With this PR, we add the syntax to use for filtration of items over the
containerd API. This package defines a syntax and parser that can be
used across types and use cases in a uniform manner.
The syntax is fairly familiar, if you've used container ecosystem
projects. At the core, we base it on the concept of protobuf field
paths, augmenting with the ability to quote portions of the field path
to match arbitrary labels. These "selectors" come in the following
syntax:
```
<fieldpath>[<operator><value>]
```
A basic example is as follows:
```
name=foo
```
This would match all objects that have a field `name` with the value
`foo`. If we only want to test if the field is present, we can omit the
operator. This is most useful for matching labels in containerd. The
following will match objects that has the field labels and have the
label "foo" defined:
```
labels.foo
```
We also allow for quoting of parts of the field path to allow matching
of arbitrary items:
```
labels."very complex label"==something
```
We also define `!=` and `~=` as operators. The `!=` operator will match
all objects that don't match the value for a field and `~=` will compile
the target value as a regular expression and match the field value
against that.
Selectors can be combined using a comma, such that the resulting
selector will require all selectors are matched for the object to match.
The following example will match objects that are named `foo` and have
the label `bar`:
```
name==foo,labels.bar
```
This filter syntax will be used across all APIs that allow listing of
objects and for filtering which event a cleint see. By using a common
syntax, we hope to keep API access uniform.
For the most part, this takes inspiration from docker, swarm and k8s,
but has the limitation that it only allows selection of an inner
product. We may expand to operators that implement `or`, `in` or
`notin`, but it is not clear that this is useful at this level of the
stack.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
