makefile: pick Python that is more likely to build with setup.py

Summary:
The `python` in `PATH` might be the fbcode Python that has `-nostdinc` in its
`CFLAGS`. That Python is problematic because it cannot compile C extensions.

Reviewed By: DurhamG, markbt

Differential Revision: D27661488

fbshipit-source-id: 243205522fcaf53d5af6a3c9afc4d28160072de5

eden/scm/Makefile

@@ -65,7 +65,7 @@ help:
 all: build
 
 local:
-	$(PYTHON) setup.py $(PURE) \
+	$(shell $(PYTHON) contrib/pick_python.py $(PYTHON)) setup.py $(PURE) \
 	  build_py -c -d . \
 	  build_clib $(COMPILERFLAG) \
 	  build_ext $(COMPILERFLAG) -i \
@@ -80,7 +80,7 @@ else
 endif
 
 local3:
-	HGNAME=$(HGNAME3) $(PYTHON3) setup3.py $(PURE) \
+	HGNAME=$(HGNAME3) $(shell $(PYTHON3) contrib/pick_python.py $(PYTHON3)) setup3.py $(PURE) \
 	  build_py -c -d . \
 	  build_clib $(COMPILERFLAG) \
 	  build_ext $(COMPILERFLAG) -i \

eden/scm/contrib/pick_python.py

@@ -0,0 +1,52 @@
+# Copyright (c) Facebook, Inc. and its affiliates.
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2.
+
+"""
+Pick a Python that is more likely to be able to build with setup.py.
+
+Arguments: Binary name (ex. python2, or python3).
+Stdout: Full path to a system python, or the first argument of the input.
+
+This script does not need to be executed using a system Python.
+"""
+
+import os
+import subprocess
+import sys
+
+
+def main(args):
+    dirs = os.environ.get("PATH").split(os.pathsep)
+    names = args or ["python3"]
+    for dir in dirs:
+        for name in names:
+            path = os.path.join(dir, name)
+            if does_python_look_good(path):
+                sys.stdout.write(path)
+                return
+    # Fallback
+    sys.stdout.write(names[0])
+
+
+def does_python_look_good(path):
+    if not os.path.isfile(path):
+        return False
+    try:
+        cflags = subprocess.check_output(
+            [path, "-c", "import sysconfig;print(sysconfig.get_config_var('CFLAGS'))"]
+        )
+        if b"-nostdinc" in cflags.split():
+            sys.stderr.write("%s: ignored, lack of C stdlib\n" % path)
+            return False
+        return True
+    except Exception:
+        return False
+
+
+if __name__ == "__main__":
+    code = main(sys.argv[1:]) or 0
+    sys.stderr.flush()
+    sys.stdout.flush()
+    sys.exit(code)