diff --git a/configure b/configure
index 11a4284e5bd..07529825d18 100755
--- a/configure
+++ b/configure
@@ -15959,12 +15959,29 @@ 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
 
diff --git a/configure.ac b/configure.ac
index fc523c6aeb4..7f855783f4e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1757,11 +1757,13 @@ 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
 
diff --git a/contrib/dblink/input/paths.source b/contrib/dblink/input/paths.source
index aab3a3b2bfb..881a65314f3 100644
--- a/contrib/dblink/input/paths.source
+++ b/contrib/dblink/input/paths.source
@@ -1,8 +1,8 @@
 -- Initialization that requires path substitution.
 
-CREATE FUNCTION putenv(text)
+CREATE FUNCTION setenv(text, text)
    RETURNS void
-   AS '@libdir@/regress@DLSUFFIX@', 'regress_putenv'
+   AS '@libdir@/regress@DLSUFFIX@', 'regress_setenv'
    LANGUAGE C STRICT;
 
 CREATE FUNCTION wait_pid(int)
@@ -11,4 +11,4 @@ CREATE FUNCTION wait_pid(int)
    LANGUAGE C STRICT;
 
 CREATE FUNCTION set_pgservicefile(text) RETURNS void LANGUAGE SQL
-    AS $$SELECT putenv('PGSERVICEFILE=@abs_srcdir@/' || $1)$$;
+    AS $$SELECT setenv('PGSERVICEFILE', '@abs_srcdir@/' || $1)$$;
diff --git a/contrib/dblink/output/paths.source b/contrib/dblink/output/paths.source
index e1097f0996f..8ed95e1f782 100644
--- a/contrib/dblink/output/paths.source
+++ b/contrib/dblink/output/paths.source
@@ -1,11 +1,11 @@
 -- Initialization that requires path substitution.
-CREATE FUNCTION putenv(text)
+CREATE FUNCTION setenv(text, text)
    RETURNS void
-   AS '@libdir@/regress@DLSUFFIX@', 'regress_putenv'
+   AS '@libdir@/regress@DLSUFFIX@', 'regress_setenv'
    LANGUAGE C STRICT;
 CREATE FUNCTION wait_pid(int)
    RETURNS void
    AS '@libdir@/regress@DLSUFFIX@'
    LANGUAGE C STRICT;
 CREATE FUNCTION set_pgservicefile(text) RETURNS void LANGUAGE SQL
-    AS $$SELECT putenv('PGSERVICEFILE=@abs_srcdir@/' || $1)$$;
+    AS $$SELECT setenv('PGSERVICEFILE', '@abs_srcdir@/' || $1)$$;
diff --git a/src/backend/utils/adt/pg_locale.c b/src/backend/utils/adt/pg_locale.c
index c39d67645c6..088c1444c3b 100644
--- a/src/backend/utils/adt/pg_locale.c
+++ b/src/backend/utils/adt/pg_locale.c
@@ -105,20 +105,6 @@ char	   *localized_full_months[12 + 1];
 static bool CurrentLocaleConvValid = false;
 static bool CurrentLCTimeValid = false;
 
-/* Environment variable storage area */
-
-#define LC_ENV_BUFSIZE (NAMEDATALEN + 20)
-
-static char lc_collate_envbuf[LC_ENV_BUFSIZE];
-static char lc_ctype_envbuf[LC_ENV_BUFSIZE];
-
-#ifdef LC_MESSAGES
-static char lc_messages_envbuf[LC_ENV_BUFSIZE];
-#endif
-static char lc_monetary_envbuf[LC_ENV_BUFSIZE];
-static char lc_numeric_envbuf[LC_ENV_BUFSIZE];
-static char lc_time_envbuf[LC_ENV_BUFSIZE];
-
 /* Cache for collation-related knowledge */
 
 typedef struct
@@ -163,7 +149,6 @@ pg_perm_setlocale(int category, const char *locale)
 {
 	char	   *result;
 	const char *envvar;
-	char	   *envbuf;
 
 #ifndef WIN32
 	result = setlocale(category, locale);
@@ -199,7 +184,7 @@ pg_perm_setlocale(int category, const char *locale)
 	 */
 	if (category == LC_CTYPE)
 	{
-		static char save_lc_ctype[LC_ENV_BUFSIZE];
+		static char save_lc_ctype[NAMEDATALEN + 20];
 
 		/* copy setlocale() return value before callee invokes it again */
 		strlcpy(save_lc_ctype, result, sizeof(save_lc_ctype));
@@ -216,16 +201,13 @@ pg_perm_setlocale(int category, const char *locale)
 	{
 		case LC_COLLATE:
 			envvar = "LC_COLLATE";
-			envbuf = lc_collate_envbuf;
 			break;
 		case LC_CTYPE:
 			envvar = "LC_CTYPE";
-			envbuf = lc_ctype_envbuf;
 			break;
 #ifdef LC_MESSAGES
 		case LC_MESSAGES:
 			envvar = "LC_MESSAGES";
-			envbuf = lc_messages_envbuf;
 #ifdef WIN32
 			result = IsoLocaleName(locale);
 			if (result == NULL)
@@ -236,26 +218,19 @@ pg_perm_setlocale(int category, const char *locale)
 #endif							/* LC_MESSAGES */
 		case LC_MONETARY:
 			envvar = "LC_MONETARY";
-			envbuf = lc_monetary_envbuf;
 			break;
 		case LC_NUMERIC:
 			envvar = "LC_NUMERIC";
-			envbuf = lc_numeric_envbuf;
 			break;
 		case LC_TIME:
 			envvar = "LC_TIME";
-			envbuf = lc_time_envbuf;
 			break;
 		default:
 			elog(FATAL, "unrecognized LC category: %d", category);
-			envvar = NULL;		/* keep compiler quiet */
-			envbuf = NULL;
-			return NULL;
+			return NULL;		/* keep compiler quiet */
 	}
 
-	snprintf(envbuf, LC_ENV_BUFSIZE - 1, "%s=%s", envvar, result);
-
-	if (putenv(envbuf))
+	if (setenv(envvar, result, 1) != 0)
 		return NULL;
 
 	return result;
diff --git a/src/bin/initdb/initdb.c b/src/bin/initdb/initdb.c
index f994c4216bc..0865f73ee0b 100644
--- a/src/bin/initdb/initdb.c
+++ b/src/bin/initdb/initdb.c
@@ -2355,8 +2355,7 @@ check_need_password(const char *authmethodlocal, const char *authmethodhost)
 void
 setup_pgdata(void)
 {
-	char	   *pgdata_get_env,
-			   *pgdata_set_env;
+	char	   *pgdata_get_env;
 
 	if (!pg_data)
 	{
@@ -2386,8 +2385,11 @@ setup_pgdata(void)
 	 * need quotes otherwise on Windows because paths there are most likely to
 	 * have embedded spaces.
 	 */
-	pgdata_set_env = psprintf("PGDATA=%s", pg_data);
-	putenv(pgdata_set_env);
+	if (setenv("PGDATA", pg_data, 1) != 0)
+	{
+		pg_log_error("could not set environment");
+		exit(1);
+	}
 }
 
 
diff --git a/src/bin/pg_ctl/pg_ctl.c b/src/bin/pg_ctl/pg_ctl.c
index fc07f1aba6e..fcdc0213e48 100644
--- a/src/bin/pg_ctl/pg_ctl.c
+++ b/src/bin/pg_ctl/pg_ctl.c
@@ -889,11 +889,10 @@ do_start(void)
 	 */
 #ifndef WIN32
 	{
-		static char env_var[32];
+		char		env_var[32];
 
-		snprintf(env_var, sizeof(env_var), "PG_GRANDPARENT_PID=%d",
-				 (int) getppid());
-		putenv(env_var);
+		snprintf(env_var, sizeof(env_var), "%d", (int) getppid());
+		setenv("PG_GRANDPARENT_PID", env_var, 1);
 	}
 #endif
 
@@ -2340,12 +2339,10 @@ main(int argc, char **argv)
 				case 'D':
 					{
 						char	   *pgdata_D;
-						char	   *env_var;
 
 						pgdata_D = pg_strdup(optarg);
 						canonicalize_path(pgdata_D);
-						env_var = psprintf("PGDATA=%s", pgdata_D);
-						putenv(env_var);
+						setenv("PGDATA", pgdata_D, 1);
 
 						/*
 						 * We could pass PGDATA just in an environment
@@ -2353,6 +2350,7 @@ main(int argc, char **argv)
 						 * 'ps' display
 						 */
 						pgdata_opt = psprintf("-D \"%s\" ", pgdata_D);
+						free(pgdata_D);
 						break;
 					}
 				case 'e':
diff --git a/src/bin/pg_upgrade/controldata.c b/src/bin/pg_upgrade/controldata.c
index 39bcaa8fe1a..7eb56e7a293 100644
--- a/src/bin/pg_upgrade/controldata.c
+++ b/src/bin/pg_upgrade/controldata.c
@@ -97,20 +97,20 @@ get_control_data(ClusterInfo *cluster, bool live_check)
 	if (getenv("LC_MESSAGES"))
 		lc_messages = pg_strdup(getenv("LC_MESSAGES"));
 
-	pg_putenv("LC_COLLATE", NULL);
-	pg_putenv("LC_CTYPE", NULL);
-	pg_putenv("LC_MONETARY", NULL);
-	pg_putenv("LC_NUMERIC", NULL);
-	pg_putenv("LC_TIME", NULL);
+	unsetenv("LC_COLLATE");
+	unsetenv("LC_CTYPE");
+	unsetenv("LC_MONETARY");
+	unsetenv("LC_NUMERIC");
+	unsetenv("LC_TIME");
 #ifndef WIN32
-	pg_putenv("LANG", NULL);
+	unsetenv("LANG");
 #else
 	/* On Windows the default locale may not be English, so force it */
-	pg_putenv("LANG", "en");
+	setenv("LANG", "en", 1);
 #endif
-	pg_putenv("LANGUAGE", NULL);
-	pg_putenv("LC_ALL", NULL);
-	pg_putenv("LC_MESSAGES", "C");
+	unsetenv("LANGUAGE");
+	unsetenv("LC_ALL");
+	setenv("LC_MESSAGES", "C", 1);
 
 	/*
 	 * Check for clean shutdown
@@ -490,17 +490,31 @@ get_control_data(ClusterInfo *cluster, bool live_check)
 	pclose(output);
 
 	/*
-	 * Restore environment variables
+	 * Restore environment variables.  Note all but LANG and LC_MESSAGES were
+	 * unset above.
 	 */
-	pg_putenv("LC_COLLATE", lc_collate);
-	pg_putenv("LC_CTYPE", lc_ctype);
-	pg_putenv("LC_MONETARY", lc_monetary);
-	pg_putenv("LC_NUMERIC", lc_numeric);
-	pg_putenv("LC_TIME", lc_time);
-	pg_putenv("LANG", lang);
-	pg_putenv("LANGUAGE", language);
-	pg_putenv("LC_ALL", lc_all);
-	pg_putenv("LC_MESSAGES", lc_messages);
+	if (lc_collate)
+		setenv("LC_COLLATE", lc_collate, 1);
+	if (lc_ctype)
+		setenv("LC_CTYPE", lc_ctype, 1);
+	if (lc_monetary)
+		setenv("LC_MONETARY", lc_monetary, 1);
+	if (lc_numeric)
+		setenv("LC_NUMERIC", lc_numeric, 1);
+	if (lc_time)
+		setenv("LC_TIME", lc_time, 1);
+	if (lang)
+		setenv("LANG", lang, 1);
+	else
+		unsetenv("LANG");
+	if (language)
+		setenv("LANGUAGE", language, 1);
+	if (lc_all)
+		setenv("LC_ALL", lc_all, 1);
+	if (lc_messages)
+		setenv("LC_MESSAGES", lc_messages, 1);
+	else
+		unsetenv("LC_MESSAGES");
 
 	pg_free(lc_collate);
 	pg_free(lc_ctype);
diff --git a/src/bin/pg_upgrade/option.c b/src/bin/pg_upgrade/option.c
index 548d648e8c4..5b566c14ef4 100644
--- a/src/bin/pg_upgrade/option.c
+++ b/src/bin/pg_upgrade/option.c
@@ -193,7 +193,7 @@ parseCommandLine(int argc, char *argv[])
 				 * Push the user name into the environment so pre-9.1
 				 * pg_ctl/libpq uses it.
 				 */
-				pg_putenv("PGUSER", os_info.user);
+				setenv("PGUSER", os_info.user, 1);
 				break;
 
 			case 'v':
@@ -245,11 +245,11 @@ parseCommandLine(int argc, char *argv[])
 		char	   *pgoptions = psprintf("%s %s", FIX_DEFAULT_READ_ONLY,
 										 getenv("PGOPTIONS"));
 
-		pg_putenv("PGOPTIONS", pgoptions);
+		setenv("PGOPTIONS", pgoptions, 1);
 		pfree(pgoptions);
 	}
 	else
-		pg_putenv("PGOPTIONS", FIX_DEFAULT_READ_ONLY);
+		setenv("PGOPTIONS", FIX_DEFAULT_READ_ONLY, 1);
 
 	/* Get values from env if not already set */
 	check_required_directory(&old_cluster.bindir, "PGBINOLD", false,
diff --git a/src/bin/pg_upgrade/pg_upgrade.h b/src/bin/pg_upgrade/pg_upgrade.h
index ee70243c2e9..1842556274f 100644
--- a/src/bin/pg_upgrade/pg_upgrade.h
+++ b/src/bin/pg_upgrade/pg_upgrade.h
@@ -436,7 +436,6 @@ void		end_progress_output(void);
 void		prep_status(const char *fmt,...) pg_attribute_printf(1, 2);
 void		check_ok(void);
 unsigned int str2uint(const char *str);
-void		pg_putenv(const char *var, const char *val);
 
 
 /* version.c */
diff --git a/src/bin/pg_upgrade/util.c b/src/bin/pg_upgrade/util.c
index a16c794261b..9c9ba29124e 100644
--- a/src/bin/pg_upgrade/util.c
+++ b/src/bin/pg_upgrade/util.c
@@ -241,39 +241,3 @@ str2uint(const char *str)
 {
 	return strtoul(str, NULL, 10);
 }
-
-
-/*
- *	pg_putenv()
- *
- *	This is like putenv(), but takes two arguments.
- *	It also does unsetenv() if val is NULL.
- */
-void
-pg_putenv(const char *var, const char *val)
-{
-	if (val)
-	{
-#ifndef WIN32
-		char	   *envstr;
-
-		envstr = psprintf("%s=%s", var, val);
-		putenv(envstr);
-
-		/*
-		 * Do not free envstr because it becomes part of the environment on
-		 * some operating systems.  See port/unsetenv.c::unsetenv.
-		 */
-#else
-		SetEnvironmentVariableA(var, val);
-#endif
-	}
-	else
-	{
-#ifndef WIN32
-		unsetenv(var);
-#else
-		SetEnvironmentVariableA(var, "");
-#endif
-	}
-}
diff --git a/src/bin/psql/command.c b/src/bin/psql/command.c
index 38b588882d1..c545341cddd 100644
--- a/src/bin/psql/command.c
+++ b/src/bin/psql/command.c
@@ -2296,17 +2296,8 @@ exec_command_setenv(PsqlScanState scan_state, bool active_branch,
 		else
 		{
 			/* Set variable to the value of the next argument */
-			char	   *newval;
-
-			newval = psprintf("%s=%s", envvar, envval);
-			putenv(newval);
+			setenv(envvar, envval, 1);
 			success = true;
-
-			/*
-			 * Do not free newval here, it will screw up the environment if
-			 * you do. See putenv man page for details. That means we leak a
-			 * bit of memory here, but not enough to worry about.
-			 */
 		}
 		free(envvar);
 		free(envval);
diff --git a/src/common/exec.c b/src/common/exec.c
index 78bb486f999..773afd080c0 100644
--- a/src/common/exec.c
+++ b/src/common/exec.c
@@ -435,9 +435,6 @@ set_pglocale_pgservice(const char *argv0, const char *app)
 {
 	char		path[MAXPGPATH];
 	char		my_exec_path[MAXPGPATH];
-	char		env_path[MAXPGPATH + sizeof("PGSYSCONFDIR=")];	/* longer than
-																 * PGLOCALEDIR */
-	char	   *dup_path;
 
 	/* don't set LC_ALL in the backend */
 	if (strcmp(app, PG_TEXTDOMAIN("postgres")) != 0)
@@ -462,28 +459,15 @@ set_pglocale_pgservice(const char *argv0, const char *app)
 	get_locale_path(my_exec_path, path);
 	bindtextdomain(app, path);
 	textdomain(app);
-
-	if (getenv("PGLOCALEDIR") == NULL)
-	{
-		/* set for libpq to use */
-		snprintf(env_path, sizeof(env_path), "PGLOCALEDIR=%s", path);
-		canonicalize_path(env_path + 12);
-		dup_path = strdup(env_path);
-		if (dup_path)
-			putenv(dup_path);
-	}
+	/* set for libpq to use, but don't override existing setting */
+	setenv("PGLOCALEDIR", path, 0);
 #endif
 
 	if (getenv("PGSYSCONFDIR") == NULL)
 	{
 		get_etc_path(my_exec_path, path);
-
 		/* set for libpq to use */
-		snprintf(env_path, sizeof(env_path), "PGSYSCONFDIR=%s", path);
-		canonicalize_path(env_path + 13);
-		dup_path = strdup(env_path);
-		if (dup_path)
-			putenv(dup_path);
+		setenv("PGSYSCONFDIR", path, 0);
 	}
 }
 
diff --git a/src/common/restricted_token.c b/src/common/restricted_token.c
index dcc88a75c59..9058c5ad252 100644
--- a/src/common/restricted_token.c
+++ b/src/common/restricted_token.c
@@ -171,7 +171,7 @@ get_restricted_token(void)
 
 		cmdline = pg_strdup(GetCommandLine());
 
-		putenv("PG_RESTRICT_EXEC=1");
+		setenv("PG_RESTRICT_EXEC", "1", 1);
 
 		if ((restrictedToken = CreateRestrictedProcess(cmdline, &pi)) == 0)
 		{
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index de8f838e536..ddaa9e8e182 100644
--- a/src/include/pg_config.h.in
+++ b/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
 
diff --git a/src/include/port.h b/src/include/port.h
index 5dfb00b07cc..c631185c8bb 100644
--- a/src/include/port.h
+++ b/src/include/port.h
@@ -447,8 +447,12 @@ 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);
+extern int	unsetenv(const char *name);
 #endif
 
 #ifndef HAVE_SRANDOM
diff --git a/src/include/port/win32_port.h b/src/include/port/win32_port.h
index 59c7f35e3df..2ffe056a0fe 100644
--- a/src/include/port/win32_port.h
+++ b/src/include/port/win32_port.h
@@ -490,7 +490,12 @@ extern void _dosmaperr(unsigned long);
 
 /* in port/win32env.c */
 extern int	pgwin32_putenv(const char *);
-extern void pgwin32_unsetenv(const char *);
+extern int	pgwin32_setenv(const char *name, const char *value, int overwrite);
+extern int	pgwin32_unsetenv(const char *name);
+
+#define putenv(x) pgwin32_putenv(x)
+#define setenv(x,y,z) pgwin32_setenv(x,y,z)
+#define unsetenv(x) pgwin32_unsetenv(x)
 
 /* in port/win32security.c */
 extern int	pgwin32_is_service(void);
@@ -499,9 +504,6 @@ extern int	pgwin32_is_admin(void);
 /* Windows security token manipulation (in src/common/exec.c) */
 extern BOOL AddUserToTokenDacl(HANDLE hToken);
 
-#define putenv(x) pgwin32_putenv(x)
-#define unsetenv(x) pgwin32_unsetenv(x)
-
 /* Things that exist in MinGW headers, but need to be added to MSVC */
 #ifdef _MSC_VER
 
diff --git a/src/interfaces/ecpg/test/pg_regress_ecpg.c b/src/interfaces/ecpg/test/pg_regress_ecpg.c
index 6e1d25b1f4a..49b2d8141f5 100644
--- a/src/interfaces/ecpg/test/pg_regress_ecpg.c
+++ b/src/interfaces/ecpg/test/pg_regress_ecpg.c
@@ -147,8 +147,9 @@ ecpg_start_test(const char *testname,
 			 outfile_stdout,
 			 outfile_stderr);
 
-	appnameenv = psprintf("PGAPPNAME=ecpg/%s", testname_dash.data);
-	putenv(appnameenv);
+	appnameenv = psprintf("ecpg/%s", testname_dash.data);
+	setenv("PGAPPNAME", appnameenv, 1);
+	free(appnameenv);
 
 	pid = spawn_process(cmd);
 
@@ -160,7 +161,6 @@ ecpg_start_test(const char *testname,
 	}
 
 	unsetenv("PGAPPNAME");
-	free(appnameenv);
 
 	free(testname_dash.data);
 
diff --git a/src/port/setenv.c b/src/port/setenv.c
new file mode 100644
index 00000000000..d8a5647fb02
--- /dev/null
+++ b/src/port/setenv.c
@@ -0,0 +1,48 @@
+/*-------------------------------------------------------------------------
+ *
+ * setenv.c
+ *	  setenv() emulation for machines without it
+ *
+ * Portions Copyright (c) 1996-2020, 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);
+}
diff --git a/src/port/unsetenv.c b/src/port/unsetenv.c
index f2028c2f834..a5f19f8db39 100644
--- a/src/port/unsetenv.c
+++ b/src/port/unsetenv.c
@@ -16,13 +16,20 @@
 #include "c.h"
 
 
-void
+int
 unsetenv(const char *name)
 {
 	char	   *envstr;
 
+	/* Error conditions, per POSIX */
+	if (name == NULL || name[0] == '\0' || strchr(name, '=') != NULL)
+	{
+		errno = EINVAL;
+		return -1;
+	}
+
 	if (getenv(name) == NULL)
-		return;					/* no work */
+		return 0;				/* no work */
 
 	/*
 	 * The technique embodied here works if libc follows the Single Unix Spec
@@ -40,11 +47,12 @@ unsetenv(const char *name)
 
 	envstr = (char *) malloc(strlen(name) + 2);
 	if (!envstr)				/* not much we can do if no memory */
-		return;
+		return -1;
 
 	/* Override the existing setting by forcibly defining the var */
 	sprintf(envstr, "%s=", name);
-	putenv(envstr);
+	if (putenv(envstr))
+		return -1;
 
 	/* Now we can clobber the variable definition this way: */
 	strcpy(envstr, "=");
@@ -53,5 +61,5 @@ unsetenv(const char *name)
 	 * This last putenv cleans up if we have multiple zero-length names as a
 	 * result of unsetting multiple things.
 	 */
-	putenv(envstr);
+	return putenv(envstr);
 }
diff --git a/src/port/win32env.c b/src/port/win32env.c
index 177488cc67e..f5bed672974 100644
--- a/src/port/win32env.c
+++ b/src/port/win32env.c
@@ -1,8 +1,10 @@
 /*-------------------------------------------------------------------------
  *
  * win32env.c
- *	  putenv() and unsetenv() for win32, which update both process environment
- *	  and caches in (potentially multiple) C run-time library (CRT) versions.
+ *	  putenv(), setenv(), and unsetenv() for win32.
+ *
+ * These functions update both the process environment and caches in
+ * (potentially multiple) C run-time library (CRT) versions.
  *
  * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
@@ -16,6 +18,11 @@
 
 #include "c.h"
 
+
+/*
+ * Note that unlike POSIX putenv(), this doesn't use the passed-in string
+ * as permanent storage.
+ */
 int
 pgwin32_putenv(const char *envval)
 {
@@ -64,7 +71,7 @@ pgwin32_putenv(const char *envval)
 	}
 	*cp = '\0';
 	cp++;
-	if (strlen(cp))
+	if (*cp)
 	{
 		/*
 		 * Only call SetEnvironmentVariable() when we are adding a variable,
@@ -110,16 +117,47 @@ pgwin32_putenv(const char *envval)
 	return _putenv(envval);
 }
 
-void
+int
+pgwin32_setenv(const char *name, const char *value, int overwrite)
+{
+	int			res;
+	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;
+
+	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);
+
+	res = pgwin32_putenv(envstr);
+	free(envstr);
+	return res;
+}
+
+int
 pgwin32_unsetenv(const char *name)
 {
+	int			res;
 	char	   *envbuf;
 
 	envbuf = (char *) malloc(strlen(name) + 2);
 	if (!envbuf)
-		return;
+		return -1;
 
 	sprintf(envbuf, "%s=", name);
-	pgwin32_putenv(envbuf);
+	res = pgwin32_putenv(envbuf);
 	free(envbuf);
+	return res;
 }
diff --git a/src/test/isolation/isolation_main.c b/src/test/isolation/isolation_main.c
index 50916b00dca..a6a64e7ec52 100644
--- a/src/test/isolation/isolation_main.c
+++ b/src/test/isolation/isolation_main.c
@@ -98,8 +98,9 @@ isolation_start_test(const char *testname,
 		exit(2);
 	}
 
-	appnameenv = psprintf("PGAPPNAME=isolation/%s", testname);
-	putenv(appnameenv);
+	appnameenv = psprintf("isolation/%s", testname);
+	setenv("PGAPPNAME", appnameenv, 1);
+	free(appnameenv);
 
 	pid = spawn_process(psql_cmd);
 
@@ -111,7 +112,6 @@ isolation_start_test(const char *testname,
 	}
 
 	unsetenv("PGAPPNAME");
-	free(appnameenv);
 
 	return pid;
 }
diff --git a/src/test/regress/pg_regress.c b/src/test/regress/pg_regress.c
index 23d7d0beb2e..866bc8c4704 100644
--- a/src/test/regress/pg_regress.c
+++ b/src/test/regress/pg_regress.c
@@ -724,18 +724,6 @@ get_expectfile(const char *testname, const char *file)
 	return NULL;
 }
 
-/*
- * Handy subroutine for setting an environment variable "var" to "val"
- */
-static void
-doputenv(const char *var, const char *val)
-{
-	char	   *s;
-
-	s = psprintf("%s=%s", var, val);
-	putenv(s);
-}
-
 /*
  * Prepare environment variables for running regression tests
  */
@@ -746,7 +734,7 @@ initialize_environment(void)
 	 * Set default application_name.  (The test_function may choose to
 	 * override this, but if it doesn't, we have something useful in place.)
 	 */
-	putenv("PGAPPNAME=pg_regress");
+	setenv("PGAPPNAME", "pg_regress", 1);
 
 	if (nolocale)
 	{
@@ -769,7 +757,7 @@ initialize_environment(void)
 		 * variables unset; see PostmasterMain().
 		 */
 #if defined(WIN32) || defined(__CYGWIN__) || defined(__darwin__)
-		putenv("LANG=C");
+		setenv("LANG", "C", 1);
 #endif
 	}
 
@@ -781,21 +769,21 @@ initialize_environment(void)
 	 */
 	unsetenv("LANGUAGE");
 	unsetenv("LC_ALL");
-	putenv("LC_MESSAGES=C");
+	setenv("LC_MESSAGES", "C", 1);
 
 	/*
 	 * Set encoding as requested
 	 */
 	if (encoding)
-		doputenv("PGCLIENTENCODING", encoding);
+		setenv("PGCLIENTENCODING", encoding, 1);
 	else
 		unsetenv("PGCLIENTENCODING");
 
 	/*
 	 * Set timezone and datestyle for datetime-related tests
 	 */
-	putenv("PGTZ=PST8PDT");
-	putenv("PGDATESTYLE=Postgres, MDY");
+	setenv("PGTZ", "PST8PDT", 1);
+	setenv("PGDATESTYLE", "Postgres, MDY", 1);
 
 	/*
 	 * Likewise set intervalstyle to ensure consistent results.  This is a bit
@@ -809,9 +797,10 @@ initialize_environment(void)
 
 		if (!old_pgoptions)
 			old_pgoptions = "";
-		new_pgoptions = psprintf("PGOPTIONS=%s %s",
+		new_pgoptions = psprintf("%s %s",
 								 old_pgoptions, my_pgoptions);
-		putenv(new_pgoptions);
+		setenv("PGOPTIONS", new_pgoptions, 1);
+		free(new_pgoptions);
 	}
 
 	if (temp_instance)
@@ -832,17 +821,17 @@ initialize_environment(void)
 		unsetenv("PGDATA");
 #ifdef HAVE_UNIX_SOCKETS
 		if (hostname != NULL)
-			doputenv("PGHOST", hostname);
+			setenv("PGHOST", hostname, 1);
 		else
 		{
 			sockdir = getenv("PG_REGRESS_SOCK_DIR");
 			if (!sockdir)
 				sockdir = make_temp_sockdir();
-			doputenv("PGHOST", sockdir);
+			setenv("PGHOST", sockdir, 1);
 		}
 #else
 		Assert(hostname != NULL);
-		doputenv("PGHOST", hostname);
+		setenv("PGHOST", hostname, 1);
 #endif
 		unsetenv("PGHOSTADDR");
 		if (port != -1)
@@ -850,7 +839,7 @@ initialize_environment(void)
 			char		s[16];
 
 			sprintf(s, "%d", port);
-			doputenv("PGPORT", s);
+			setenv("PGPORT", s, 1);
 		}
 	}
 	else
@@ -864,7 +853,7 @@ initialize_environment(void)
 		 */
 		if (hostname != NULL)
 		{
-			doputenv("PGHOST", hostname);
+			setenv("PGHOST", hostname, 1);
 			unsetenv("PGHOSTADDR");
 		}
 		if (port != -1)
@@ -872,10 +861,10 @@ initialize_environment(void)
 			char		s[16];
 
 			sprintf(s, "%d", port);
-			doputenv("PGPORT", s);
+			setenv("PGPORT", s, 1);
 		}
 		if (user != NULL)
-			doputenv("PGUSER", user);
+			setenv("PGUSER", user, 1);
 
 		/*
 		 * However, we *don't* honor PGDATABASE, since we certainly don't wish
@@ -2431,7 +2420,7 @@ regression_main(int argc, char *argv[], init_function ifunc, test_function tfunc
 				fprintf(stderr, _("port %d apparently in use, trying %d\n"), port, port + 1);
 				port++;
 				sprintf(s, "%d", port);
-				doputenv("PGPORT", s);
+				setenv("PGPORT", s, 1);
 			}
 			else
 				break;
diff --git a/src/test/regress/pg_regress_main.c b/src/test/regress/pg_regress_main.c
index dd8ad245648..5e503efa4a7 100644
--- a/src/test/regress/pg_regress_main.c
+++ b/src/test/regress/pg_regress_main.c
@@ -91,8 +91,9 @@ psql_start_test(const char *testname,
 		exit(2);
 	}
 
-	appnameenv = psprintf("PGAPPNAME=pg_regress/%s", testname);
-	putenv(appnameenv);
+	appnameenv = psprintf("pg_regress/%s", testname);
+	setenv("PGAPPNAME", appnameenv, 1);
+	free(appnameenv);
 
 	pid = spawn_process(psql_cmd);
 
@@ -104,7 +105,6 @@ psql_start_test(const char *testname,
 	}
 
 	unsetenv("PGAPPNAME");
-	free(appnameenv);
 
 	return pid;
 }
diff --git a/src/test/regress/regress.c b/src/test/regress/regress.c
index 09bc42a8c0f..b8b3af4e956 100644
--- a/src/test/regress/regress.c
+++ b/src/test/regress/regress.c
@@ -624,22 +624,18 @@ make_tuple_indirect(PG_FUNCTION_ARGS)
 	PG_RETURN_POINTER(newtup->t_data);
 }
 
-PG_FUNCTION_INFO_V1(regress_putenv);
+PG_FUNCTION_INFO_V1(regress_setenv);
 
 Datum
-regress_putenv(PG_FUNCTION_ARGS)
+regress_setenv(PG_FUNCTION_ARGS)
 {
-	MemoryContext oldcontext;
-	char	   *envbuf;
+	char	   *envvar = text_to_cstring(PG_GETARG_TEXT_PP(0));
+	char	   *envval = text_to_cstring(PG_GETARG_TEXT_PP(1));
 
 	if (!superuser())
 		elog(ERROR, "must be superuser to change environment variables");
 
-	oldcontext = MemoryContextSwitchTo(TopMemoryContext);
-	envbuf = text_to_cstring((text *) PG_GETARG_POINTER(0));
-	MemoryContextSwitchTo(oldcontext);
-
-	if (putenv(envbuf) != 0)
+	if (setenv(envvar, envval, 1) != 0)
 		elog(ERROR, "could not set environment variable: %m");
 
 	PG_RETURN_VOID();
diff --git a/src/tools/msvc/Solution.pm b/src/tools/msvc/Solution.pm
index 22d6abd3674..95d4e826b1d 100644
--- a/src/tools/msvc/Solution.pm
+++ b/src/tools/msvc/Solution.pm
@@ -348,6 +348,7 @@ 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,