--- a/mercurial/configitems.py	Mon Jun 29 20:53:32 2020 +0900
+++ b/mercurial/configitems.py	Thu Jun 18 15:13:38 2020 +0200
@@ -1384,6 +1384,9 @@
     b'ui', b'timeout.warn', default=0,
 )
 coreconfigitem(
+    b'ui', b'timestamp-output', default=False,
+)
+coreconfigitem(
     b'ui', b'traceback', default=False,
 )
 coreconfigitem(

--- a/mercurial/ui.py	Mon Jun 29 20:53:32 2020 +0900
+++ b/mercurial/ui.py	Thu Jun 18 15:13:38 2020 +0200
@@ -9,6 +9,7 @@
 
 import collections
 import contextlib
+import datetime
 import errno
 import getpass
 import inspect
@@ -242,6 +243,7 @@
         self._terminfoparams = {}
         self._styles = {}
         self._uninterruptible = False
+        self.showtimestamp = False
 
         if src:
             self._fout = src._fout
@@ -561,6 +563,7 @@
             self._reportuntrusted = self.debugflag or self.configbool(
                 b"ui", b"report_untrusted"
             )
+            self.showtimestamp = self.configbool(b'ui', b'timestamp-output')
             self.tracebackflag = self.configbool(b'ui', b'traceback')
             self.logblockedtimes = self.configbool(b'ui', b'logblockedtimes')
 
@@ -1217,7 +1220,20 @@
             ) * 1000
 
     def _writemsg(self, dest, *args, **opts):
+        timestamp = self.showtimestamp and opts.get('type') in {
+            b'debug',
+            b'error',
+            b'note',
+            b'status',
+            b'warning',
+        }
+        if timestamp:
+            args = (
+                b'[%s] ' % bytes(datetime.datetime.now().isoformat(), 'ASCII'),
+            ) + args
         _writemsgwith(self._write, dest, *args, **opts)
+        if timestamp:
+            dest.flush()
 
     def _writemsgnobuf(self, dest, *args, **opts):
         _writemsgwith(self._writenobuf, dest, *args, **opts)

--- a/tests/test-progress.t	Mon Jun 29 20:53:32 2020 +0900
+++ b/tests/test-progress.t	Thu Jun 18 15:13:38 2020 +0200
@@ -18,7 +18,8 @@
   > @command(b'loop',
   >     [(b'', b'total', b'', b'override for total'),
   >     (b'', b'nested', False, b'show nested results'),
-  >     (b'', b'parallel', False, b'show parallel sets of results')],
+  >     (b'', b'parallel', False, b'show parallel sets of results'),
+  >     (b'', b'warn', False, b'show warning if step divisible by 3')],
   >     b'hg loop LOOPS',
   >     norepo=True)
   > def loop(ui, loops, **opts):
@@ -32,6 +33,7 @@
   >     if opts.get('nested', None):
   >         nested = True
   >     loops = abs(loops)
+  >     showwarn = opts.get('warn', False)
   > 
   >     progress = ui.makeprogress(topiclabel, unit=b'loopnum', total=total)
   >     other = ui.makeprogress(b'other', unit=b'othernum', total=total)
@@ -48,6 +50,8 @@
   >             for j in range(nested_steps):
   >                 nested.update(j, item=b'nested.%d' % j)
   >             nested.complete()
+  >         if showwarn and i % 3 == 0:
+  >             ui.warn(b'reached step %d\n' %i)
   >     progress.complete()
   > 
   > topiclabel = b'loop'
@@ -179,6 +183,42 @@
   loop [ <=>                                            ] 5/4\r (no-eol) (esc)
                                                               \r (no-eol) (esc)
 
+test interaction with ui.warn
+
+  $ hg loop --warn 6
+  \r (no-eol) (esc)
+  loop [                                                ] 0/6\r (no-eol) (esc)
+                                                              \r (no-eol) (esc)
+  reached step 0
+  \r (no-eol) (esc)
+  loop [=======>                                        ] 1/6\r (no-eol) (esc)
+  loop [===============>                                ] 2/6\r (no-eol) (esc)
+  loop [=======================>                        ] 3/6\r (no-eol) (esc)
+                                                              \r (no-eol) (esc)
+  reached step 3
+  \r (no-eol) (esc)
+  loop [===============================>                ] 4/6\r (no-eol) (esc)
+  loop [=======================================>        ] 5/6\r (no-eol) (esc)
+                                                              \r (no-eol) (esc)
+
+test interaction with ui.timestamp-output
+
+  $ hg loop --warn --config ui.timestamp-output=true 6
+  \r (no-eol) (esc)
+  loop [                                                ] 0/6\r (no-eol) (esc)
+                                                              \r (no-eol) (esc)
+  \[20[2-9][0-9]-[01][0-9]-[0-3][0-9]T[0-5][0-9]:[0-5][0-9]:[0-5][0-9]\.[0-9][0-9][0-9][0-9][0-9][0-9]\] reached step 0 (re)
+  \r (no-eol) (esc)
+  loop [=======>                                        ] 1/6\r (no-eol) (esc)
+  loop [===============>                                ] 2/6\r (no-eol) (esc)
+  loop [=======================>                        ] 3/6\r (no-eol) (esc)
+                                                              \r (no-eol) (esc)
+  \[20[2-9][0-9]-[01][0-9]-[0-3][0-9]T[0-5][0-9]:[0-5][0-9]:[0-5][0-9]\.[0-9][0-9][0-9][0-9][0-9][0-9]\] reached step 3 (re)
+  \r (no-eol) (esc)
+  loop [===============================>                ] 4/6\r (no-eol) (esc)
+  loop [=======================================>        ] 5/6\r (no-eol) (esc)
+                                                              \r (no-eol) (esc)
+
 test immediate progress completion
 
   $ hg -y loop 0

--- a/tests/test-pull.t	Mon Jun 29 20:53:32 2020 +0900
+++ b/tests/test-pull.t	Thu Jun 18 15:13:38 2020 +0200
@@ -142,9 +142,9 @@
   pulling from ssh://fakehost%7Ctouch%24%7BIFS%7Downed/path
   abort: no suitable response from remote hg!
   [255]
-  $ hg pull 'ssh://fakehost%7Ctouch%20owned/path'
-  pulling from ssh://fakehost%7Ctouch%20owned/path
-  abort: no suitable response from remote hg!
+  $ hg --config ui.timestamp-output=true pull 'ssh://fakehost%7Ctouch%20owned/path'
+  \[20[2-9][0-9]-[01][0-9]-[0-3][0-9]T[0-5][0-9]:[0-5][0-9]:[0-5][0-9]\.[0-9][0-9][0-9][0-9][0-9][0-9]\] pulling from ssh://fakehost%7Ctouch%20owned/path (re)
+  \[20[2-9][0-9]-[01][0-9]-[0-3][0-9]T[0-5][0-9]:[0-5][0-9]:[0-5][0-9]\.[0-9][0-9][0-9][0-9][0-9][0-9]\] abort: no suitable response from remote hg! (re)
   [255]
 
   $ [ ! -f owned ] || echo 'you got owned'