debugcommands: introduce actions to perform deterministic reads

"readavailable" is useful as a debugging device to see what data is
available on a pipe. But the mechanism isn't deterministic because
what's available on a pipe is highly conditional on timing, system
load, OS behavior, etc. This makes it not suitable for tests.

We introduce "ereadline," "read," and "eread" for performing
deterministic I/O operations (at least on blocking file descriptors).

We stop short of converting existing consumers of "readavailable"
in tests because we're working out race conditions and deadlocks
on Windows. But the goal is to eventually move tests away from
"readavailable" to these new APIs.

Differential Revision: https://phab.mercurial-scm.org/D2720

mercurial/debugcommands.py

@@ -2715,6 +2715,21 b' def debugwireproto(ui, repo, **opts):'
 
     Read a line of output from the server. If there are multiple output
     pipes, reads only the main pipe.
+
+    ereadline
+    ---------
+
+    Like ``readline``, but read from the stderr pipe, if available.
+
+    read <X>
+    --------
+
+    ``read()`` N bytes from the server's main output pipe.
+
+    eread <X>
+    ---------
+
+    ``read()`` N bytes from the server's stderr pipe, if available.
     """
     opts = pycompat.byteskwargs(opts)
 
@@ -2855,6 +2870,14 b' def debugwireproto(ui, repo, **opts):'
             stderr.read()
         elif action == 'readline':
             stdout.readline()
+        elif action == 'ereadline':
+            stderr.readline()
+        elif action.startswith('read '):
+            count = int(action.split(' ', 1)[1])
+            stdout.read(count)
+        elif action.startswith('eread '):
+            count = int(action.split(' ', 1)[1])
+            stderr.read(count)
         else:
             raise error.Abort(_('unknown action: %s') % action)