sane-ctype: fix compiler error on Amazon Linux 2

Compiling Git fails on Amazon Linux 2 when using GCC 7.3.1 with the
following compiler error:

    In file included from compat/posix.h:449:0,
                     from git-compat-util.h:26,
                     from daemon.c:3:
    compat/../sane-ctype.h:29:60: error: expected expression before ']' token
     #define sane_istest(x,mask) ((sane_ctype[(unsigned char)(x)] & (mask)) != 0)
                                                                ^
    compat/../sane-ctype.h:29:72: error: expected ')' before '!=' token
     #define sane_istest(x,mask) ((sane_ctype[(unsigned char)(x)] & (mask)) != 0)
                                                                            ^
    compat/../sane-ctype.h:29:60: error: expected expression before ']' token
     #define sane_istest(x,mask) ((sane_ctype[(unsigned char)(x)] & (mask)) != 0)
                                                                ^
    ... lots of similar lines ...

    compat/../sane-ctype.h:45:50: error: expected declaration specifiers or '...' before numeric constant
     #define toupper(x) sane_case((unsigned char)(x), 0)
                                                      ^
    /usr/include/ctype.h:142:12: error: expected identifier or '(' before 'int'
     extern int isascii (int __c) __THROW;
                ^
    compat/../sane-ctype.h:30:26: error: expected ')' before '&' token
     #define isascii(x) (((x) & ~0x7f) == 0)
                              ^
    compat/../sane-ctype.h:30:35: error: expected ')' before '==' token
     #define isascii(x) (((x) & ~0x7f) == 0)
                                       ^
    In file included from /usr/include/features.h:423:0,
                     from /usr/include/unistd.h:25,
                     from compat/posix.h:90,
                     from git-compat-util.h:26,
                     from daemon.c:3:
    compat/../sane-ctype.h:44:30: error: expected declaration specifiers or '...' before '(' token
     #define tolower(x) sane_case((unsigned char)(x), 0x20)
                                  ^
    compat/../sane-ctype.h:44:50: error: expected declaration specifiers or '...' before numeric constant
     #define tolower(x) sane_case((unsigned char)(x), 0x20)
                                                      ^
    compat/../sane-ctype.h:45:30: error: expected declaration specifiers or '...' before '(' token
     #define toupper(x) sane_case((unsigned char)(x), 0)
                                  ^
    compat/../sane-ctype.h:45:50: error: expected declaration specifiers or '...' before numeric constant
     #define toupper(x) sane_case((unsigned char)(x), 0)
                                                      ^

This error bisect back to 75a044f (git-compat-util.h: split out
POSIX-emulating bits, 2025-02-18), where lots of bits got split out of
"git-compat-util.h" into a new "compat/posix.h" header.

The compiler error isn't immediately obvious, doubly so because the
actual errors are ~3x as long as the above snippet. But what happens
here is that we transitively include <ctype.h> after we have included
our own "sane-ctype.h" header. Consequently, the function declarations
that exist in <ctype.h> for isascii(3p) et al will be mangled by our
macros of the same type. The result is of course completely broken.

It's unclear why this issue only happens on Amazon Linux 2. My guess is
that it's either specific to the compiler version or specific to the
glibc version. We don't explicitly include <ctypes.h> anywhere, but it's
being transitively included. So chances are that later versions of the
toolchain reorganized their headers so that <ctypes.h> is not included
transitively anymore.

Fix the issue by explicitly including <ctype.h> in "sane-ctype.h". This
ensures that the header guards will be activated and that any subsequent
include of the same header will become a no-op. With this we can then
safely override the function declarations with our own macros.

Reported-by: Stan Hu <[email protected]>
Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

sane-ctype.h

@@ -1,6 +1,15 @@
 #ifndef SANE_CTYPE_H
 #define SANE_CTYPE_H
 
+/*
+ * Explicitly include <ctype.h> so that its header guards kick in from here on.
+ * This ensures that the file won't get included after "sane-ctype.h", as that
+ * would otherwise lead to a compiler error because the function declarations
+ * for `int isascii(int c)` et al would be mangled by our macros with the same
+ * name.
+ */
+#include <ctype.h>
+
 /* Sane ctype - no locale, and works with signed chars */
 #undef isascii
 #undef isspace