Treat EPERM as a non-error case when checking to see if old postmaster

is still alive.  This improves our odds of not getting fooled by an
unrelated process when checking a stale lock file.  Other checks already
in place, plus one newly added in checkDataDir(), ensure that we cannot
attempt to usurp the place of a postmaster belonging to a different userid,
so there is no need to error out.  Add comments indicating the importance
of these other checks.

src/backend/postmaster/postmaster.c

@@ -37,7 +37,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/postmaster/postmaster.c,v 1.446 2005/03/10 07:14:03 neilc Exp $
+ *	  $PostgreSQL: pgsql/src/backend/postmaster/postmaster.c,v 1.447 2005/03/18 03:48:49 tgl Exp $
  *
  * NOTES
  *
@@ -952,9 +952,32 @@ checkDataDir(void)
 					DataDir)));
 	}
 
+	/*
+	 * Check that the directory belongs to my userid; if not, reject.
+	 *
+	 * This check is an essential part of the interlock that prevents two
+	 * postmasters from starting in the same directory (see CreateLockFile()).
+	 * Do not remove or weaken it.
+	 *
+	 * XXX can we safely enable this check on Windows?
+	 */
+#if !defined(WIN32) && !defined(__CYGWIN__)
+	if (stat_buf.st_uid != geteuid())
+		ereport(FATAL,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("data directory \"%s\" has wrong ownership",
+						DataDir),
+				 errhint("The server must be started by the user that owns the data directory.")));
+#endif
+
 	/*
 	 * Check if the directory has group or world access.  If so, reject.
 	 *
+	 * It would be possible to allow weaker constraints (for example, allow
+	 * group access) but we cannot make a general assumption that that is
+	 * okay; for example there are platforms where nearly all users customarily
+	 * belong to the same group.  Perhaps this test should be configurable.
+	 *
 	 * XXX temporarily suppress check when on Windows, because there may not
 	 * be proper support for Unix-y file permissions.  Need to think of a
 	 * reasonable check to apply on Windows.