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
# Here we create a simple DAG which has just enough of the required
# topology to test all the bisection status labels:
#
# 13--14
# /
# 0--1--2--3---------9--10--11--12
# \ /
# 4--5--6--7--8
$ hg init
$ echo '0' >a
$ hg add a
$ hg ci -u test -d '0 0' -m '0'
$ echo '1' >a
$ hg ci -u test -d '1 0' -m '1'
branch 2-3
$ echo '2' >b
$ hg add b
$ hg ci -u test -d '2 0' -m '2'
$ echo '3' >b
$ hg ci -u test -d '3 0' -m '3'
branch 4-8
$ hg up -r 1
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ echo '4' >c
$ hg add c
$ hg ci -u test -d '4 0' -m '4'
created new head
$ echo '5' >c
$ hg ci -u test -d '5 0' -m '5'
$ echo '6' >c
$ hg ci -u test -d '6 0' -m '6'
$ echo '7' >c
$ hg ci -u test -d '7 0' -m '7'
$ echo '8' >c
$ hg ci -u test -d '8 0' -m '8'
merge
$ hg merge -r 3
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ hg ci -u test -d '9 0' -m '9=8+3'
$ echo '10' >a
$ hg ci -u test -d '10 0' -m '10'
$ echo '11' >a
$ hg ci -u test -d '11 0' -m '11'
$ echo '12' >a
$ hg ci -u test -d '12 0' -m '12'
unrelated branch
$ hg up -r 3
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ echo '13' >d
$ hg add d
$ hg ci -u test -d '13 0' -m '13'
created new head
$ echo '14' >d
$ hg ci -u test -d '14 0' -m '14'
mark changesets
$ hg bisect --reset
$ hg bisect --good 4
$ hg bisect --good 6
$ hg bisect --bad 12
Testing changeset 9:2197c557e14c (6 changesets remaining, ~2 tests)
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ hg bisect --bad 10
Testing changeset 8:e74a86251f58 (4 changesets remaining, ~2 tests)
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ hg bisect --skip 7
Testing changeset 8:e74a86251f58 (4 changesets remaining, ~2 tests)
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
test template
$ hg log --template '{rev}:{node|short} {bisect}\n'
14:cbf2f3105bbf
13:e07efca37c43
12:98c6b56349c0 bad
11:03f491376e63 bad (implicit)
10:c012b15e2409 bad
9:2197c557e14c untested
8:e74a86251f58 untested
7:a5f87041c899 skipped
6:7d997bedcd8d good
5:2dd1875f1028 good (implicit)
4:2a1daef14cd4 good
3:8417d459b90c ignored
2:e1355ee1f23e ignored
1:ce7c85e06a9f good (implicit)
0:b4e73ffab476 good (implicit)
$ hg log --template '{bisect|shortbisect} {rev}:{node|short}\n'
14:cbf2f3105bbf
13:e07efca37c43
B 12:98c6b56349c0
B 11:03f491376e63
B 10:c012b15e2409
U 9:2197c557e14c
U 8:e74a86251f58
S 7:a5f87041c899
G 6:7d997bedcd8d
G 5:2dd1875f1028
G 4:2a1daef14cd4
I 3:8417d459b90c
I 2:e1355ee1f23e
G 1:ce7c85e06a9f
G 0:b4e73ffab476
test style
$ hg log --style bisect
changeset: 14:cbf2f3105bbf
bisect:
tag: tip
user: test
date: Thu Jan 01 00:00:14 1970 +0000
summary: 14
changeset: 13:e07efca37c43
bisect:
parent: 3:8417d459b90c
user: test
date: Thu Jan 01 00:00:13 1970 +0000
summary: 13
changeset: 12:98c6b56349c0
bisect: bad
user: test
date: Thu Jan 01 00:00:12 1970 +0000
summary: 12
changeset: 11:03f491376e63
bisect: bad (implicit)
user: test
date: Thu Jan 01 00:00:11 1970 +0000
summary: 11
changeset: 10:c012b15e2409
bisect: bad
user: test
date: Thu Jan 01 00:00:10 1970 +0000
summary: 10
changeset: 9:2197c557e14c
bisect: untested
parent: 8:e74a86251f58
parent: 3:8417d459b90c
user: test
date: Thu Jan 01 00:00:09 1970 +0000
summary: 9=8+3
changeset: 8:e74a86251f58
bisect: untested
user: test
date: Thu Jan 01 00:00:08 1970 +0000
summary: 8
changeset: 7:a5f87041c899
bisect: skipped
user: test
date: Thu Jan 01 00:00:07 1970 +0000
summary: 7
changeset: 6:7d997bedcd8d
bisect: good
user: test
date: Thu Jan 01 00:00:06 1970 +0000
summary: 6
changeset: 5:2dd1875f1028
bisect: good (implicit)
user: test
date: Thu Jan 01 00:00:05 1970 +0000
summary: 5
changeset: 4:2a1daef14cd4
bisect: good
parent: 1:ce7c85e06a9f
user: test
date: Thu Jan 01 00:00:04 1970 +0000
summary: 4
changeset: 3:8417d459b90c
bisect: ignored
user: test
date: Thu Jan 01 00:00:03 1970 +0000
summary: 3
changeset: 2:e1355ee1f23e
bisect: ignored
user: test
date: Thu Jan 01 00:00:02 1970 +0000
summary: 2
changeset: 1:ce7c85e06a9f
bisect: good (implicit)
user: test
date: Thu Jan 01 00:00:01 1970 +0000
summary: 1
changeset: 0:b4e73ffab476
bisect: good (implicit)
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: 0
$ hg log --quiet --style bisect
14:cbf2f3105bbf
13:e07efca37c43
B 12:98c6b56349c0
B 11:03f491376e63
B 10:c012b15e2409
U 9:2197c557e14c
U 8:e74a86251f58
S 7:a5f87041c899
G 6:7d997bedcd8d
G 5:2dd1875f1028
G 4:2a1daef14cd4
I 3:8417d459b90c
I 2:e1355ee1f23e
G 1:ce7c85e06a9f
G 0:b4e73ffab476
$ hg --config extensions.color= --color=debug log --quiet --style bisect
[log.bisect| ] 14:cbf2f3105bbf
[log.bisect| ] 13:e07efca37c43
[log.bisect bisect.bad|B] 12:98c6b56349c0
[log.bisect bisect.bad|B] 11:03f491376e63
[log.bisect bisect.bad|B] 10:c012b15e2409
[log.bisect bisect.untested|U] 9:2197c557e14c
[log.bisect bisect.untested|U] 8:e74a86251f58
[log.bisect bisect.skipped|S] 7:a5f87041c899
[log.bisect bisect.good|G] 6:7d997bedcd8d
[log.bisect bisect.good|G] 5:2dd1875f1028
[log.bisect bisect.good|G] 4:2a1daef14cd4
[log.bisect bisect.ignored|I] 3:8417d459b90c
[log.bisect bisect.ignored|I] 2:e1355ee1f23e
[log.bisect bisect.good|G] 1:ce7c85e06a9f
[log.bisect bisect.good|G] 0:b4e73ffab476