tests/test-editor-filename.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

Test temp file used with an editor has the expected suffix.

  $ hg init

Create an editor that writes its arguments to stdout and set it to $HGEDITOR.

  $ cat > editor.sh << EOF
  > echo "\$@"
  > exit 1
  > EOF
  $ hg add editor.sh
  $ HGEDITOR="sh $TESTTMP/editor.sh"
  $ export HGEDITOR

Verify that the path for a commit editor has the expected suffix.

  $ hg commit
  *.commit.hg.txt (glob)
  abort: edit failed: sh exited with status 1
  [255]

Verify that the path for a histedit editor has the expected suffix.

  $ cat >> $HGRCPATH <<EOF
  > [extensions]
  > rebase=
  > histedit=
  > EOF
  $ hg commit --message 'At least one commit for histedit.'
  $ hg histedit
  *.histedit.hg.txt (glob)
  abort: edit failed: sh exited with status 1
  [255]

Verify that when performing an action that has the side-effect of creating an
editor for a diff, the file ends in .diff.

  $ echo 1 > one
  $ echo 2 > two
  $ hg add
  adding one
  adding two
  $ hg commit --interactive --config ui.interactive=true --config ui.interface=text << EOF
  > y
  > e
  > q
  > EOF
  diff --git a/one b/one
  new file mode 100644
  examine changes to 'one'? [Ynesfdaq?] y
  
  @@ -0,0 +1,1 @@
  +1
  record change 1/2 to 'one'? [Ynesfdaq?] e
  
  *.diff (glob)
  editor exited with exit code 1
  record change 1/2 to 'one'? [Ynesfdaq?] q
  
  abort: user quit
  [255]