util: always force line buffered stdout when stdout is a tty (BC)

pager replaced stdout with a line buffered version to work around glibc
deciding on a buffering strategy on the first write to stdout. This is going
to make my next patch hard, as replacing stdout will make tracking time
spent blocked on it more challenging.

Move the line buffering requirement to util.py, and remove it from pager.
This means that the abuse of ui.formatted=True and pager set to cat or equivalent
no longer results in a line-buffered output to a pipe, hence (BC), although
I don't expect anyone to be affected

hgext/pager.py

@@ -87,14 +87,10 b' def _runpager(ui, p):'
                              close_fds=util.closefds, stdin=subprocess.PIPE,
                              stdout=util.stdout, stderr=util.stderr)
 
-    # back up original file objects and descriptors
-    olduifout = ui.fout
-    oldstdout = util.stdout
+    # back up original file descriptors
     stdoutfd = os.dup(util.stdout.fileno())
     stderrfd = os.dup(util.stderr.fileno())
 
-    # create new line-buffered stdout so that output can show up immediately
-    ui.fout = util.stdout = newstdout = os.fdopen(util.stdout.fileno(), 'wb', 1)
     os.dup2(pager.stdin.fileno(), util.stdout.fileno())
     if ui._isatty(util.stderr):
         os.dup2(pager.stdin.fileno(), util.stderr.fileno())
@@ -103,15 +99,10 b' def _runpager(ui, p):'
     def killpager():
         if util.safehasattr(signal, "SIGINT"):
             signal.signal(signal.SIGINT, signal.SIG_IGN)
-        pager.stdin.close()
-        ui.fout = olduifout
-        util.stdout = oldstdout
-        # close new stdout while it's associated with pager; otherwise stdout
-        # fd would be closed when newstdout is deleted
-        newstdout.close()
-        # restore original fds: stdout is open again
+        # restore original fds, closing pager.stdin copies in the process
         os.dup2(stdoutfd, util.stdout.fileno())
         os.dup2(stderrfd, util.stderr.fileno())
+        pager.stdin.close()
         pager.wait()
 
 def uisetup(ui):

mercurial/util.py

@@ -63,9 +63,21 b' urlparse = pycompat.urlparse'
 urlreq = pycompat.urlreq
 xmlrpclib = pycompat.xmlrpclib
 
+def isatty(fp):
+    try:
+        return fp.isatty()
+    except AttributeError:
+        return False
+
+# glibc determines buffering on first write to stdout - if we replace a TTY
+# destined stdout with a pipe destined stdout (e.g. pager), we want line
+# buffering
+if isatty(stdout):
+    stdout = os.fdopen(stdout.fileno(), 'wb', 1)
+
 if pycompat.osname == 'nt':
     from . import windows as platform
-    stdout = platform.winstdout(pycompat.stdout)
+    stdout = platform.winstdout(stdout)
 else:
     from . import posix as platform
 
@@ -2750,12 +2762,6 b' def removeauth(u):'
     u.user = u.passwd = None
     return str(u)
 
-def isatty(fp):
-    try:
-        return fp.isatty()
-    except AttributeError:
-        return False
-
 timecount = unitcountfn(
     (1, 1e3, _('%.0f s')),
     (100, 1, _('%.1f s')),