Modify the stats regression test to delay until the stats file actually

changes (with an upper limit of 30 seconds), and record the delay time in
the postmaster log.  This should give us some info about what's happening
with the intermittent stats failures in buildfarm.  After an idea of
Andrew Dunstan's.

src/test/regress/expected/stats.out

@@ -30,26 +30,72 @@ SELECT t.seq_scan, t.seq_tup_read, t.idx_scan, t.idx_tup_fetch,
 -- enable statistics
 SET stats_block_level = on;
 SET stats_row_level = on;
--- do something
+-- do a seqscan
 SELECT count(*) FROM tenk2;
  count 
 -------
  10000
 (1 row)
 
+-- do an indexscan
 SELECT count(*) FROM tenk2 WHERE unique1 = 1;
  count 
 -------
      1
 (1 row)
 
--- let stats collector catch up
-SELECT pg_sleep(2.0);
- pg_sleep 
-----------
+-- All of the thrashing here is to wait for the stats collector to update,
+-- without waiting too long (in fact, we'd like to try to measure how long
+-- we wait).  Watching for change in the stats themselves wouldn't work
+-- because the backend only reads them once per transaction.  The stats file
+-- mod timestamp isn't too helpful because it may have resolution of only one
+-- second, or even worse.  So, we touch a new table and then watch for change
+-- in the size of the stats file.  Ugh.
+-- save current stats-file size
+CREATE TEMP TABLE prevfilesize AS
+  SELECT size FROM pg_stat_file('global/pgstat.stat');
+-- make and touch a previously nonexistent table
+CREATE TABLE stats_hack (f1 int);
+SELECT * FROM stats_hack;
+ f1 
+----
+(0 rows)
+
+-- wait for stats collector to update
+create function wait_for_stats() returns void as $$
+declare
+  start_time timestamptz := clock_timestamp();
+  oldsize bigint;
+  newsize bigint;
+begin
+  -- fetch previous stats-file size
+  select size into oldsize from prevfilesize;
+
+  -- we don't want to wait forever; loop will exit after 30 seconds
+  for i in 1 .. 300 loop
+
+    -- look for update of stats file
+    select size into newsize from pg_stat_file('global/pgstat.stat');
+
+    exit when newsize != oldsize;
+
+    -- wait a little
+    perform pg_sleep(0.1);
+
+  end loop;
+
+  -- report time waited in postmaster log (where it won't change test output)
+  raise log 'wait_for_stats delayed % seconds',
+    extract(epoch from clock_timestamp() - start_time);
+end
+$$ language plpgsql;
+SELECT wait_for_stats();
+ wait_for_stats 
+----------------
  
 (1 row)
 
+DROP TABLE stats_hack;
 -- check effects
 SELECT st.seq_scan >= pr.seq_scan + 1,
        st.seq_tup_read >= pr.seq_tup_read + cl.reltuples,

src/test/regress/sql/stats.sql

@@ -25,12 +25,59 @@ SELECT t.seq_scan, t.seq_tup_read, t.idx_scan, t.idx_tup_fetch,
 SET stats_block_level = on;
 SET stats_row_level = on;
 
--- do something
+-- do a seqscan
 SELECT count(*) FROM tenk2;
+-- do an indexscan
 SELECT count(*) FROM tenk2 WHERE unique1 = 1;
 
--- let stats collector catch up
-SELECT pg_sleep(2.0);
+-- All of the thrashing here is to wait for the stats collector to update,
+-- without waiting too long (in fact, we'd like to try to measure how long
+-- we wait).  Watching for change in the stats themselves wouldn't work
+-- because the backend only reads them once per transaction.  The stats file
+-- mod timestamp isn't too helpful because it may have resolution of only one
+-- second, or even worse.  So, we touch a new table and then watch for change
+-- in the size of the stats file.  Ugh.
+
+-- save current stats-file size
+CREATE TEMP TABLE prevfilesize AS
+  SELECT size FROM pg_stat_file('global/pgstat.stat');
+
+-- make and touch a previously nonexistent table
+CREATE TABLE stats_hack (f1 int);
+SELECT * FROM stats_hack;
+
+-- wait for stats collector to update
+create function wait_for_stats() returns void as $$
+declare
+  start_time timestamptz := clock_timestamp();
+  oldsize bigint;
+  newsize bigint;
+begin
+  -- fetch previous stats-file size
+  select size into oldsize from prevfilesize;
+
+  -- we don't want to wait forever; loop will exit after 30 seconds
+  for i in 1 .. 300 loop
+
+    -- look for update of stats file
+    select size into newsize from pg_stat_file('global/pgstat.stat');
+
+    exit when newsize != oldsize;
+
+    -- wait a little
+    perform pg_sleep(0.1);
+
+  end loop;
+
+  -- report time waited in postmaster log (where it won't change test output)
+  raise log 'wait_for_stats delayed % seconds',
+    extract(epoch from clock_timestamp() - start_time);
+end
+$$ language plpgsql;
+
+SELECT wait_for_stats();
+
+DROP TABLE stats_hack;
 
 -- check effects
 SELECT st.seq_scan >= pr.seq_scan + 1,