Un-break peer authentication.

Commit 613c6d2 sloppily replaced a
lookup of the UID obtained from getpeereid() with a lookup of the
server's own user name, thus totally destroying peer authentication.
Revert.  Per report from Christoph Berg.

In passing, make sure get_user_name() zeroes *errstr on success on
Windows as well as non-Windows.  I don't think any callers actually
depend on this ATM, but we should be consistent across platforms.

Author: tglsfdc

src/backend/libpq/auth.c

@@ -21,7 +21,6 @@
 #include <arpa/inet.h>
 #include <unistd.h>
 
-#include "common/username.h"
 #include "libpq/auth.h"
 #include "libpq/crypt.h"
 #include "libpq/ip.h"
@@ -1560,8 +1559,7 @@ auth_peer(hbaPort *port)
 	char		ident_user[IDENT_USERNAME_MAX + 1];
 	uid_t		uid;
 	gid_t		gid;
-	const char *user_name;
-	char	   *errstr;
+	struct passwd *pass;
 
 	errno = 0;
 	if (getpeereid(port->sock, &uid, &gid) != 0)
@@ -1578,15 +1576,17 @@ auth_peer(hbaPort *port)
 		return STATUS_ERROR;
 	}
 
-	user_name = get_user_name(&errstr);
-	if (!user_name)
+	pass = getpwuid(uid);
+
+	if (pass == NULL)
 	{
-		ereport(LOG, (errmsg_internal("%s", errstr)));
-		pfree(errstr);
+		ereport(LOG,
+				(errmsg("local user with ID %d does not exist",
+						(int) uid)));
 		return STATUS_ERROR;
 	}
 
-	strlcpy(ident_user, user_name, IDENT_USERNAME_MAX + 1);
+	strlcpy(ident_user, pass->pw_name, IDENT_USERNAME_MAX + 1);
 
 	return check_usermap(port->hba->usermap, port->user_name, ident_user, false);
 }

src/common/username.c

@@ -54,6 +54,8 @@ get_user_name(char **errstr)
 	static char username[256 + 1];
 	DWORD		len = sizeof(username) - 1;
 
+	*errstr = NULL;
+
 	if (!GetUserName(username, &len))
 	{
 		*errstr = psprintf(_("user name lookup failure: %s"), strerror(errno));