batch: Use github.com/expr-lang/expr

[breml]

replace include/exclude regex with include expression

@gibmat, @stgraber I looked into the task of improving the way we define a batch. In the respective task (Figure out better way of defining a batch) we listed 3 options:

• https://github.com/Knetic/govaluate (MIT license), no longer maintained, last commit 8 years ago
• https://github.com/PaesslerAG/gval (BSD-3-Clause license), not very popular
• https://github.com/hashicorp/go-bexpr (MPL-2.0 license), not very popular

After quickly checking on them, I found a 4th option, that was mentioned in an issue on govaluate and I found, that this actually the best option in my opinion.

• https://github.com/expr-lang/expr (MIT license), most popular, actively maintained

This PR implements a PoC with the above mentioned expression package. Since this package provides complete boolean expressions, we are able to replace both, the include and the exclude regex we used so far. The expression package also allows to match against regex, so there is no reduction in functionality at all.

By passing the instance to the expression evaluation as argument, one does have access to all the methods (and fields, if the argument is a struct), which allows as for more fine grained logic if an instance should be included or excluded from a batch.
The functionality of the expr package can be extended by custom function. I also added an example for this by adding the filepath.Dir and filepath.Base functions additionally.

I think, the provided test cases do explain the usage the best.

Additional resources:

• https://github.com/expr-lang/expr
• https://expr-lang.org/docs/language-definition

Side note: While integrating the package into our code base, I discovered an issue with the way interfaces are handled. There is already an open issue for this, so I added some context there as well: expr-lang/expr#744

[stgraber]

Looks pretty good from the reference link.

I actually expect regexp queries to be pretty rare overall, I suspect we're likely to want something closer to this as an example batch filter:

source in ["vcenter01", "vcenter02", "vcenter03"] && ("/DEV/foo" in path || "/DEV/bar" in path) && instance.resources.cpus <= 4 && instance.resources.memory <= 1024*1024*1024*8 && instance.resources.disks == 1 && !instance.resources.shared_disks && instance.os in ["Ubuntu 22.04", "Ubuntu 24.04"]

Which it seems like this project can handle just fine, including support for some extra string management functions.

[stgraber]

How good is their error reporting? We're going to be exposing this to end users, so it'd be nice to have clear error messages so they can easily tell what's wrong with their filter and can fix it.

[breml]

How good is their error reporting? We're going to be exposing this to end users, so it'd be nice to have clear error messages so they can easily tell what's wrong with their filter and can fix it.

The error returned from the expression package look like this:

Example 1:

unknown name GetInventoryPath1 (1:1)
| GetInventoryPath1() == "a/b/c"
| ^

Example 2:

literal not terminated (1:29)
 | GetInventoryPath() == "a/b/c
 | ............................^

It will be our responsibility to prefix these errors with additional meaningful details about the step, that failed (e.g. compiling the expression or running it).

[breml]

@stgraber, @gibmat I addressed the review comments and I extended the poc such that the more complex / realistic example provided by @stgraber in #41 (comment) becomes possible to filter for (see testcase https://github.com/FuturFusion/migration-manager/pull/41/files#diff-e3d7bfdcdd97202d2d2778d99715109ec9cb180125c0c318a51059193f027856R118-R124).