pg_waldump: Limit the number of lines shown at the start

Hello.

I'm occasionally annoyed by the behavior of pg_waldump that it shows
all records in the specified segment before entering waiting state.

tail -f shows only the last few dozen lines at the start.  This
behavior I think is useful also for pg_waldump -f.

It was as simple than expected as attached.

Any thoughts, opinions?

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center
diff --git a/src/bin/pg_waldump/pg_waldump.c b/src/bin/pg_waldump/pg_waldump.c
index 31e99c2a6d..dacdb600bf 100644
--- a/src/bin/pg_waldump/pg_waldump.c
+++ b/src/bin/pg_waldump/pg_waldump.c
@@ -72,6 +72,8 @@ typedef struct XLogDumpStats
 
 #define fatal_error(...) do { pg_log_fatal(__VA_ARGS__); exit(EXIT_FAILURE); } while(0)
 
+#define FOLLOW_START_LINES 20
+
 static void
 print_rmgr_list(void)
 {
@@ -752,6 +754,10 @@ main(int argc, char **argv)
     XLogRecPtr    first_record;
     char       *waldir = NULL;
     char       *errormsg;
+    XLogRecPtr    last_recs[FOLLOW_START_LINES];
+    int            last_idx = 0;
+    bool        storing;
+    bool        rounded;
 
     static struct option long_options[] = {
         {"bkp-details", no_argument, NULL, 'b'},
@@ -1065,10 +1071,41 @@ main(int argc, char **argv)
                (uint32) (first_record >> 32), (uint32) first_record,
                (uint32) (first_record - private.startptr));
 
+    storing = config.follow;
+    rounded = false;
+
     for (;;)
     {
         /* try to read the next record */
         record = XLogReadRecord(xlogreader_state, &errormsg);
+
+        if (storing)
+        {
+            if (record)
+            {
+                /* remember LSNs of the last FOLLOW_START_LINES records  */
+                last_recs[last_idx++] = xlogreader_state->ReadRecPtr;
+                last_idx = last_idx % FOLLOW_START_LINES;
+                
+                if (last_idx == 0)
+                    rounded = true;
+            }
+            else
+            {
+                XLogRecPtr startPtr;
+
+                /* find the first record to show */
+                if (!rounded)
+                    startPtr = last_recs[0];
+                else
+                    startPtr = last_recs[last_idx];
+
+                XLogFindNextRecord(xlogreader_state, startPtr);
+                storing = false;
+            }
+            continue;
+        }
+
         if (!record)
         {
             if (!config.follow || private.endptr_reached)