Reduce the amount of memory "clobbered" for every process title change,

on platforms that need this.  This is done by only writing past the
previously stored message, if it was longer.

src/backend/utils/misc/ps_status.c

@@ -5,7 +5,7 @@
  * to contain some useful information. Mechanism differs wildly across
  * platforms.
  *
- * $PostgreSQL: pgsql/src/backend/utils/misc/ps_status.c,v 1.34 2007/01/05 22:19:46 momjian Exp $
+ * $PostgreSQL: pgsql/src/backend/utils/misc/ps_status.c,v 1.35 2007/02/16 21:34:04 momjian Exp $
  *
  * Copyright (c) 2000-2007, PostgreSQL Global Development Group
  * various details abducted from various places
@@ -91,6 +91,7 @@ static const size_t ps_buffer_size = PS_BUFFER_SIZE;
 #else							/* PS_USE_CLOBBER_ARGV */
 static char *ps_buffer;			/* will point to argv area */
 static size_t ps_buffer_size;	/* space determined at run time */
+static size_t last_status_len;	/* use to minimize length of clobber */
 #endif   /* PS_USE_CLOBBER_ARGV */
 
 static size_t ps_buffer_fixed_size;		/* size of the constant prefix */
@@ -153,8 +154,8 @@ save_ps_display_args(int argc, char **argv)
 		}
 
 		ps_buffer = argv[0];
-		ps_buffer_size = end_of_area - argv[0];
-
+		last_status_len = ps_buffer_size = end_of_area - argv[0];
+		
 		/*
 		 * move the environment out of the way
 		 */
@@ -329,7 +330,10 @@ set_ps_display(const char *activity, bool force)
 
 		/* pad unused memory */
 		buflen = strlen(ps_buffer);
-		MemSet(ps_buffer + buflen, PS_PADDING, ps_buffer_size - buflen);
+		/* clobber remainder of old status string */
+		if (last_status_len > buflen)
+			MemSet(ps_buffer + buflen, PS_PADDING, last_status_len - buflen);
+		last_status_len = buflen;
 	}
 #endif   /* PS_USE_CLOBBER_ARGV */