Add fallback implementation for setenv()

This fixes the code compilation on Windows with MSVC and Kerberos, as
a missing implementation of setenv() causes a compilation failure of the
GSSAPI code.  This was only reproducible when building the code with
Kerberos, something that buildfarm animal hamerkop has fixed recently.

This issue only happens on 12 and 13, as this code has been introduced
in b0b39f7.  HEAD is already able to compile properly thanks to
7ca37fb0, and this commit is a minimal cherry-pick of it.

Thanks to Tom Lane for the discussion.

Discussion: https://postgr.es/m/YLDtm5WGjPxm6ua4@paquier.xyz
Backpatch-through: 12

configure

@@ -15858,12 +15858,29 @@ case $host_os in
         # Windows uses a specialised env handler
         mingw*)
 
+$as_echo "#define HAVE_SETENV 1" >>confdefs.h
+
+
 $as_echo "#define HAVE_UNSETENV 1" >>confdefs.h
 
+                ac_cv_func_setenv=yes
                 ac_cv_func_unsetenv=yes
                 ;;
         *)
-                ac_fn_c_check_func "$LINENO" "unsetenv" "ac_cv_func_unsetenv"
+                ac_fn_c_check_func "$LINENO" "setenv" "ac_cv_func_setenv"
+if test "x$ac_cv_func_setenv" = xyes; then :
+  $as_echo "#define HAVE_SETENV 1" >>confdefs.h
+
+else
+  case " $LIBOBJS " in
+  *" setenv.$ac_objext "* ) ;;
+  *) LIBOBJS="$LIBOBJS setenv.$ac_objext"
+ ;;
+esac
+
+fi
+
+ac_fn_c_check_func "$LINENO" "unsetenv" "ac_cv_func_unsetenv"
 if test "x$ac_cv_func_unsetenv" = xyes; then :
   $as_echo "#define HAVE_UNSETENV 1" >>confdefs.h
 

configure.in

@@ -1747,11 +1747,13 @@ fi
 case $host_os in
         # Windows uses a specialised env handler
         mingw*)
+                AC_DEFINE(HAVE_SETENV, 1, [Define to 1 because replacement version used.])
                 AC_DEFINE(HAVE_UNSETENV, 1, [Define to 1 because replacement version used.])
+                ac_cv_func_setenv=yes
                 ac_cv_func_unsetenv=yes
                 ;;
         *)
-                AC_REPLACE_FUNCS([unsetenv])
+                AC_REPLACE_FUNCS([setenv unsetenv])
                 ;;
 esac
 

src/include/pg_config.h.in

@@ -473,6 +473,9 @@
 /* Define to 1 if you have the <security/pam_appl.h> header file. */
 #undef HAVE_SECURITY_PAM_APPL_H
 
+/* Define to 1 if you have the `setenv' function. */
+#undef HAVE_SETENV
+
 /* Define to 1 if you have the `setproctitle' function. */
 #undef HAVE_SETPROCTITLE
 

src/include/port.h

@@ -429,6 +429,10 @@ extern size_t strnlen(const char *str, size_t maxlen);
 extern long random(void);
 #endif
 
+#ifndef HAVE_SETENV
+extern int setenv(const char *name, const char *value, int overwrite);
+#endif
+
 #ifndef HAVE_UNSETENV
 extern void unsetenv(const char *name);
 #endif

src/include/port/win32_port.h

@@ -462,6 +462,7 @@ extern void _dosmaperr(unsigned long);
 
 /* in port/win32env.c */
 extern int	pgwin32_putenv(const char *);
+extern int	pgwin32_setenv(const char *name, const char *value, int overwrite);
 extern void pgwin32_unsetenv(const char *);
 
 /* in port/win32security.c */
@@ -472,6 +473,7 @@ extern int	pgwin32_is_admin(void);
 extern BOOL AddUserToTokenDacl(HANDLE hToken);
 
 #define putenv(x) pgwin32_putenv(x)
+#define setenv(x,y,z) pgwin32_setenv(x,y,z)
 #define unsetenv(x) pgwin32_unsetenv(x)
 
 /* Things that exist in MinGW headers, but need to be added to MSVC */

src/port/setenv.c

@@ -0,0 +1,48 @@
+/*-------------------------------------------------------------------------
+ *
+ * setenv.c
+ *	  setenv() emulation for machines without it
+ *
+ * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/port/setenv.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "c.h"
+
+
+int
+setenv(const char *name, const char *value, int overwrite)
+{
+	char	   *envstr;
+
+	/* Error conditions, per POSIX */
+	if (name == NULL || name[0] == '\0' || strchr(name, '=') != NULL ||
+		value == NULL)
+	{
+		errno = EINVAL;
+		return -1;
+	}
+
+	/* No work if variable exists and we're not to replace it */
+	if (overwrite == 0 && getenv(name) != NULL)
+		return 0;
+
+	/*
+	 * Add or replace the value using putenv().  This will leak memory if the
+	 * same variable is repeatedly redefined, but there's little we can do
+	 * about that when sitting atop putenv().
+	 */
+	envstr = (char *) malloc(strlen(name) + strlen(value) + 2);
+	if (!envstr)				/* not much we can do if no memory */
+		return -1;
+
+	sprintf(envstr, "%s=%s", name, value);
+
+	return putenv(envstr);
+}

src/tools/msvc/Mkvcbuild.pm

@@ -101,7 +101,7 @@ sub mkvcbuild
 	  dirent.c dlopen.c getopt.c getopt_long.c link.c
 	  pread.c pwrite.c pg_bitutils.c
 	  pg_strong_random.c pgcheckdir.c pgmkdirp.c pgsleep.c pgstrcasecmp.c
-	  pqsignal.c mkdtemp.c qsort.c qsort_arg.c quotes.c system.c
+	  pqsignal.c mkdtemp.c qsort.c qsort_arg.c quotes.c setenv.c system.c
 	  sprompt.c strerror.c tar.c thread.c
 	  win32env.c win32error.c win32security.c win32setlocale.c);
 

src/tools/msvc/Solution.pm

@@ -341,6 +341,7 @@ sub GenerateFiles
 		HAVE_RL_FILENAME_QUOTING_FUNCTION        => undef,
 		HAVE_RL_RESET_SCREEN_SIZE                => undef,
 		HAVE_SECURITY_PAM_APPL_H                 => undef,
+		HAVE_SETENV                              => undef,
 		HAVE_SETPROCTITLE                        => undef,
 		HAVE_SETPROCTITLE_FAST                   => undef,
 		HAVE_SETSID                              => undef,