cborutil: implement sans I/O decoder
The vendored CBOR package decodes by calling read(n) on an object.
There are a number of disadvantages to this:
* Uses blocking I/O. If sufficient data is not available, the decoder
will hang until it is.
* No support for partial reads. If the read(n) returns less data than
requested, the decoder raises an error.
* Requires the use of a file like object. If the original data is in
say a buffer, we need to "cast" it to e.g. a BytesIO to appease the
decoder.
In addition, the vendored CBOR decoder doesn't provide flexibility
that we desire. Specifically:
* It buffers indefinite length bytestrings instead of streaming them.
* It doesn't allow limiting the set of types that can be decoded. This
property is useful when implementing a "hardened" decoder that is
less susceptible to abusive input.
* It doesn't provide sufficient "hook points" and introspection to
institute checks around behavior. These are useful for implementing
a "hardened" decoder.
This all adds up to a reasonable set of justifications for writing our
own decoder.
So, this commit implements our own CBOR decoder.
At the heart of the decoder is a function that decodes a single "item"
from a buffer. This item can be a complete simple value or a special
value, such as "start of array." Using this function, we can build a
decoder that effectively iterates over the stream of decoded items and
builds up higher-level values, such as arrays, maps, sets, and indefinite
length bytestrings. And we can do this without performing I/O in the
decoder itself.
The core of the sans I/O decoder will probably not be used directly.
Instead, it is expected that we'll build utility functions for invoking
the decoder given specific input types. This will allow extreme
flexibility in how data is delivered to the decoder.
I'm pretty happy with the state of the decoder modulo the TODO items
to track wanted features to help with a "hardened" decoder. The one
thing I could be convinced to change is the handling of semantic tags.
Since we only support a single semantic tag (sets), I thought it would
be easier to handle them inline in decodeitem(). This is simpler now.
But if we add support for other semantic tags, it will likely be easier
to move semantic tag handling outside of decodeitem(). But, properly
supporting semantic tags opens up a whole can of worms, as many
semantic tags imply new types. I'm optimistic we won't need these in
Mercurial. But who knows.
I'm also pretty happy with the test coverage. Writing comprehensive
tests for partial decoding did flush out a handful of bugs. One
general improvement to testing would be fuzz testing for partial
decoding. I may implement that later. I also anticipate switching the
wire protocol code to this new decoder will flush out any lingering
bugs.
Differential Revision: https://phab.mercurial-scm.org/D4414
$ hg init repo
$ cd repo
$ echo a > a
$ hg add a
$ hg commit -m test
Do we ever miss a sub-second change?:
$ for i in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20; do
> hg co -qC 0
> echo b > a
> hg st
> done
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
$ echo test > b
$ mkdir dir1
$ echo test > dir1/c
$ echo test > d
$ echo test > e
#if execbit
A directory will typically have the execute bit -- make sure it doesn't get
confused with a file with the exec bit set
$ chmod +x e
#endif
$ hg add b dir1 d e
adding dir1/c
$ hg commit -m test2
$ cat >> $TESTTMP/dirstaterace.py << EOF
> from mercurial import (
> context,
> extensions,
> )
> def extsetup():
> extensions.wrapfunction(context.workingctx, '_checklookup', overridechecklookup)
> def overridechecklookup(orig, self, files):
> # make an update that changes the dirstate from underneath
> self._repo.ui.system(r"sh '$TESTTMP/dirstaterace.sh'",
> cwd=self._repo.root)
> return orig(self, files)
> EOF
$ hg debugrebuilddirstate
$ hg debugdirstate
n 0 -1 unset a
n 0 -1 unset b
n 0 -1 unset d
n 0 -1 unset dir1/c
n 0 -1 unset e
XXX Note that this returns M for files that got replaced by directories. This is
definitely a bug, but the fix for that is hard and the next status run is fine
anyway.
$ cat > $TESTTMP/dirstaterace.sh <<EOF
> rm b && rm -r dir1 && rm d && mkdir d && rm e && mkdir e
> EOF
$ hg status --config extensions.dirstaterace=$TESTTMP/dirstaterace.py
M d
M e
! b
! dir1/c
$ hg debugdirstate
n 644 2 * a (glob)
n 0 -1 unset b
n 0 -1 unset d
n 0 -1 unset dir1/c
n 0 -1 unset e
$ hg status
! b
! d
! dir1/c
! e
$ rmdir d e
$ hg update -C -q .
Test that dirstate changes aren't written out at the end of "hg
status", if .hg/dirstate is already changed simultaneously before
acquisition of wlock in workingctx._poststatusfixup().
This avoidance is important to keep consistency of dirstate in race
condition (see issue5584 for detail).
$ hg parents -q
1:* (glob)
$ hg debugrebuilddirstate
$ hg debugdirstate
n 0 -1 unset a
n 0 -1 unset b
n 0 -1 unset d
n 0 -1 unset dir1/c
n 0 -1 unset e
$ cat > $TESTTMP/dirstaterace.sh <<EOF
> # This script assumes timetable of typical issue5584 case below:
> #
> # 1. "hg status" loads .hg/dirstate
> # 2. "hg status" confirms clean-ness of FILE
> # 3. "hg update -C 0" updates the working directory simultaneously
> # (FILE is removed, and FILE is dropped from .hg/dirstate)
> # 4. "hg status" acquires wlock
> # (.hg/dirstate is re-loaded = no FILE entry in dirstate)
> # 5. "hg status" marks FILE in dirstate as clean
> # (FILE entry is added to in-memory dirstate)
> # 6. "hg status" writes dirstate changes into .hg/dirstate
> # (FILE entry is written into .hg/dirstate)
> #
> # To reproduce similar situation easily and certainly, #2 and #3
> # are swapped. "hg cat" below ensures #2 on "hg status" side.
>
> hg update -q -C 0
> hg cat -r 1 b > b
> EOF
"hg status" below should excludes "e", of which exec flag is set, for
portability of test scenario, because unsure but missing "e" is
treated differently in _checklookup() according to runtime platform.
- "missing(!)" on POSIX, "pctx[f].cmp(self[f])" raises ENOENT
- "modified(M)" on Windows, "self.flags(f) != pctx.flags(f)" is True
$ hg status --config extensions.dirstaterace=$TESTTMP/dirstaterace.py --debug -X path:e
skip updating dirstate: identity mismatch
M a
! d
! dir1/c
$ hg parents -q
0:* (glob)
$ hg files
a
$ hg debugdirstate
n * * * a (glob)
$ rm b
#if fsmonitor
Create fsmonitor state.
$ hg status
$ f --type .hg/fsmonitor.state
.hg/fsmonitor.state: file
Test that invalidating fsmonitor state in the middle (which doesn't require the
wlock) causes the fsmonitor update to be skipped.
hg debugrebuilddirstate ensures that the dirstaterace hook will be called, but
it also invalidates the fsmonitor state. So back it up and restore it.
$ mv .hg/fsmonitor.state .hg/fsmonitor.state.tmp
$ hg debugrebuilddirstate
$ mv .hg/fsmonitor.state.tmp .hg/fsmonitor.state
$ cat > $TESTTMP/dirstaterace.sh <<EOF
> rm .hg/fsmonitor.state
> EOF
$ hg status --config extensions.dirstaterace=$TESTTMP/dirstaterace.py --debug
skip updating fsmonitor.state: identity mismatch
$ f .hg/fsmonitor.state
.hg/fsmonitor.state: file not found
#endif
Set up a rebase situation for issue5581.
$ echo c2 > a
$ echo c2 > b
$ hg add b
$ hg commit -m c2
created new head
$ echo c3 >> a
$ hg commit -m c3
$ hg update 2
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo c4 >> a
$ echo c4 >> b
$ hg commit -m c4
created new head
Configure a merge tool that runs status in the middle of the rebase. The goal of
the status call is to trigger a potential bug if fsmonitor's state is written
even though the wlock is held by another process. The output of 'hg status' in
the merge tool goes to /dev/null because we're more interested in the results of
'hg status' run after the rebase.
$ cat >> $TESTTMP/mergetool-race.sh << EOF
> echo "custom merge tool"
> printf "c2\nc3\nc4\n" > \$1
> hg --cwd "$TESTTMP/repo" status > /dev/null
> echo "custom merge tool end"
> EOF
$ cat >> $HGRCPATH << EOF
> [extensions]
> rebase =
> [merge-tools]
> test.executable=sh
> test.args=$TESTTMP/mergetool-race.sh \$output
> EOF
$ hg rebase -s . -d 3 --tool test
rebasing 4:b08445fd6b2a "c4" (tip)
merging a
custom merge tool
custom merge tool end
saved backup bundle to $TESTTMP/repo/.hg/strip-backup/* (glob)
This hg status should be empty, whether or not fsmonitor is enabled (issue5581).
$ hg status