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
#require cvs
This is https://bz.mercurial-scm.org/1148
and https://bz.mercurial-scm.org/1447
$ cvscall()
> {
> cvs -f "$@" > /dev/null
> }
$ cat <<EOF >> $HGRCPATH
> [extensions]
> convert =
> [convert]
> cvsps.cache = 0
> EOF
create cvs repository
$ mkdir cvsrepo
$ cd cvsrepo
$ CVSROOT=`pwd`
$ export CVSROOT
$ CVS_OPTIONS=-f
$ export CVS_OPTIONS
$ cd ..
$ rmdir cvsrepo
$ cvscall -q -d "$CVSROOT" init
Create a new project
$ mkdir src
$ cd src
$ echo "1" > a
$ echo "1" > b
$ cvscall import -m "init" src v0 r0 | sort
$ cd ..
$ cvscall co src
cvs checkout: Updating src
$ cd src
Branch the project
$ cvscall tag -b BRANCH
cvs tag: Tagging .
$ cvscall up -r BRANCH > /dev/null
cvs update: Updating .
Modify file a, then b, then a
$ sleep 1
$ echo "2" > a
$ cvscall ci -m "mod a"
cvs commit: Examining .
$ echo "2" > b
$ cvscall ci -m "mod b"
cvs commit: Examining .
$ sleep 1
$ echo "3" > a
$ cvscall ci -m "mod a again"
cvs commit: Examining .
Convert
$ cd ..
$ hg convert src
assuming destination src-hg
initializing destination src-hg repository
connecting to $TESTTMP/cvsrepo
scanning source...
collecting CVS rlog
7 log entries
creating changesets
5 changeset entries
sorting...
converting...
4 Initial revision
3 init
2 mod a
1 mod b
0 mod a again
updating tags
Check the result
$ hg -R src-hg log -G --template '{rev} ({branches}) {desc} files: {files}\n'
o 5 () update tags files: .hgtags
|
| o 4 (BRANCH) mod a again files: a
| |
| o 3 (BRANCH) mod b files: b
| |
| o 2 (BRANCH) mod a files: a
| |
| o 1 (v0) init files:
|/
o 0 () Initial revision files: a b
issue 1447
$ cvscall()
> {
> cvs -f "$@" > /dev/null
> sleep 1
> }
$ cvsci()
> {
> cvs -f ci "$@" >/dev/null
> sleep 1
> }
$ cvscall -Q -d `pwd`/cvsmaster2 init
$ cd cvsmaster2
$ CVSROOT=`pwd`
$ export CVSROOT
$ mkdir foo
$ cd ..
$ cvscall -Q co -d cvswork2 foo
$ cd cvswork2
$ echo foo > a.txt
$ echo bar > b.txt
$ cvscall -Q add a.txt b.txt
$ cvsci -m "Initial commit"
cvs commit: Examining .
$ echo foo > b.txt
$ cvsci -m "Fix b on HEAD"
cvs commit: Examining .
$ echo bar > a.txt
$ cvsci -m "Small fix in a on HEAD"
cvs commit: Examining .
$ cvscall -Q tag -b BRANCH
$ cvscall -Q up -P -rBRANCH
$ echo baz > b.txt
$ cvsci -m "Change on BRANCH in b"
cvs commit: Examining .
$ hg debugcvsps -x --parents foo
collecting CVS rlog
5 log entries
creating changesets
4 changeset entries
---------------------
PatchSet 1
Date: * (glob)
Author: * (glob)
Branch: HEAD
Tag: (none)
Log:
Initial commit
Members:
a.txt:INITIAL->1.1
b.txt:INITIAL->1.1
---------------------
PatchSet 2
Date: * (glob)
Author: * (glob)
Branch: HEAD
Tag: (none)
Branchpoints: BRANCH
Parent: 1
Log:
Fix b on HEAD
Members:
b.txt:1.1->1.2
---------------------
PatchSet 3
Date: * (glob)
Author: * (glob)
Branch: HEAD
Tag: (none)
Branchpoints: BRANCH
Parent: 2
Log:
Small fix in a on HEAD
Members:
a.txt:1.1->1.2
---------------------
PatchSet 4
Date: * (glob)
Author: * (glob)
Branch: BRANCH
Tag: (none)
Parent: 3
Log:
Change on BRANCH in b
Members:
b.txt:1.2->1.2.2.1
$ cd ..