autodetecting libpng installation on macos

setup.py

@@ -66,6 +66,7 @@ import platform
 from distutils.errors import CompileError
 import distutils.command.build_ext
 import setuptools.command.build_ext
+from setuptools.command.build_ext import build_ext as _build_ext
 import multiprocessing
 
 # for Jenkins we do not want to be greedy
@@ -137,6 +138,24 @@ def quote_path(path):
     else:
         return path
 
+import subprocess
+
+def check_libpng():
+    """ Check if libpng is available (Linux & Macos only)"""
+    try:
+        subprocess.check_call(["libpng-config", "--version"], stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+        return True
+    except FileNotFoundError as e:
+        # libpng is not present.
+        if True: # change to False to enable installation without libpng
+            raise RuntimeError("libpng missing. libpng-config is not available") from e
+        return False
+
+def libpng_cflags():
+    return subprocess.check_output(["libpng-config", "--cflags"]).decode().split()
+
+def libpng_ldflags():
+    return subprocess.check_output(["libpng-config", "--ldflags"]).decode().split()
 
 # TODO: delete (Obsolete)
 # patch get_ext_filename
@@ -161,6 +180,12 @@ distutils.command.build_ext.build_ext.get_ext_filename = patched_get_ext_filenam
 
 # end patch get_ext_filename
 
+# TODO: customize this object instead of introspecting and patching distutils (if possible)
+class klayout_build_ext(_build_ext):
+    """ Customize build extension class to check for dependencies before installing."""
+    def finalize_options(self) -> None:
+        ret = super().finalize_options()
+        return ret
 
 # patch CCompiler's library_filename for _dbpi libraries (SOs)
 # Its default is to have .so for shared objects, instead of dylib,
@@ -222,7 +247,6 @@ def always_link_shared_object(
         target_lang,
     )
 
-
 setuptools.command.build_ext.libtype = "shared"
 setuptools.command.build_ext.link_shared_object = always_link_shared_object
 
@@ -321,6 +345,9 @@ class Config(object):
                 "-Wno-strict-aliasing",  # Avoids many "type-punned pointer" warnings
                 "-std=c++11",  # because we use unordered_map/unordered_set
             ]
+        if platform.system() == "Darwin" and mod == "_tl":
+            if check_libpng():
+                args += libpng_cflags()
         return args
 
     def libraries(self, mod):
@@ -330,9 +357,14 @@ class Config(object):
         if platform.system() == "Windows":
             if mod == "_tl":
                 return ["libcurl", "expat", "pthreadVCE2", "zlib", "wsock32", "libpng16"]
+        elif platform.system() == "Darwin":
+            if mod == "_tl":
+                libs = ["curl", "expat"]  # libpng is included by libpng_ldflags
+                return libs
         else:
             if mod == "_tl":
-                return ["curl", "expat", "png"]
+                libs = ["curl", "expat", "png"]
+                return libs
         return []
 
     def link_args(self, mod):
@@ -361,6 +393,8 @@ class Config(object):
                     "-Wl,-install_name,@rpath/%s" % self.libname_of(mod, is_lib=True),
                 ]
             args += ["-Wl,-rpath,@loader_path/"]
+            if mod == "_tl" and check_libpng():
+                args += libpng_ldflags()
             return args
         else:
             # this makes the libraries suitable for linking with a path -
@@ -385,8 +419,7 @@ class Config(object):
         """
         Returns the macros to use for building
         """
-        return [
+        macros = [
-            ("HAVE_PNG", 1),
             ("HAVE_CURL", 1),
             ("HAVE_EXPAT", 1),
             ("KLAYOUT_MAJOR_VERSION", self.major_version()),
@@ -394,6 +427,12 @@ class Config(object):
             ("GSI_ALIAS_INSPECT", 1),
         ]
 
+        if platform.system() == "Darwin" and check_libpng():
+                macros += [("HAVE_PNG", 1)]
+        else:
+            macros += [("HAVE_PNG", 1)]
+        return macros
+
     def minor_version(self):
         """
         Gets the version string
@@ -937,5 +976,6 @@ if __name__ == "__main__":
         include_package_data=True,
         ext_modules=[_tl, _gsi, _pya, _rba, _db, _lib, _rdb, _lym, _laybasic, _layview, _ant, _edt, _img]
             + db_plugins
-            + [tl, db, lib, rdb, lay]
+            + [tl, db, lib, rdb, lay],
+        cmdclass={'build_ext': klayout_build_ext}
     )