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
Minimal hgk check
$ echo "[extensions]" >> $HGRCPATH
$ echo "hgk=" >> $HGRCPATH
$ hg init repo
$ cd repo
$ echo a > a
$ hg ci -Am adda
adding a
$ hg debug-cat-file commit 0
tree a0c8bcbbb45c
parent 000000000000
author test 0 0
revision 0
branch default
phase draft
adda
$ echo b > b
$ hg ci -Am addb
adding b
$ hg log -T '{node}\n'
102a90ea7b4a3361e4082ed620918c261189a36a
07f4944404050f47db2e5c5071e0e84e7a27bba9
$ hg debug-diff-tree 07f494440405 102a90ea7b4a
:000000 100664 000000000000 1e88685f5dde N b b
$ hg debug-diff-tree 07f494440405 102a90ea7b4a --patch
diff --git a/b b/b
new file mode 100644
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+b
Ensure that diff-tree output isn't affected by diffopts
$ hg --config diff.noprefix=True debug-diff-tree 07f494440405 102a90ea7b4a
:000000 100664 000000000000 1e88685f5dde N b b
$ hg --config diff.noprefix=True debug-diff-tree --patch 07f494440405 102a90ea7b4a
diff --git a/b b/b
new file mode 100644
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+b
$ cd ..