Merge branch 'jk/use-wunreachable-code-for-devs'

Enable -Wunreachable-code for developer builds.

* jk/use-wunreachable-code-for-devs:
  config.mak.dev: enable -Wunreachable-code
  git-compat-util: add NOT_CONSTANT macro and use it in atfork_prepare()
  run-command: use errno to check for sigfillset() error

Author: gitster

Makefile

@@ -995,6 +995,7 @@ LIB_OBJS += common-init.o
 LIB_OBJS += compat/nonblock.o
 LIB_OBJS += compat/obstack.o
 LIB_OBJS += compat/terminal.o
+LIB_OBJS += compiler-tricks/not-constant.o
 LIB_OBJS += config.o
 LIB_OBJS += connect.o
 LIB_OBJS += connected.o

compiler-tricks/not-constant.c

@@ -0,0 +1,2 @@
+#include <git-compat-util.h>
+int false_but_the_compiler_does_not_know_it_;

config.mak.dev

@@ -39,6 +39,7 @@ DEVELOPER_CFLAGS += -Wunused
 DEVELOPER_CFLAGS += -Wvla
 DEVELOPER_CFLAGS += -Wwrite-strings
 DEVELOPER_CFLAGS += -fno-common
+DEVELOPER_CFLAGS += -Wunreachable-code
 
 ifneq ($(filter clang4,$(COMPILER_FEATURES)),)
 DEVELOPER_CFLAGS += -Wtautological-constant-out-of-range-compare

git-compat-util.h

@@ -1583,4 +1583,13 @@ static inline void *container_of_or_null_offset(void *ptr, size_t offset)
 	((uintptr_t)&(ptr)->member - (uintptr_t)(ptr))
 #endif /* !__GNUC__ */
 
+/*
+ * Prevent an overly clever compiler from optimizing an expression
+ * out, triggering a false positive when building with the
+ * -Wunreachable-code option. false_but_the_compiler_does_not_know_it_
+ * is defined in a compilation unit separate from where the macro is
+ * used, initialized to 0, and never modified.
+ */
+#define NOT_CONSTANT(expr) ((expr) || false_but_the_compiler_does_not_know_it_)
+extern int false_but_the_compiler_does_not_know_it_;
 #endif

meson.build

@@ -264,6 +264,7 @@ libgit_sources = [
   'compat/nonblock.c',
   'compat/obstack.c',
   'compat/terminal.c',
+  'compiler-tricks/not-constant.c',
   'config.c',
   'connect.c',
   'connected.c',
@@ -719,6 +720,7 @@ if get_option('warning_level') in ['2','3', 'everything'] and compiler.get_argum
     '-Woverflow',
     '-Wpointer-arith',
     '-Wstrict-prototypes',
+    '-Wunreachable-code',
     '-Wunused',
     '-Wvla',
     '-Wwrite-strings',

run-command.c

@@ -515,7 +515,13 @@ static void atfork_prepare(struct atfork_state *as)
 {
 	sigset_t all;
 
-	if (sigfillset(&all))
+	/*
+	 * POSIX says sigfillset() can fail, but an overly clever
+	 * compiler can see through the header files and decide
+	 * it cannot fail on a particular platform it is compiling for,
+	 * triggering -Wunreachable-code false positive.
+	 */
+	if (NOT_CONSTANT(sigfillset(&all)))
 		die_errno("sigfillset");
 #ifdef NO_PTHREADS
 	if (sigprocmask(SIG_SETMASK, &all, &as->old))