| commit | 19db71413329da3f5d22b5fc7681471f3d971d88 | [log] [tgz] |
|---|---|---|
| author | Mark Schaller <mschaller@google.com> | Thu Sep 01 17:58:02 2016 +0000 |
| committer | Klaus Aehlig <aehlig@google.com> | Fri Sep 02 08:27:01 2016 +0000 |
| tree | 30378f5692e8388c86ddcfda56a8ab3d5c5a1f1d | |
| parent | d9db4754a5b3d50ac50131d637140c07696567a2 [diff] |
Skylark: Replaced late bound attributes with computed default attributes Motivation: Compared to computed default attributes, late bound attributes are evaluated in a later phase (analysis instead of loading phase). While both mechanisms provide access to other attributes of a rule, only late bound attributes additionally provide access to the build configuration. However, late bound attributes could be used to return new labels that have not been loaded before. Since this happens in the analysis phase, it can break one of Blaze's underlying principles, thus introducing a serious correctness bug. We decided to replace late bound attributes in Skylark with computed default attributes since this moves the evaluation of values into the loading phase, thus fixing this bug. Moreover, none of the existing users of this mechanism required access to the build configuration, which means that the user impact of this change is minimal. Implementation details: Unlike attributes of non-Skylark rules, however, Skylark computed defaults need to be able to depend on more than two configurable attributes since all attributes of Skylark rules are configurable by default. This has two implications: 1. Unlike "normal" Skylark attributes, Skylark computed defaults need to know about the existence of other attributes in order to declare a dependency on them. This CL takes advantage of work previously done to require parameter names to be the names of attributes used by the computed default function. 2. Since Bazel computes the combinations of all possible attribute values, a Skylark rule with a computed default that depends on a lot of configurable attributes could crash Bazel. Consequently, this CL also introduces an upper bound (64) for the number of valid combinations. Caveats: 1. Getting the depended-on attributes' names from function paramters is mildly surprising. Alternatives: The best solution would be to keep SkylarkLateBound, but restrict it in a way that it can only return already loaded labels. This is not possible right now. -- MOS_MIGRATED_REVID=131967238
{Fast, Correct} - Choose two
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