tests/test-diff-change.t

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

Testing diff --change

  $ hg init a
  $ cd a

  $ echo "first" > file.txt
  $ hg add file.txt
  $ hg commit -m 'first commit' # 0

  $ echo "second" > file.txt
  $ hg commit -m 'second commit' # 1

  $ echo "third" > file.txt
  $ hg commit -m 'third commit' # 2

  $ hg diff --nodates --change 1
  diff -r 4bb65dda5db4 -r e9b286083166 file.txt
  --- a/file.txt
  +++ b/file.txt
  @@ -1,1 +1,1 @@
  -first
  +second

  $ hg diff --change e9b286083166
  diff -r 4bb65dda5db4 -r e9b286083166 file.txt
  --- a/file.txt	Thu Jan 01 00:00:00 1970 +0000
  +++ b/file.txt	Thu Jan 01 00:00:00 1970 +0000
  @@ -1,1 +1,1 @@
  -first
  +second

  $ cd ..

Test dumb revspecs: top-level "x:y", "x:", ":y" and ":" ranges should be handled
as pairs even if x == y, but not for "f(x:y)" nor "x::y" (issue3474, issue4774)

  $ hg clone -q a dumbspec
  $ cd dumbspec
  $ echo "wdir" > file.txt

  $ hg diff -r 2:2
  $ hg diff -r 2:.
  $ hg diff -r 2:
  $ hg diff -r :0
  $ hg diff -r '2:first(2:2)'
  $ hg diff -r 'first(2:2)' --nodates
  diff -r bf5ff72eb7e0 file.txt
  --- a/file.txt
  +++ b/file.txt
  @@ -1,1 +1,1 @@
  -third
  +wdir
  $ hg diff -r '(2:2)' --nodates
  diff -r bf5ff72eb7e0 file.txt
  --- a/file.txt
  +++ b/file.txt
  @@ -1,1 +1,1 @@
  -third
  +wdir
  $ hg diff -r 2::2 --nodates
  diff -r bf5ff72eb7e0 file.txt
  --- a/file.txt
  +++ b/file.txt
  @@ -1,1 +1,1 @@
  -third
  +wdir
  $ hg diff -r "2 and 1"
  abort: empty revision range
  [255]

  $ cd ..

  $ hg clone -qr0 a dumbspec-rev0
  $ cd dumbspec-rev0
  $ echo "wdir" > file.txt

  $ hg diff -r :
  $ hg diff -r 'first(:)' --nodates
  diff -r 4bb65dda5db4 file.txt
  --- a/file.txt
  +++ b/file.txt
  @@ -1,1 +1,1 @@
  -first
  +wdir

  $ cd ..

Testing diff --change when merge:

  $ cd a

  $ for i in 1 2 3 4 5 6 7 8 9 10; do
  >    echo $i >> file.txt
  > done
  $ hg commit -m "lots of text" # 3

  $ sed -e 's,^2$,x,' file.txt > file.txt.tmp
  $ mv file.txt.tmp file.txt
  $ hg commit -m "change 2 to x" # 4

  $ hg up -r 3
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ sed -e 's,^8$,y,' file.txt > file.txt.tmp
  $ mv file.txt.tmp file.txt
  $ hg commit -m "change 8 to y"
  created new head

  $ hg up -C -r 4
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg merge -r 5
  merging file.txt
  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg commit -m "merge 8 to y" # 6

  $ hg diff --change 5
  diff -r ae119d680c82 -r 9085c5c02e52 file.txt
  --- a/file.txt	Thu Jan 01 00:00:00 1970 +0000
  +++ b/file.txt	Thu Jan 01 00:00:00 1970 +0000
  @@ -6,6 +6,6 @@
   5
   6
   7
  -8
  +y
   9
   10

must be similar to 'hg diff --change 5':

  $ hg diff -c 6
  diff -r 273b50f17c6d -r 979ca961fd2e file.txt
  --- a/file.txt	Thu Jan 01 00:00:00 1970 +0000
  +++ b/file.txt	Thu Jan 01 00:00:00 1970 +0000
  @@ -6,6 +6,6 @@
   5
   6
   7
  -8
  +y
   9
   10

  $ cd ..