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

6 files changed, 61 insertions, 1 deletions

diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 244886a5314..9ac86d1c0a2 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -495,6 +495,9 @@
 /* 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/pg_config.h.win32 b/src/include/pg_config.h.win32
index 2fc4be5f14c..e4590cf2e5a 100644
--- a/src/include/pg_config.h.win32
+++ b/src/include/pg_config.h.win32
@@ -371,6 +371,9 @@
 /* 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. */
+#define HAVE_SETENV 1
+
 /* 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 ebcee1c55cf..8a5c6389e6f 100644
--- a/src/include/port.h
+++ b/src/include/port.h
@@ -437,6 +437,10 @@ 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
diff --git a/src/include/port/win32_port.h b/src/include/port/win32_port.h
index f4841fb3975..f9daaf66d7f 100644
--- a/src/include/port/win32_port.h
+++ b/src/include/port/win32_port.h
@@ -471,6 +471,7 @@ 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 */
@@ -481,6 +482,7 @@ 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 */
diff --git a/src/port/setenv.c b/src/port/setenv.c
new file mode 100644
index 00000000000..440bc832ce8
--- /dev/null
+++ b/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);
+}
diff --git a/src/tools/msvc/Mkvcbuild.pm b/src/tools/msvc/Mkvcbuild.pm
index b13e57fa531..131a4c76c4f 100644
--- a/src/tools/msvc/Mkvcbuild.pm
+++ b/src/tools/msvc/Mkvcbuild.pm
@@ -100,7 +100,7 @@ sub mkvcbuild
 	  dirent.c dlopen.c getopt.c getopt_long.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);