Merge branch 'nd/warn-more-for-devs'

The build procedure "make DEVELOPER=YesPlease" learned to enable a
bit more warning options depending on the compiler used to help
developers more.  There also is "make DEVOPTS=tokens" knob
available now, for those who want to help fixing warnings we
usually ignore, for example.

* nd/warn-more-for-devs:
  Makefile: add a DEVOPTS to get all of -Wextra
  Makefile: add a DEVOPTS to suppress -Werror under DEVELOPER
  Makefile: detect compiler and enable more warnings in DEVELOPER=1
  connect.c: mark die_initial_contact() NORETURN

Makefile

@@ -464,6 +464,26 @@ all::
 # When using RUNTIME_PREFIX, define HAVE_WPGMPTR if your platform offers
 # the global variable _wpgmptr containing the absolute path of the current
 # executable (this is the case on Windows).
+#
+# Define DEVELOPER to enable more compiler warnings. Compiler version
+# and family are auto detected, but could be overridden by defining
+# COMPILER_FEATURES (see config.mak.dev)
+#
+# When DEVELOPER is set, DEVOPTS can be used to control compiler
+# options.  This variable contains keywords separated by
+# whitespace. The following keywords are are recognized:
+#
+#    no-error:
+#
+#        suppresses the -Werror that implicitly comes with
+#        DEVELOPER=1. Useful for getting the full set of errors
+#        without immediately dying, or for logging them.
+#
+#    extra-all:
+#
+#        The DEVELOPER mode enables -Wextra with a few exceptions. By
+#        setting this flag the exceptions are removed, and all of
+#        -Wextra is used.
 
 GIT-VERSION-FILE: FORCE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN
@@ -472,15 +492,6 @@ GIT-VERSION-FILE: FORCE
 # CFLAGS and LDFLAGS are for the users to override from the command line.
 
 CFLAGS = -g -O2 -Wall
-DEVELOPER_CFLAGS = -Werror \
-	-Wdeclaration-after-statement \
-	-Wno-format-zero-length \
-	-Wold-style-definition \
-	-Woverflow \
-	-Wpointer-arith \
-	-Wstrict-prototypes \
-	-Wunused \
-	-Wvla
 LDFLAGS =
 ALL_CFLAGS = $(CPPFLAGS) $(CFLAGS)
 ALL_LDFLAGS = $(LDFLAGS)
@@ -1098,7 +1109,7 @@ include config.mak.uname
 -include config.mak
 
 ifdef DEVELOPER
-CFLAGS += $(DEVELOPER_CFLAGS)
+include config.mak.dev
 endif
 
 comma := ,

config.mak.dev

@@ -0,0 +1,42 @@
+ifeq ($(filter no-error,$(DEVOPTS)),)
+CFLAGS += -Werror
+endif
+CFLAGS += -Wdeclaration-after-statement
+CFLAGS += -Wno-format-zero-length
+CFLAGS += -Wold-style-definition
+CFLAGS += -Woverflow
+CFLAGS += -Wpointer-arith
+CFLAGS += -Wstrict-prototypes
+CFLAGS += -Wunused
+CFLAGS += -Wvla
+
+ifndef COMPILER_FEATURES
+COMPILER_FEATURES := $(shell ./detect-compiler $(CC))
+endif
+
+ifneq ($(filter clang4,$(COMPILER_FEATURES)),)
+CFLAGS += -Wtautological-constant-out-of-range-compare
+endif
+
+ifneq ($(or $(filter gcc6,$(COMPILER_FEATURES)),$(filter clang4,$(COMPILER_FEATURES))),)
+CFLAGS += -Wextra
+# if a function is public, there should be a prototype and the right
+# header file should be included. If not, it should be static.
+CFLAGS += -Wmissing-prototypes
+ifeq ($(filter extra-all,$(DEVOPTS)),)
+# These are disabled because we have these all over the place.
+CFLAGS += -Wno-empty-body
+CFLAGS += -Wno-missing-field-initializers
+CFLAGS += -Wno-sign-compare
+CFLAGS += -Wno-unused-function
+CFLAGS += -Wno-unused-parameter
+endif
+endif
+
+# uninitialized warnings on gcc 4.9.2 in xdiff/xdiffi.c and config.c
+# not worth fixing since newer compilers correctly stop complaining
+ifneq ($(filter gcc4,$(COMPILER_FEATURES)),)
+ifeq ($(filter gcc5,$(COMPILER_FEATURES)),)
+CFLAGS += -Wno-uninitialized
+endif
+endif

connect.c

@@ -48,7 +48,7 @@ int check_ref_type(const struct ref *ref, int flags)
 	return check_ref(ref->name, flags);
 }
 
-static void die_initial_contact(int unexpected)
+static NORETURN void die_initial_contact(int unexpected)
 {
 	/*
 	 * A hang-up after seeing some response from the other end

detect-compiler

@@ -0,0 +1,53 @@
+#!/bin/sh
+#
+# Probe the compiler for vintage, version, etc. This is used for setting
+# optional make knobs under the DEVELOPER knob.
+
+CC="$*"
+
+# we get something like (this is at least true for gcc and clang)
+#
+# FreeBSD clang version 3.4.1 (tags/RELEASE...)
+get_version_line() {
+	$CC -v 2>&1 | grep ' version '
+}
+
+get_family() {
+	get_version_line | sed 's/^\(.*\) version [0-9][^ ]* .*/\1/'
+}
+
+get_version() {
+	get_version_line | sed 's/^.* version \([0-9][^ ]*\) .*/\1/'
+}
+
+print_flags() {
+	family=$1
+	version=$(get_version | cut -f 1 -d .)
+
+	# Print a feature flag not only for the current version, but also
+	# for any prior versions we encompass. This avoids needing to do
+	# numeric comparisons in make, which are awkward.
+	while test "$version" -gt 0
+	do
+		echo $family$version
+		version=$((version - 1))
+	done
+}
+
+case "$(get_family)" in
+gcc)
+	print_flags gcc
+	;;
+clang)
+	print_flags clang
+	;;
+"FreeBSD clang")
+	print_flags clang
+	;;
+"Apple LLVM")
+	print_flags clang
+	;;
+*)
+	: unknown compiler family
+	;;
+esac