Fix incorrect error message reported for non-existent users

Previously, lookups of non-existent user names could return "Success";
it will now return "User does not exist" by resetting errno.  This also
centralizes the user name lookup code in libpgport.

Report and analysis by Nicolas Marchildon;  patch by me

src/port/Makefile

@@ -33,7 +33,7 @@ LIBS += $(PTHREAD_LIBS)
 OBJS = $(LIBOBJS) chklocale.o dirmod.o erand48.o fls.o inet_net_ntop.o \
 	noblock.o path.o pgcheckdir.o pg_crc.o pgmkdirp.o pgsleep.o \
 	pgstrcasecmp.o pqsignal.o \
-	qsort.o qsort_arg.o quotes.o sprompt.o tar.o thread.o
+	qsort.o qsort_arg.o quotes.o sprompt.o tar.o thread.o username.o
 
 # foo_srv.o and foo.o are both built from foo.c, but only foo.o has -DFRONTEND
 OBJS_SRV = $(OBJS:%.o=%_srv.o)

src/port/username.c

@@ -0,0 +1,84 @@
+/*-------------------------------------------------------------------------
+ *
+ * username.c
+ *	  get user name
+ *
+ * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  src/port/username.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#ifndef FRONTEND
+#include "postgres.h"
+#else
+#include "postgres_fe.h"
+#endif
+
+#include <errno.h>
+#include <pwd.h>
+#include <unistd.h>
+#include <sys/types.h>
+
+
+/*
+ * Returns the current user name in a static buffer, or NULL on error and
+ * sets errstr
+ */
+const char *
+get_user_name(char **errstr)
+{
+#ifndef WIN32
+	struct passwd *pw;
+	uid_t user_id = geteuid();
+
+	*errstr = NULL;
+
+	errno = 0;	/* clear errno before call */
+	pw = getpwuid(user_id);
+	if (!pw)
+	{
+		*errstr = psprintf(_("failed to look up effective user id %d: %s"),
+				(int) user_id, errno ? strerror(errno) :
+				_("user does not exist"));
+		return NULL;
+	}
+
+	return pw->pw_name;
+#else
+	/* UNLEN = 256, 'static' variable remains after function exit */
+	static char username[256 + 1]; 
+	DWORD		len = sizeof(username) - 1;
+
+	if (!GetUserName(username, &len))
+	{
+		*errstr = psprintf(_("user name lookup failure: %s"), strerror(errno));
+		return NULL;
+	}
+
+	return username;
+#endif
+}
+
+
+/*
+ * Returns the current user name in a static buffer or exits
+ */
+const char *
+get_user_name_or_exit(const char *progname)
+{
+	const char *user_name;
+	char 	   *errstr;
+
+	user_name = get_user_name(&errstr);
+
+	if (!user_name)
+	{
+		fprintf(stderr, "%s: %s\n", progname, errstr);
+		exit(1);
+	}
+	return user_name;
+}