bpo-31536: Avoid wholesale rebuild after `make regen-all` (#3678)

* bpo-31536: Avoid wholesale rebuild after `make regen-all`

* Add NEWS

4 files changed, 86 insertions, 51 deletions

diff --git a/Makefile.pre.in b/Makefile.pre.in
index 5a001ec..8f4918d 100644
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -230,6 +230,7 @@ PYTHON=		python$(EXE)
 BUILDPYTHON=	python$(BUILDEXE)
 
 PYTHON_FOR_REGEN=@PYTHON_FOR_REGEN@
+UPDATE_FILE=@PYTHON_FOR_REGEN@ $(srcdir)/Tools/scripts/update_file.py
 PYTHON_FOR_BUILD=@PYTHON_FOR_BUILD@
 _PYTHON_HOST_PLATFORM=@_PYTHON_HOST_PLATFORM@
 BUILD_GNU_TYPE=	@build@
@@ -691,12 +692,14 @@ regen-importlib: Programs/_freeze_importlib
 	# from Lib/importlib/_bootstrap_external.py using _freeze_importlib
 	./Programs/_freeze_importlib \
 	    $(srcdir)/Lib/importlib/_bootstrap_external.py \
-	    $(srcdir)/Python/importlib_external.h
+	    $(srcdir)/Python/importlib_external.h.new
+	$(UPDATE_FILE) $(srcdir)/Python/importlib_external.h $(srcdir)/Python/importlib_external.h.new
 	# Regenerate Python/importlib.h from Lib/importlib/_bootstrap.py
 	# using _freeze_importlib
 	./Programs/_freeze_importlib \
 	    $(srcdir)/Lib/importlib/_bootstrap.py \
-	    $(srcdir)/Python/importlib.h
+	    $(srcdir)/Python/importlib.h.new
+	$(UPDATE_FILE) $(srcdir)/Python/importlib.h $(srcdir)/Python/importlib.h.new
 
 
 ############################################################################
@@ -770,8 +773,10 @@ regen-grammar: $(PGEN)
 	# from Grammar/Grammar using pgen
 	@$(MKDIR_P) Include
 	$(PGEN) $(srcdir)/Grammar/Grammar \
-		$(srcdir)/Include/graminit.h \
-		$(srcdir)/Python/graminit.c
+		$(srcdir)/Include/graminit.h.new \
+		$(srcdir)/Python/graminit.c.new
+	$(UPDATE_FILE) $(srcdir)/Include/graminit.h $(srcdir)/Include/graminit.h.new
+	$(UPDATE_FILE) $(srcdir)/Python/graminit.c $(srcdir)/Python/graminit.c.new
 
 Parser/grammar.o:	$(srcdir)/Parser/grammar.c \
 				$(srcdir)/Include/token.h \
@@ -789,13 +794,15 @@ regen-ast:
 	# Regenerate Include/Python-ast.h using Parser/asdl_c.py -h
 	$(MKDIR_P) $(srcdir)/Include
 	$(PYTHON_FOR_REGEN) $(srcdir)/Parser/asdl_c.py \
-		-h $(srcdir)/Include \
+		-h $(srcdir)/Include/Python-ast.h.new \
 		$(srcdir)/Parser/Python.asdl
+	$(UPDATE_FILE) $(srcdir)/Include/Python-ast.h $(srcdir)/Include/Python-ast.h.new
 	# Regenerate Python/Python-ast.c using Parser/asdl_c.py -c
 	$(MKDIR_P) $(srcdir)/Python
 	$(PYTHON_FOR_REGEN) $(srcdir)/Parser/asdl_c.py \
-		-c $(srcdir)/Python \
+		-c $(srcdir)/Python/Python-ast.c.new \
 		$(srcdir)/Parser/Python.asdl
+	$(UPDATE_FILE) $(srcdir)/Python/Python-ast.c $(srcdir)/Python/Python-ast.c.new
 
 .PHONY: regen-opcode
 regen-opcode:
@@ -803,7 +810,8 @@ regen-opcode:
 	# using Tools/scripts/generate_opcode_h.py
 	$(PYTHON_FOR_REGEN) $(srcdir)/Tools/scripts/generate_opcode_h.py \
 		$(srcdir)/Lib/opcode.py \
-		$(srcdir)/Include/opcode.h
+		$(srcdir)/Include/opcode.h.new
+	$(UPDATE_FILE) $(srcdir)/Include/opcode.h $(srcdir)/Include/opcode.h.new
 
 Python/compile.o Python/symtable.o Python/ast.o: $(srcdir)/Include/graminit.h $(srcdir)/Include/Python-ast.h
 
@@ -860,7 +868,8 @@ regen-opcode-targets:
 	# Regenerate Python/opcode_targets.h from Lib/opcode.py
 	# using Python/makeopcodetargets.py
 	$(PYTHON_FOR_REGEN) $(srcdir)/Python/makeopcodetargets.py \
-		$(srcdir)/Python/opcode_targets.h
+		$(srcdir)/Python/opcode_targets.h.new
+	$(UPDATE_FILE) $(srcdir)/Python/opcode_targets.h $(srcdir)/Python/opcode_targets.h.new
 
 Python/ceval.o: $(srcdir)/Python/opcode_targets.h $(srcdir)/Python/ceval_gil.h
 
@@ -887,7 +896,8 @@ regen-typeslots:
 	# using Objects/typeslots.py
 	$(PYTHON_FOR_REGEN) $(srcdir)/Objects/typeslots.py \
 		< $(srcdir)/Include/typeslots.h \
-		$(srcdir)/Objects/typeslots.inc
+		$(srcdir)/Objects/typeslots.inc.new
+	$(UPDATE_FILE) $(srcdir)/Objects/typeslots.inc $(srcdir)/Objects/typeslots.inc.new
 
 ############################################################################
 # Header files
diff --git a/Misc/NEWS.d/next/Build/2017-09-20-21-32-21.bpo-31536.KUDjno.rst b/Misc/NEWS.d/next/Build/2017-09-20-21-32-21.bpo-31536.KUDjno.rst
new file mode 100644
index 0000000..414f1a4
--- /dev/null
+++ b/Misc/NEWS.d/next/Build/2017-09-20-21-32-21.bpo-31536.KUDjno.rst
@@ -0,0 +1 @@
+Avoid wholesale rebuild after `make regen-all` if nothing changed.
diff --git a/Parser/asdl_c.py b/Parser/asdl_c.py
index a43d2e7..0286d66 100644
--- a/Parser/asdl_c.py
+++ b/Parser/asdl_c.py
@@ -1285,59 +1285,55 @@ def main(srcfile, dump_module=False):
         print(mod)
     if not asdl.check(mod):
         sys.exit(1)
-    if INC_DIR:
-        p = "%s/%s-ast.h" % (INC_DIR, mod.name)
-        f = open(p, "w")
-        f.write(auto_gen_msg)
-        f.write('#include "asdl.h"\n\n')
-        c = ChainOfVisitors(TypeDefVisitor(f),
-                            StructVisitor(f),
-                            PrototypeVisitor(f),
-                            )
-        c.visit(mod)
-        f.write("PyObject* PyAST_mod2obj(mod_ty t);\n")
-        f.write("mod_ty PyAST_obj2mod(PyObject* ast, PyArena* arena, int mode);\n")
-        f.write("int PyAST_Check(PyObject* obj);\n")
-        f.close()
-
-    if SRC_DIR:
-        p = os.path.join(SRC_DIR, str(mod.name) + "-ast.c")
-        f = open(p, "w")
-        f.write(auto_gen_msg)
-        f.write('#include <stddef.h>\n')
-        f.write('\n')
-        f.write('#include "Python.h"\n')
-        f.write('#include "%s-ast.h"\n' % mod.name)
-        f.write('\n')
-        f.write("static PyTypeObject AST_type;\n")
-        v = ChainOfVisitors(
-            PyTypesDeclareVisitor(f),
-            PyTypesVisitor(f),
-            Obj2ModPrototypeVisitor(f),
-            FunctionVisitor(f),
-            ObjVisitor(f),
-            Obj2ModVisitor(f),
-            ASTModuleVisitor(f),
-            PartingShots(f),
-            )
-        v.visit(mod)
-        f.close()
+    if H_FILE:
+        with open(H_FILE, "w") as f:
+            f.write(auto_gen_msg)
+            f.write('#include "asdl.h"\n\n')
+            c = ChainOfVisitors(TypeDefVisitor(f),
+                                StructVisitor(f),
+                                PrototypeVisitor(f),
+                                )
+            c.visit(mod)
+            f.write("PyObject* PyAST_mod2obj(mod_ty t);\n")
+            f.write("mod_ty PyAST_obj2mod(PyObject* ast, PyArena* arena, int mode);\n")
+            f.write("int PyAST_Check(PyObject* obj);\n")
+
+    if C_FILE:
+        with open(C_FILE, "w") as f:
+            f.write(auto_gen_msg)
+            f.write('#include <stddef.h>\n')
+            f.write('\n')
+            f.write('#include "Python.h"\n')
+            f.write('#include "%s-ast.h"\n' % mod.name)
+            f.write('\n')
+            f.write("static PyTypeObject AST_type;\n")
+            v = ChainOfVisitors(
+                PyTypesDeclareVisitor(f),
+                PyTypesVisitor(f),
+                Obj2ModPrototypeVisitor(f),
+                FunctionVisitor(f),
+                ObjVisitor(f),
+                Obj2ModVisitor(f),
+                ASTModuleVisitor(f),
+                PartingShots(f),
+                )
+            v.visit(mod)
 
 if __name__ == "__main__":
     import getopt
 
-    INC_DIR = ''
-    SRC_DIR = ''
+    H_FILE = ''
+    C_FILE = ''
     dump_module = False
     opts, args = getopt.getopt(sys.argv[1:], "dh:c:")
     for o, v in opts:
         if o == '-h':
-            INC_DIR = v
+            H_FILE = v
         if o == '-c':
-            SRC_DIR = v
+            C_FILE = v
         if o == '-d':
             dump_module = True
-    if INC_DIR and SRC_DIR:
+    if H_FILE and C_FILE:
         print('Must specify exactly one output file')
         sys.exit(1)
     elif len(args) != 1:
diff --git a/Tools/scripts/update_file.py b/Tools/scripts/update_file.py
new file mode 100644
index 0000000..224585c
--- /dev/null
+++ b/Tools/scripts/update_file.py
@@ -0,0 +1,28 @@
+"""
+A script that replaces an old file with a new one, only if the contents
+actually changed.  If not, the new file is simply deleted.
+
+This avoids wholesale rebuilds when a code (re)generation phase does not
+actually change the in-tree generated code.
+"""
+
+import os
+import sys
+
+
+def main(old_path, new_path):
+    with open(old_path, 'rb') as f:
+        old_contents = f.read()
+    with open(new_path, 'rb') as f:
+        new_contents = f.read()
+    if old_contents != new_contents:
+        os.replace(new_path, old_path)
+    else:
+        os.unlink(new_path)
+
+
+if __name__ == '__main__':
+    if len(sys.argv) != 3:
+        print("Usage: %s <path to be updated> <path with new contents>" % (sys.argv[0],))
+        sys.exit(1)
+    main(sys.argv[1], sys.argv[2])