posix: implement readpipe using non-blocking I/O (issue4336)

On Linux, fstat().st_size of a pipe always returns 0, even if the
pipe has data available for reading. This meant that reading from
and subsequently printing the stderr pipe content after wireproto
commands over SSH meant that available data wasn't being printed.

We now implement pipe reading on POSIX by doing a non-blocking
read for all available data.

mercurial/posix.py

@@ -8,6 +8,7 b''
 from i18n import _
 import encoding
 import os, sys, errno, stat, getpass, pwd, grp, socket, tempfile, unicodedata
+import fcntl
 
 posixfile = open
 normpath = os.path.normpath
@@ -432,7 +433,7 b' def gethgcmd():'
 
 def termwidth():
     try:
-        import termios, array, fcntl
+        import termios, array
         for dev in (sys.stderr, sys.stdout, sys.stdin):
             try:
                 try:
@@ -570,13 +571,24 b' def statisexec(st):'
 
 def readpipe(pipe):
     """Read all available data from a pipe."""
-    chunks = []
-    while True:
-        size = os.fstat(pipe.fileno()).st_size
-        if not size:
-            break
+    # We can't fstat() a pipe because Linux will always report 0.
+    # So, we set the pipe to non-blocking mode and read everything
+    # that's available.
+    flags = fcntl.fcntl(pipe, fcntl.F_GETFL)
+    flags |= os.O_NONBLOCK
+    oldflags = fcntl.fcntl(pipe, fcntl.F_SETFL, flags)
 
-        s = pipe.read(size)
-        if not s:
-            break
-        chunks.append(s)
+    try:
+        chunks = []
+        while True:
+            try:
+                s = pipe.read()
+                if not s:
+                    break
+                chunks.append(s)
+            except IOError:
+                break
+
+        return ''.join(chunks)
+    finally:
+        fcntl.fcntl(pipe, fcntl.F_SETFL, oldflags)