PR: Improve calls to libpng-config on Ubuntu/Debian (#2398)

* Improve detection of libpng-config on Ubuntu/Debian

* Do not disable libdir on Mac

setup.py

@@ -2,6 +2,7 @@ import os
 import io
 import re
 import sys
+import csv
 from setuptools import setup, find_packages
 from pkg_resources import parse_version, get_distribution, DistributionNotFound
 import subprocess
@@ -135,6 +136,26 @@ def find_library(name, vision_include):
     return library_found, conda_installed, include_folder, lib_folder
 
 
+def get_linux_distribution():
+    release_data = {}
+    with open("/etc/os-release") as f:
+        reader = csv.reader(f, delimiter="=")
+        for row in reader:
+            if row:
+                release_data[row[0]] = row[1]
+    if release_data["ID"] in ["debian", "raspbian"]:
+        with open("/etc/debian_version") as f:
+            debian_version = f.readline().strip()
+        major_version = debian_version.split(".")[0]
+        version_split = release_data["VERSION"].split(" ", maxsplit=1)
+        if version_split[0] == major_version:
+            # Just major version shown, replace it with the full version
+            release_data["VERSION"] = " ".join(
+                [debian_version] + version_split[1:])
+    print("{} {}".format(release_data["NAME"], release_data["VERSION"]))
+    return release_data
+
+
 def get_extensions():
     this_dir = os.path.dirname(os.path.abspath(__file__))
     extensions_dir = os.path.join(this_dir, 'torchvision', 'csrc')
@@ -246,6 +267,14 @@ def get_extensions():
     image_library = []
     image_link_flags = []
 
+    # Detect if build is running under conda/conda-build
+    conda = distutils.spawn.find_executable('conda')
+    is_conda = conda is not None
+
+    build_prefix = os.environ.get('BUILD_PREFIX', None)
+    is_conda_build = build_prefix is not None
+    running_under_conda = is_conda or is_conda_build
+
     # Locating libPNG
     libpng = distutils.spawn.find_executable('libpng-config')
     pngfix = distutils.spawn.find_executable('pngfix')
@@ -262,14 +291,26 @@ def get_extensions():
             png_version = parse_version(png_version)
             if png_version >= parse_version("1.6.0"):
                 print('Building torchvision with PNG image support')
-                png_lib = subprocess.run([libpng, '--libdir'],
-                                         stdout=subprocess.PIPE)
+                linux = sys.platform == 'linux'
+                not_debian = False
+                libpng_on_conda = False
+                if linux:
+                    bin_folder = os.path.dirname(sys.executable)
+                    png_bin_folder = os.path.dirname(libpng)
+                    libpng_on_conda = (
+                        running_under_conda and bin_folder == png_bin_folder)
+                    release_info = get_linux_distribution()
+                    not_debian = release_info["NAME"] not in {'Ubuntu', 'Debian'}
+                if not linux or libpng_on_conda or not_debian:
+                    png_lib = subprocess.run([libpng, '--libdir'],
+                                             stdout=subprocess.PIPE)
+                    png_lib = png_lib.stdout.strip().decode('utf-8')
+                    image_library += [png_lib]
                 png_include = subprocess.run([libpng, '--I_opts'],
                                              stdout=subprocess.PIPE)
                 png_include = png_include.stdout.strip().decode('utf-8')
                 _, png_include = png_include.split('-I')
                 print('libpng include path: {0}'.format(png_include))
-                image_library += [png_lib.stdout.strip().decode('utf-8')]
                 image_include += [png_include]
                 image_link_flags.append('png')
             else: