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
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
make a test repository that looks like this:
o 2:28bc7b1afd6a
|
| @ 1:d7fe2034f71b
|/
o 0/62ecad8b70e5
$ hg init r0
$ cd r0
$ touch f0
$ hg ci -m0 -Aq
$ touch f1
$ hg ci -m1 -Aq
$ hg update 0 -q
$ touch f2
$ hg ci -m2 -Aq
$ hg update 1 -q
make some patches with a parent: 1:d7fe2034f71b -> p0 -> p1
$ echo cp0 >> fp0
$ hg add fp0
$ hg ci -m p0 -d "0 0"
$ hg export -r. > p0
$ hg strip -qn .
$ hg qimport p0
adding p0 to series file
$ hg qpush
applying p0
now at: p0
$ echo cp1 >> fp1
$ hg add fp1
$ hg qnew p1 -d "0 0"
$ hg qpop -aq
patch queue now empty
qpush --exact when at the parent
$ hg update 1 -q
$ hg qpush -e
applying p0
now at: p0
$ hg parents -qr qbase
1:d7fe2034f71b
$ hg qpop -aq
patch queue now empty
$ hg qpush -e p0
applying p0
now at: p0
$ hg parents -qr qbase
1:d7fe2034f71b
$ hg qpop -aq
patch queue now empty
$ hg qpush -e p1
applying p0
applying p1
now at: p1
$ hg parents -qr qbase
1:d7fe2034f71b
$ hg qpop -aq
patch queue now empty
qpush --exact when at another rev
$ hg update 0 -q
$ hg qpush -e
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
applying p0
now at: p0
$ hg parents -qr qbase
1:d7fe2034f71b
$ hg qpop -aq
patch queue now empty
$ hg update 0 -q
$ hg qpush -e p0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
applying p0
now at: p0
$ hg parents -qr qbase
1:d7fe2034f71b
$ hg qpop -aq
patch queue now empty
$ hg update 0 -q
$ hg qpush -e p1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
applying p0
applying p1
now at: p1
$ hg parents -qr qbase
1:d7fe2034f71b
$ hg qpop -aq
patch queue now empty
$ hg update 0 -q
$ hg qpush -ea
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
applying p0
applying p1
now at: p1
$ hg parents -qr qbase
1:d7fe2034f71b
$ hg qpop -aq
patch queue now empty
qpush --exact while crossing branches
$ hg update 2 -q
$ hg qpush -e
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
applying p0
now at: p0
$ hg parents -qr qbase
1:d7fe2034f71b
$ hg qpop -aq
patch queue now empty
$ hg update 2 -q
$ hg qpush -e p0
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
applying p0
now at: p0
$ hg parents -qr qbase
1:d7fe2034f71b
$ hg qpop -aq
patch queue now empty
$ hg update 2 -q
$ hg qpush -e p1
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
applying p0
applying p1
now at: p1
$ hg parents -qr qbase
1:d7fe2034f71b
$ hg qpop -aq
patch queue now empty
$ hg update 2 -q
$ hg qpush -ea
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
applying p0
applying p1
now at: p1
$ hg parents -qr qbase
1:d7fe2034f71b
$ hg qpop -aq
patch queue now empty
qpush --exact --force with changes to an unpatched file
$ hg update 1 -q
$ echo c0 >> f0
$ hg qpush -e
abort: local changes found
[255]
$ hg qpush -ef
applying p0
now at: p0
$ cat f0
c0
$ rm f0
$ touch f0
$ hg qpop -aq
patch queue now empty
$ hg update 1 -q
$ echo c0 >> f0
$ hg qpush -e p1
abort: local changes found
[255]
$ hg qpush -e p1 -f
applying p0
applying p1
now at: p1
$ cat f0
c0
$ rm f0
$ touch f0
$ hg qpop -aq
patch queue now empty
qpush --exact --force with changes to a patched file
$ hg update 1 -q
$ echo cp0-bad >> fp0
$ hg add fp0
$ hg qpush -e
abort: local changes found
[255]
$ hg qpush -ef
applying p0
file fp0 already exists
1 out of 1 hunks FAILED -- saving rejects to file fp0.rej
patch failed, unable to continue (try -v)
patch failed, rejects left in working directory
errors during apply, please fix and qrefresh p0
[2]
$ cat fp0
cp0-bad
$ cat fp0.rej
--- fp0
+++ fp0
@@ -0,0 +1,1 @@
+cp0
$ hg qpop -aqf
patch queue now empty
$ rm fp0
$ rm fp0.rej
$ hg update 1 -q
$ echo cp1-bad >> fp1
$ hg add fp1
$ hg qpush -e p1
abort: local changes found
[255]
$ hg qpush -e p1 -f
applying p0
applying p1
file fp1 already exists
1 out of 1 hunks FAILED -- saving rejects to file fp1.rej
patch failed, unable to continue (try -v)
patch failed, rejects left in working directory
errors during apply, please fix and qrefresh p1
[2]
$ cat fp1
cp1-bad
$ cat fp1.rej
--- fp1
+++ fp1
@@ -0,0 +1,1 @@
+cp1
$ hg qpop -aqf
patch queue now empty
$ hg forget fp1
$ rm fp1
$ rm fp1.rej
qpush --exact when already at a patch
$ hg update 1
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg qpush -e p0
applying p0
now at: p0
$ hg qpush -e p1
abort: cannot push --exact with applied patches
[255]
$ hg qpop -aq
patch queue now empty
qpush --exact --move should fail
$ hg qpush -e --move p1
abort: cannot use --exact and --move together
[255]
qpush --exact a patch without a parent recorded
$ hg qpush -q
now at: p0
$ grep -v '# Parent' .hg/patches/p0 > p0.new
$ mv p0.new .hg/patches/p0
$ hg qpop -aq
patch queue now empty
$ hg qpush -e
abort: p0 does not have a parent recorded
[255]
$ hg qpush -e p0
abort: p0 does not have a parent recorded
[255]
$ hg qpush -e p1
abort: p0 does not have a parent recorded
[255]
$ hg qpush -ea
abort: p0 does not have a parent recorded
[255]
$ cd ..