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 unix-permissions no-root no-windows
Prepare
$ hg init a
$ echo a > a/a
$ hg -R a ci -A -m a
adding a
$ hg clone a b
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
Test that raising an exception in the release function doesn't cause the lock to choke
$ cat > testlock.py << EOF
> from mercurial import error, registrar
>
> cmdtable = {}
> command = registrar.command(cmdtable)
>
> def acquiretestlock(repo, releaseexc):
> def unlock():
> if releaseexc:
> raise error.Abort(b'expected release exception')
> l = repo._lock(repo.vfs, b'testlock', False, unlock, None, b'test lock')
> return l
>
> @command(b'testlockexc')
> def testlockexc(ui, repo):
> testlock = acquiretestlock(repo, True)
> try:
> testlock.release()
> finally:
> try:
> testlock = acquiretestlock(repo, False)
> except error.LockHeld:
> raise error.Abort(b'lockfile on disk even after releasing!')
> testlock.release()
> EOF
$ cat >> $HGRCPATH << EOF
> [extensions]
> testlock=$TESTTMP/testlock.py
> EOF
$ hg -R b testlockexc
abort: expected release exception
[255]
One process waiting for another
$ cat > hooks.py << EOF
> import time
> def sleepone(**x): time.sleep(1)
> def sleephalf(**x): time.sleep(0.5)
> EOF
$ echo b > b/b
$ hg -R b ci -A -m b --config hooks.precommit="python:`pwd`/hooks.py:sleepone" > stdout &
$ hg -R b up -q --config hooks.pre-update="python:`pwd`/hooks.py:sleephalf" \
> > preup-stdout 2>preup-stderr
$ wait
$ cat preup-stdout
$ cat preup-stderr
waiting for lock on working directory of b held by process '*' on host '*' (glob)
got lock after * seconds (glob)
$ cat stdout
adding b
On processs waiting on another, warning after a long time.
$ echo b > b/c
$ hg -R b ci -A -m b --config hooks.precommit="python:`pwd`/hooks.py:sleepone" > stdout &
$ hg -R b up -q --config hooks.pre-update="python:`pwd`/hooks.py:sleephalf" \
> --config ui.timeout.warn=250 \
> > preup-stdout 2>preup-stderr
$ wait
$ cat preup-stdout
$ cat preup-stderr
$ cat stdout
adding c
On processs waiting on another, warning disabled.
$ echo b > b/d
$ hg -R b ci -A -m b --config hooks.precommit="python:`pwd`/hooks.py:sleepone" > stdout &
$ hg -R b up -q --config hooks.pre-update="python:`pwd`/hooks.py:sleephalf" \
> --config ui.timeout.warn=-1 \
> > preup-stdout 2>preup-stderr
$ wait
$ cat preup-stdout
$ cat preup-stderr
$ cat stdout
adding d
check we still print debug output
On processs waiting on another, warning after a long time (debug output on)
$ echo b > b/e
$ hg -R b ci -A -m b --config hooks.precommit="python:`pwd`/hooks.py:sleepone" > stdout &
$ hg -R b up --config hooks.pre-update="python:`pwd`/hooks.py:sleephalf" \
> --config ui.timeout.warn=250 --debug\
> > preup-stdout 2>preup-stderr
$ wait
$ cat preup-stdout
calling hook pre-update: hghook_pre-update.sleephalf
waiting for lock on working directory of b held by process '*' on host '*' (glob)
got lock after * seconds (glob)
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cat preup-stderr
$ cat stdout
adding e
On processs waiting on another, warning disabled, (debug output on)
$ echo b > b/f
$ hg -R b ci -A -m b --config hooks.precommit="python:`pwd`/hooks.py:sleepone" > stdout &
$ hg -R b up --config hooks.pre-update="python:`pwd`/hooks.py:sleephalf" \
> --config ui.timeout.warn=-1 --debug\
> > preup-stdout 2>preup-stderr
$ wait
$ cat preup-stdout
calling hook pre-update: hghook_pre-update.sleephalf
waiting for lock on working directory of b held by process '*' on host '*' (glob)
got lock after * seconds (glob)
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cat preup-stderr
$ cat stdout
adding f
Pushing to a local read-only repo that can't be locked
$ chmod 100 a/.hg/store
$ hg -R b push a
pushing to a
searching for changes
abort: could not lock repository a: Permission denied
[255]
$ chmod 700 a/.hg/store