emscripten`
+    * `nix-shell -p emscripten`
+
+* **Declarative usage**:
+
+    This mode is far more power full since this makes use of `nix` for dependency management of emscripten libraries and targets by using the `mkDerivation` which is implemented by `pkgs.emscriptenStdenv` and `pkgs.buildEmscriptenPackage`. The source for the packages is in `pkgs/top-level/emscripten-packages.nix` and the abstraction behind it in `pkgs/development/em-modules/generic/default.nix`.
+    * build and install all packages: 
+        * `nix-env -iA emscriptenPackages` 
+          
+    * dev-shell for zlib implementation hacking: 
+        * `nix-shell -A emscriptenPackages.zlib` 
+
+
+## Imperative usage
+
+A few things to note:
+
+* `export EMCC_DEBUG=2` is nice for debugging
+* `~/.emscripten`, the build artifact cache sometimes creates issues and needs to be removed from time to time
+
+
+## Declarative usage
+
+Let's see two different examples from `pkgs/top-level/emscripten-packages.nix`:
+
+* `pkgs.zlib.override`
+* `pkgs.buildEmscriptenPackage`
+
+Both are interesting concepts.
+
+A special requirement of the `pkgs.buildEmscriptenPackage` is the `doCheck = true` is a default meaning that each emscriptenPackage requires a `checkPhase` implemented.
+
+* Use `export EMCC_DEBUG=2` from within a emscriptenPackage's `phase` to get more detailed debug output what is going wrong.
+* ~/.emscripten cache is requiring us to set `HOME=$TMPDIR` in individual phases. This makes compilation slower but also makes it more deterministic.
+
+### Usage 1: pkgs.zlib.override
+
+This example uses `zlib` from nixpkgs but instead of compiling **C** to **ELF** it compiles **C** to **JS** since we were using `pkgs.zlib.override` and changed stdenv to `pkgs.emscriptenStdenv`. A few adaptions and hacks were set in place to make it working. One advantage is that when `pkgs.zlib` is updated, it will automatically update this package as well. However, this can also be the downside...
+
+See the `zlib` example:
+
+    zlib = (pkgs.zlib.override {
+      stdenv = pkgs.emscriptenStdenv;
+    }).overrideDerivation
+    (old: rec {
+      buildInputs = old.buildInputs ++ [ pkgconfig ];
+      # we need to reset this setting!
+      NIX_CFLAGS_COMPILE="";
+      configurePhase = ''
+        # FIXME: Some tests require writing at $HOME
+        HOME=$TMPDIR
+        runHook preConfigure
+
+        #export EMCC_DEBUG=2
+        emconfigure ./configure --prefix=$out --shared
+
+        runHook postConfigure
+      '';
+      dontStrip = true;
+      outputs = [ "out" ];
+      buildPhase = ''
+        emmake make
+      '';
+      installPhase = ''
+        emmake make install
+      '';
+      checkPhase = ''
+        echo "================= testing zlib using node ================="
+
+        echo "Compiling a custom test"
+        set -x
+        emcc -O2 -s EMULATE_FUNCTION_POINTER_CASTS=1 test/example.c -DZ_SOLO \
+        libz.so.${old.version} -I . -o example.js
+
+        echo "Using node to execute the test"
+        ${pkgs.nodejs}/bin/node ./example.js 
+
+        set +x
+        if [ $? -ne 0 ]; then
+          echo "test failed for some reason"
+          exit 1;
+        else
+          echo "it seems to work! very good."
+        fi
+        echo "================= /testing zlib using node ================="
+      '';
+
+      postPatch = pkgs.stdenv.lib.optionalString pkgs.stdenv.isDarwin ''
+        substituteInPlace configure \
+          --replace '/usr/bin/libtool' 'ar' \
+          --replace 'AR="libtool"' 'AR="ar"' \
+          --replace 'ARFLAGS="-o"' 'ARFLAGS="-r"'
+      '';
+    });
+
+### Usage 2: pkgs.buildEmscriptenPackage
+
+This `xmlmirror` example features a emscriptenPackage which is defined completely from this context and no `pkgs.zlib.override` is used. 
+
+    xmlmirror = pkgs.buildEmscriptenPackage rec {
+      name = "xmlmirror";
+
+      buildInputs = [ pkgconfig autoconf automake libtool gnumake libxml2 nodejs openjdk json_c ];
+      nativeBuildInputs = [ pkgconfig zlib ];
+
+      src = pkgs.fetchgit {
+        url = "https://gitlab.com/odfplugfest/xmlmirror.git";
+        rev = "4fd7e86f7c9526b8f4c1733e5c8b45175860a8fd";
+        sha256 = "1jasdqnbdnb83wbcnyrp32f36w3xwhwp0wq8lwwmhqagxrij1r4b";
+      };
+
+      configurePhase = ''
+        rm -f fastXmlLint.js*
+        # a fix for ERROR:root:For asm.js, TOTAL_MEMORY must be a multiple of 16MB, was 234217728
+        # https://gitlab.com/odfplugfest/xmlmirror/issues/8
+        sed -e "s/TOTAL_MEMORY=234217728/TOTAL_MEMORY=268435456/g" -i Makefile.emEnv
+        # https://github.com/kripken/emscripten/issues/6344
+        # https://gitlab.com/odfplugfest/xmlmirror/issues/9
+        sed -e "s/\$(JSONC_LDFLAGS) \$(ZLIB_LDFLAGS) \$(LIBXML20_LDFLAGS)/\$(JSONC_LDFLAGS) \$(LIBXML20_LDFLAGS) \$(ZLIB_LDFLAGS) /g" -i Makefile.emEnv
+        # https://gitlab.com/odfplugfest/xmlmirror/issues/11
+        sed -e "s/-o fastXmlLint.js/-s EXTRA_EXPORTED_RUNTIME_METHODS='[\"ccall\", \"cwrap\"]' -o fastXmlLint.js/g" -i Makefile.emEnv
+      '';
+
+      buildPhase = ''
+        HOME=$TMPDIR
+        make -f Makefile.emEnv
+      '';
+
+      outputs = [ "out" "doc" ];
+
+      installPhase = ''
+        mkdir -p $out/share
+        mkdir -p $doc/share/${name}
+
+        cp Demo* $out/share
+        cp -R codemirror-5.12 $out/share
+        cp fastXmlLint.js* $out/share
+        cp *.xsd $out/share
+        cp *.js $out/share
+        cp *.xhtml $out/share
+        cp *.html $out/share
+        cp *.json $out/share
+        cp *.rng $out/share
+        cp README.md $doc/share/${name}
+      '';
+      checkPhase = ''
+
+      '';
+    }; 
+
+### Declarative debugging
+
+Use `nix-shell -I nixpkgs=/some/dir/nixpkgs -A emscriptenPackages.libz` and from there you can go trough the individual steps. This makes it easy to build a good `unit test` or list the files of the project.
+
+1. `nix-shell -I nixpkgs=/some/dir/nixpkgs -A emscriptenPackages.libz`
+2. `cd /tmp/`
+3. `unpackPhase`
+4. cd libz-1.2.3
+5. `configurePhase`
+6. `buildPhase`
+7. ... happy hacking...
+
+## Summary
+
+Using this toolchain makes it easy to leverage `nix` from NixOS, MacOSX or even Windows (WSL+ubuntu+nix). This toolchain is reproducible, behaves like the rest of the packages from nixpkgs and contains a set of well working examples to learn and adapt from.
+
+If in trouble, ask the maintainers.
+
diff --git a/doc/languages-frameworks/index.xml b/doc/languages-frameworks/index.xml
index fc15d847d15..6743c131201 100644
--- a/doc/languages-frameworks/index.xml
+++ b/doc/languages-frameworks/index.xml
@@ -30,6 +30,7 @@ such as Perl or Haskell.  These are described in this chapter.</para>
 <xi:include href="rust.xml" />
 <xi:include href="texlive.xml" />
 <xi:include href="vim.xml" />
+<xi:include href="emscripten.xml" />
 
 
 </chapter>
diff --git a/pkgs/development/compilers/emscripten-fastcomp/emscripten-fastcomp.nix b/pkgs/development/compilers/emscripten-fastcomp/emscripten-fastcomp.nix
index f35ca26584f..5317cf25d41 100644
--- a/pkgs/development/compilers/emscripten-fastcomp/emscripten-fastcomp.nix
+++ b/pkgs/development/compilers/emscripten-fastcomp/emscripten-fastcomp.nix
@@ -1,7 +1,7 @@
-{ stdenv, fetchFromGitHub, cmake, python, ... }:
+{ emscriptenVersion, stdenv, llvm, fetchFromGitHub, cmake, python, gtest, ... }:
 
 let
-  rev = "1.37.16";
+  rev = emscriptenVersion;
   gcc = if stdenv.cc.isGNU then stdenv.cc.cc else stdenv.cc.cc.gcc;
 in
 stdenv.mkDerivation rec {
@@ -10,18 +10,18 @@ stdenv.mkDerivation rec {
   src = fetchFromGitHub {
     owner = "kripken";
     repo = "emscripten-fastcomp";
-    sha256 = "0wj9sc0gciaiidcjv6wb0qn6ks06xds7q34351masc7qpvd217by";
+    sha256 = "04j698gmp686b5lricjakm5hyh2z2kh28m1ffkghmkyz4zkzmx98";
     inherit rev;
   };
 
   srcFL = fetchFromGitHub {
     owner = "kripken";
     repo = "emscripten-fastcomp-clang";
-    sha256 = "1akdgxzxhzjbhp4d14ajcrp9jrf39x004a726ly2gynqc185l4j7";
+    sha256 = "1ici51mmpgg80xk3y8f376nbbfak6rz27qdy98l8lxkrymklp5g5";
     inherit rev;
   };
 
-  nativeBuildInputs = [ cmake python ];
+  nativeBuildInputs = [ cmake python gtest ];
   preConfigure = ''
     cp -Lr ${srcFL} tools/clang
     chmod +w -R tools/clang
@@ -30,9 +30,10 @@ stdenv.mkDerivation rec {
     "-DCMAKE_BUILD_TYPE=Release"
     "-DLLVM_TARGETS_TO_BUILD='X86;JSBackend'"
     "-DLLVM_INCLUDE_EXAMPLES=OFF"
-    "-DLLVM_INCLUDE_TESTS=OFF"
-    # "-DCLANG_INCLUDE_EXAMPLES=OFF"
-    "-DCLANG_INCLUDE_TESTS=OFF"
+    "-DLLVM_INCLUDE_TESTS=ON"
+    #"-DLLVM_CONFIG=${llvm}/bin/llvm-config"
+    "-DLLVM_BUILD_TESTS=ON"
+    "-DCLANG_INCLUDE_TESTS=ON"
   ] ++ (stdenv.lib.optional stdenv.isLinux
     # necessary for clang to find crtend.o
     "-DGCC_INSTALL_PREFIX=${gcc}"
diff --git a/pkgs/development/compilers/emscripten/default.nix b/pkgs/development/compilers/emscripten/default.nix
index ae80ad6fc80..ac972c99a95 100644
--- a/pkgs/development/compilers/emscripten/default.nix
+++ b/pkgs/development/compilers/emscripten/default.nix
@@ -1,9 +1,9 @@
-{ stdenv, fetchFromGitHub, emscriptenfastcomp, python, nodejs, closurecompiler
-, jre, binaryen, enableWasm ? true
+{ emscriptenVersion, stdenv, fetchFromGitHub, emscriptenfastcomp, python, nodejs, closurecompiler, pkgs
+, jre, binaryen, enableWasm ? true ,  python2Packages, cmake
 }:
 
 let
-  rev = "1.37.16";
+  rev = emscriptenVersion;
   appdir = "share/emscripten";
 in
 
@@ -13,10 +13,12 @@ stdenv.mkDerivation {
   src = fetchFromGitHub {
     owner = "kripken";
     repo = "emscripten";
-    sha256 = "1qyhjx5zza01vnwmj6qzxbkagxknn4kzb6gw12fqw5q8pa8fy4zy";
+    sha256 = "02p0cp86vd1mydlpq544xbydggpnrq9dhbxx7h08j235frjm5cdc";
     inherit rev;
   };
 
+  buildInputs = [ nodejs cmake python ];
+
   buildCommand = ''
     mkdir -p $out/${appdir}
     cp -r $src/* $out/${appdir}
@@ -38,9 +40,23 @@ stdenv.mkDerivation {
     echo "COMPILER_ENGINE = NODE_JS" >> $out/${appdir}/config
     echo "CLOSURE_COMPILER = '${closurecompiler}/share/java/closure-compiler-v${closurecompiler.version}.jar'" >> $out/${appdir}/config
     echo "JAVA = '${jre}/bin/java'" >> $out/${appdir}/config
+    # to make the test(s) below work
+    echo "SPIDERMONKEY_ENGINE = []" >> $out/${appdir}/config
   ''
   + stdenv.lib.optionalString enableWasm ''
     echo "BINARYEN_ROOT = '${binaryen}'" >> $out/share/emscripten/config
+  ''
+  +
+  ''
+    echo "--------------- running test -----------------"
+    # quick hack to get the test working
+    HOME=$TMPDIR
+    cp $out/${appdir}/config $HOME/.emscripten
+    export PATH=$PATH:$out/bin
+
+    #export EMCC_DEBUG=2  
+    ${python}/bin/python $src/tests/runner.py test_hello_world
+    echo "--------------- /running test -----------------"
   '';
 
   meta = with stdenv.lib; {
diff --git a/pkgs/development/em-modules/generic/default.nix b/pkgs/development/em-modules/generic/default.nix
index 16b8f1df595..332fab8e14a 100644
--- a/pkgs/development/em-modules/generic/default.nix
+++ b/pkgs/development/em-modules/generic/default.nix
@@ -1,4 +1,4 @@
-{ pkgs, lib, emscripten }:
+{ pkgs, lib, emscripten, python }:
 
 { buildInputs ? [], nativeBuildInputs ? []
 
@@ -11,8 +11,8 @@ pkgs.stdenv.mkDerivation (
   {
 
   name = "emscripten-${args.name}";
-  buildInputs = [ emscripten ] ++ buildInputs;
-  nativeBuildInputs = [ emscripten ] ++ nativeBuildInputs;
+  buildInputs = [ emscripten python ] ++ buildInputs;
+  nativeBuildInputs = [ emscripten python ] ++ nativeBuildInputs;
 
   # fake conftest results with emscripten's python magic
   EMCONFIGURE_JS=2;
@@ -38,9 +38,18 @@ pkgs.stdenv.mkDerivation (
     runHook postBuild
   '';
 
+  doCheck = true;
+
   checkPhase = args.checkPhase or ''
     runHook preCheck
 
+    echo "Please provide a test for your emscripten based library/tool, see libxml2 as an exmple on how to use emcc/node to verify your build"
+    echo ""
+    echo "In normal C 'unresolved symbols' would yield an error and a breake of execution. In contrast, in emscripten they are only a warning which is ok given that emscripten assumptions about shared libraries."
+    echo "  -> https://github.com/kripken/emscripten/wiki/Linking"
+    echo "So just assume the dependencies were built using hydra, then YOU WILL NEVER see the warning and your code depending on a library will always fail!"
+    exit 1
+
     runHook postCheck
   '';
 
diff --git a/pkgs/top-level/all-packages.nix b/pkgs/top-level/all-packages.nix
index 0676d59d25a..054febb6223 100644
--- a/pkgs/top-level/all-packages.nix
+++ b/pkgs/top-level/all-packages.nix
@@ -2079,6 +2079,8 @@ with pkgs;
 
   buildEmscriptenPackage = callPackage ../development/em-modules/generic { };
 
+  emscriptenVersion = "1.37.36";
+
   emscripten = callPackage ../development/compilers/emscripten { };
 
   emscriptenfastcompPackages = callPackage ../development/compilers/emscripten-fastcomp { };
diff --git a/pkgs/top-level/emscripten-packages.nix b/pkgs/top-level/emscripten-packages.nix
index 7e63041867d..5b5b32b89da 100644
--- a/pkgs/top-level/emscripten-packages.nix
+++ b/pkgs/top-level/emscripten-packages.nix
@@ -1,49 +1,92 @@
 { pkgs }:
 
+with pkgs;
+
 # emscripten toolchain abstraction for nix
 # https://github.com/NixOS/nixpkgs/pull/16208
 
-with pkgs; rec {
-  json_c = pkgs.json_c.override {
-    stdenv = emscriptenStdenv;
-  };
+rec {
+  json_c = (pkgs.json_c.override {
+    stdenv = pkgs.emscriptenStdenv;
+  }).overrideDerivation
+    (old: {
+      nativeBuildInputs = [ autoreconfHook pkgconfig ];
+      propagatedBuildInputs = [ zlib ];
+      buildInputs = old.buildInputs ++ [ automake autoconf ];
+      configurePhase = ''
+        HOME=$TMPDIR
+        emconfigure ./configure --prefix=$out 
+      '';
+      checkPhase = ''
+        echo "================= testing json_c using node ================="
+
+        echo "Compiling a custom test"
+        set -x
+        emcc -O2 -s EMULATE_FUNCTION_POINTER_CASTS=1 tests/test1.c \
+          `pkg-config zlib --cflags` \
+          `pkg-config zlib --libs` \
+          -I . \
+          .libs/libjson-c.so \
+          -o ./test1.js
+
+        echo "Using node to execute the test which basically outputs an error on stderr which we grep for" 
+        ${pkgs.nodejs}/bin/node ./test1.js 
+
+        set +x
+        if [ $? -ne 0 ]; then
+          echo "test1.js execution failed -> unit test failed, please fix"
+          exit 1;
+        else
+          echo "test1.js execution seems to work! very good."
+        fi
+        echo "================= /testing json_c using node ================="
+      '';
+    });
   
   libxml2 = (pkgs.libxml2.override {
     stdenv = emscriptenStdenv;
     pythonSupport = false;
   }).overrideDerivation
     (old: { 
-      nativeBuildInputs = old.nativeBuildInputs ++ [ autoreconfHook pkgconfig ];
-      buildInputs = old.buildInputs ++ [ zlib nodejs ];
+      propagatedBuildInputs = [ zlib ];
+      buildInputs = old.buildInputs ++ [ pkgconfig ];
+
       # just override it with nothing so it does not fail
       autoreconfPhase = "echo autoreconfPhase not used..."; 
+      configurePhase = ''
+        HOME=$TMPDIR
+        emconfigure ./configure --prefix=$out --without-python
+      '';
       checkPhase = ''
-        echo "================= testing xmllint using node ================="
+        echo "================= testing libxml2 using node ================="
+
+        echo "Compiling a custom test"
+        set -x
         emcc -O2 -s EMULATE_FUNCTION_POINTER_CASTS=1 xmllint.o \
         ./.libs/libxml2.a `pkg-config zlib --cflags` `pkg-config zlib --libs` -o ./xmllint.test.js \
         --embed-file ./test/xmlid/id_err1.xml  
-        # test/xmlid/id_err2.xml:3: validity error : xml:id : attribute type should be ID
-        # <!ATTLIST foo xml:id CDATA #IMPLIED>
-        #                                    ^
-        node ./xmllint.test.js --noout test/xmlid/id_err1.xml 2>&1  | grep 0bar   
+
+        echo "Using node to execute the test which basically outputs an error on stderr which we grep for" 
+        ${pkgs.nodejs}/bin/node ./xmllint.test.js --noout test/xmlid/id_err1.xml 2>&1 | grep 0bar   
+
+        set +x
         if [ $? -ne 0 ]; then
           echo "xmllint unit test failed, please fix this package"
           exit 1;
         else
           echo "since there is no stupid text containing 'foo xml:id' it seems to work! very good."
         fi
-        echo "================= /testing xmllint using node ================="
+        echo "================= /testing libxml2 using node ================="
       '';
     });            
   
-  xmlmirror = buildEmscriptenPackage rec {
+  xmlmirror = pkgs.buildEmscriptenPackage rec {
     name = "xmlmirror";
 
-  nativeBuildInputs = [ pkgconfig ];
-    buildInputs = [ autoconf automake libtool gnumake libxml2 nodejs 
-       python openjdk json_c zlib ];
+    buildInputs = [ pkgconfig autoconf automake libtool gnumake libxml2 nodejs openjdk json_c ];
+    nativeBuildInputs = [ pkgconfig zlib ];
 
-    src = fetchgit {
+    src = pkgs.fetchgit {
       url = "https://gitlab.com/odfplugfest/xmlmirror.git";
       rev = "4fd7e86f7c9526b8f4c1733e5c8b45175860a8fd";
       sha256 = "1jasdqnbdnb83wbcnyrp32f36w3xwhwp0wq8lwwmhqagxrij1r4b";
@@ -51,6 +94,14 @@ with pkgs; rec {
      
     configurePhase = ''
       rm -f fastXmlLint.js*
+      # a fix for ERROR:root:For asm.js, TOTAL_MEMORY must be a multiple of 16MB, was 234217728
+      # https://gitlab.com/odfplugfest/xmlmirror/issues/8
+      sed -e "s/TOTAL_MEMORY=234217728/TOTAL_MEMORY=268435456/g" -i Makefile.emEnv
+      # https://github.com/kripken/emscripten/issues/6344
+      # https://gitlab.com/odfplugfest/xmlmirror/issues/9
+      sed -e "s/\$(JSONC_LDFLAGS) \$(ZLIB_LDFLAGS) \$(LIBXML20_LDFLAGS)/\$(JSONC_LDFLAGS) \$(LIBXML20_LDFLAGS) \$(ZLIB_LDFLAGS) /g" -i Makefile.emEnv
+      # https://gitlab.com/odfplugfest/xmlmirror/issues/11
+      sed -e "s/-o fastXmlLint.js/-s EXTRA_EXPORTED_RUNTIME_METHODS='[\"ccall\", \"cwrap\"]' -o fastXmlLint.js/g" -i Makefile.emEnv
     '';
     
     buildPhase = ''
@@ -75,25 +126,58 @@ with pkgs; rec {
       cp *.rng $out/share
       cp README.md $doc/share/${name}
     '';
-    
-    postInstall = ''
+    checkPhase = ''
+      
     '';
   };  
 
   zlib = (pkgs.zlib.override {
-    stdenv = emscriptenStdenv;
+    stdenv = pkgs.emscriptenStdenv;
   }).overrideDerivation
-    (old: { 
+    (old: rec { 
+      buildInputs = old.buildInputs ++ [ pkgconfig ];
+      # we need to reset this setting!
+      NIX_CFLAGS_COMPILE="";
       configurePhase = ''
         # FIXME: Some tests require writing at $HOME
         HOME=$TMPDIR
         runHook preConfigure
 
-        emconfigure ./configure --prefix=$out
+        #export EMCC_DEBUG=2
+        emconfigure ./configure --prefix=$out --shared
 
         runHook postConfigure
       '';
-      postPatch = stdenv.lib.optionalString stdenv.isDarwin ''
+      dontStrip = true;
+      outputs = [ "out" ];
+      buildPhase = ''
+        emmake make
+      '';
+      installPhase = ''
+        emmake make install
+      '';
+      checkPhase = ''
+        echo "================= testing zlib using node ================="
+
+        echo "Compiling a custom test"
+        set -x
+        emcc -O2 -s EMULATE_FUNCTION_POINTER_CASTS=1 test/example.c -DZ_SOLO \
+        libz.so.${old.version} -I . -o example.js
+
+        echo "Using node to execute the test"
+        ${pkgs.nodejs}/bin/node ./example.js 
+
+        set +x
+        if [ $? -ne 0 ]; then
+          echo "test failed for some reason"
+          exit 1;
+        else
+          echo "it seems to work! very good."
+        fi
+        echo "================= /testing zlib using node ================="
+      '';
+
+      postPatch = pkgs.stdenv.lib.optionalString pkgs.stdenv.isDarwin ''
         substituteInPlace configure \
           --replace '/usr/bin/libtool' 'ar' \
           --replace 'AR="libtool"' 'AR="ar"' \