worker: don't expose readinto() on _blockingreader since pickle is picky

The `pickle` module expects the input to be buffered and a whole
object to be available when `pickle.load()` is called, which is not
necessarily true when we send data from workers back to the parent
process (i.e., it seems like a bad assumption for the `pickle` module
to make). We added a workaround for that in
https://phab.mercurial-scm.org/D8076, which made `read()` continue
until all the requested bytes have been read.

As we found out at work after a lot of investigation (I've spent the
last two days on this), the native version of `pickle.load()` has
started calling `readinto()` on the input since Python 3.8. That
started being called in
https://github.com/python/cpython/commit/91f4380cedbae32b49adbea2518014a5624c6523
(and only by the C version of `pickle.load()`)). Before that, it was
only `read()` and `readline()` that were called. The problem with that
was that `readinto()` on our `_blockingreader` was simply delegating
to the underlying, *unbuffered* object. The symptom we saw was that
`hg fix` started failing sometimes on Python 3.8 on Mac. It failed
very relyable in some cases. I still haven't figured out under what
circumstances it fails and I've been unable to reproduce it in test
cases (I've tried writing larger amounts of data, using different
numbers of workers, and making the formatters sleep). I have, however,
been able to reproduce it 3-4 times on Linux, but then it stopped
reproducing on the following few hundred attempts.

To fix the problem, we can simply remove the implementation of
`readinto()`, since the unpickler will then fall back to calling
`read()`. The fallback was added a bit later, in
https://github.com/python/cpython/commit/b19f7ecfa3adc6ba1544225317b9473649815b38. However,
that commit also added checking that what `read()` returns is a
`bytes`, so we also need to convert the `bytearray` we use into
that. I was able to add a test for that failure at least.

Differential Revision: https://phab.mercurial-scm.org/D8928

tests/test-fix-pickle.t

@@ -0,0 +1,45 b''
+A script that implements uppercasing all letters in a file.
+
+  $ UPPERCASEPY="$TESTTMP/uppercase.py"
+  $ cat > $UPPERCASEPY <<EOF
+  > import sys
+  > from mercurial.utils.procutil import setbinary
+  > setbinary(sys.stdin)
+  > setbinary(sys.stdout)
+  > sys.stdout.write(sys.stdin.read().upper())
+  > EOF
+  $ TESTLINES="foo\nbar\nbaz\n"
+  $ printf $TESTLINES | "$PYTHON" $UPPERCASEPY
+  FOO
+  BAR
+  BAZ
+
+This file attempts to test our workarounds for pickle's lack of
+support for short reads.
+
+  $ cat >> $HGRCPATH <<EOF
+  > [extensions]
+  > fix =
+  > [fix]
+  > uppercase-whole-file:command="$PYTHON" $UPPERCASEPY
+  > uppercase-whole-file:pattern=set:**
+  > EOF
+
+  $ hg init repo
+  $ cd repo
+
+# Create a file that's large enough that it seems to not fit in
+# pickle's buffer, making it use the code path that expects our
+# _blockingreader's read() method to return bytes.
+  $ echo "some stuff" > file
+  $ for i in $($TESTDIR/seq.py 13); do
+  >   cat file file > tmp
+  >   mv -f tmp file
+  > done
+  $ hg commit -Am "add large file"
+  adding file
+
+Check that we don't get a crash
+
+  $ hg fix -r .
+  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/*-fix.hg (glob)

mercurial/worker.py

@@ -71,8 +71,12 b' if pycompat.ispy3:'
         def __init__(self, wrapped):
             self._wrapped = wrapped
 
-        def __getattr__(self, attr):
-            return getattr(self._wrapped, attr)
+        # Do NOT implement readinto() by making it delegate to
+        # _wrapped.readinto(), since that is unbuffered. The unpickler is fine
+        # with just read() and readline(), so we don't need to implement it.
+
+        def readline(self):
+            return self._wrapped.readline()
 
         # issue multiple reads until size is fulfilled
         def read(self, size=-1):
@@ -91,7 +95,7 b' if pycompat.ispy3:'
 
             del view
             del buf[pos:]
-            return buf
+            return bytes(buf)
 
 
 else: