Context

Motivation

Imagine you have a test which can run only for Fedora 33 and newer. Or your tests’ require depend on which distribution you are running. For these cases you need just a slight tweak to your metadata but you can’t really use the Virtual cases as only one of them should run.

This is exactly where adjusting metadata based on the given Context will help you. Let’s see some examples to demonstrate the usage on a real-life use case.

Disable test by setting the enabled attribute:

enabled: true
adjust:
    enabled: false
    when: distro < fedora-33
    because: The feature was added in Fedora-33

Tweak the require attribute for an older distro:

require:
  - procps-ng
adjust:
    require: procps
    when: distro ~= centos-6

Syntax

To get a better idea of the when condition syntax including supported operators consult the following grammar outline:

condition ::= expression (bool expression)*
bool ::= and | or
expression ::= dimension binary_operator values
expression ::= dimension unary_operator
expression ::= 'true' | 'false'
dimension ::= [[:alnum:]]+
binary_operator ::= '==' | '!=' | '<' | '<=' | '>' | '>=' |
    '~=' | '~!=' | '~<' | '~<=' | '~>' | '~>='
unary_operator ::= 'is defined' | 'is not defined'
values ::= value (',' value)*
value ::= [[:alnum:]]+

Let’s demonstrate the syntax on a couple of real-life examples:

# check distro, compare specific release
distro == fedora
distro >= fedora-33

# use boolean operators to build more complex expressions
distro == fedora and arch = x86_64
distro >= fedora-33 or distro >= rhel-8

# check whether a dimension is defined
collection is not defined

# disable adjust rule (e.g. during debugging / experimenting)
false and <original rule>

# always enabled adjust rule (same as if the `when` key is omitted)
true

The comma operator can be used to shorten the or expressions:

# the following two lines are equivalent
arch == x86_64 or arch == ppc64
arch == x86_64, ppc64

# works for less/greater than comparison as well
distro < fedora-33 or distro < rhel-8
distro < fedora-33, rhel-8

Warning

Do not use the comma operator with the != comparison. It is currently implemented with the or logic which is a bit weird, confusing to the users and it will be most probably changed to and in the future so that it can be interpreted as “none of the values in the list is equal”.

Lazy Evaluation

Operator and takes precedence over or and rule evaluation is lazy. It stops immediately when we know the final result.

Boolean Operations

When a dimension or outcome of the operation is not defined, the expression is treated as CannotDecide.

Boolean operations with CannotDecide:

CannotDecide  and  True         ==  CannotDecide
CannotDecide  and  False        ==  False
CannotDecide  or   True         ==  True
CannotDecide  or   False        ==  CannotDecide
CannotDecide  and  CannotDecide ==  CannotDecide
CannotDecide  or   CannotDecide ==  CannotDecide

Dimensions

Each Dimension is a view on the Context in which metadata can be adjusted. For example it can be arch, distro, component, product or pipeline in which we run tests and so on.

Values are always converted to a string representation. Each value is treated as if it was a component with version. Name of the dimension doesn’t matter, all are treated equally.

Values are case-sensitive by default, which means that values like centos and CentOS are considered different. When calling the adjust() method on the tree, case_sensitive=False can be used to make the value comparison case insensitive.

The characters : or . or - are used as version separators and are handled in the same way. The following examples demonstrate how the name and version parts are parsed:

centos-8.3.0
    name: centos
    version: 8, 3, 0

python3-3.8.5-5.fc32
    name: python3
    version: 3, 8, 5, 5, fc32

x86_64
    name: x86_64
    version: no version parts

Comparison

Value on the left always comes from dimension, it describes what is known about the context and should be as specific as possible (this is up to the calling tool). Value on the right comes from the rule and the creator of this rule sets how precise they want to be.

When the left side is not specific enough its missing version parts are treated as if they were lower than the right side. However, the left side needs to contain at least one version part:

git-2.3.4 < git-3   # True
git-2 < git-3.2.1   # True
git < git-3.2.1     # CannotDecide

Equality vs Comparison

It is always possible to evaluate whether two values are (not) equal. When the name and common version parts requested by the right side match then the two values are equal:

git-2.3.4 == git-2.3.4
git-2.3.4 == git-2.3
git-2.3.4 == git-2
git-2.3.4 == git
git-2.3.4 != git-1
git-2.3.4 != fmf

However, comparing order of two values is defined only if they match by name. If names don’t match then values cannot be compared and the expression has CannotDecide outcome:

git-2.3.4 >= git-2     # True
git-2.3.4 >= git-3     # False
git-2.3.4 >= fmf-2     # CannotDecide

Major Version

Comparing distributions across their major versions can be tricky. One cannot easily say that e.g. centos-8.0 > centos-7.9. In this case centos-8.0 was released sooner than centos-7.9 so is it really newer?

Quite often new features are implemented in given minor version such as centos-7.9 or centos-8.2 which does not mean they are available in centos-8.1 so it is not possible to apply a single rule such as distro >= centos-7.9 to cover this case.

Another usage for this operators is to check for features specific to a particular major version or a module stream.

The following operators make it possible to compare only within the same major version:

'~=' | '~!=' | '~<' | '~<=' | '~>' | '~>='

If their major versions are different then their minor versions cannot be compared and as such are skipped during evaluation. The following example shows how the special less than operator ~< would be evaluated for given centos versions. Note that the right side defines if the minor comparison is evaluated or not.

~<	centos-7.9	centos-8.2	centos-8
centos-7.8	True	CannotDecide	True
centos-7.9	False	CannotDecide	True
centos-7	CannotDecide	CannotDecide	True
centos-8.1	CannotDecide	True	False
centos-8.2	CannotDecide	False	False
centos-8	CannotDecide	CannotDecide	False

Here is a couple of examples to get a better idea of how the comparison works for some special cases:

fedora < fedora-33 ---> cannot (left side has no version parts)
fedora-33 == fedora ---> True (right side wants only name)
fedora-33 < fedora-rawhide ---> True (rawhide is newer than any number)

centos-8.4.0 == centos ---> True
centos-8.4.0 < centos-9 ---> True
centos-8.4.0 ~< centos-9 ---> True (no minor comparison requested)
centos-8.4.0 ~< centos-9.2 ---> cannot (minor comparison requested)