The script crate currently has a build script, and Cargo will consider all files
in the script crate as inputs to the build script as it otherwise doesn't know
[what the input files are][cargo-1162]. This means that if any file in the
source tree of the script crate changes (or is created) then Cargo will think it
needs to re-run the build script and rebuild the crate.
[cargo-1162]: https://github.com/rust-lang/cargo/issues/1162
The build script of the script crate is invoking python, and consequently Python
is generating some bytecode files in the source tree. On the second build of
Servo, Cargo will see these new files, think that something has changed, and
will re-run the build script of the script crate.
This change passes the `-B` flag to python to avoid generating these bytecode
files, which should avoid tampering with the source tree and appease Cargo by
ensuring that it doesn't get rebuilt.
---
As a helpful tip to if this comes up again, this was discovered by using the
changes in rust-lang/cargo@c447e9d plus the change in rust-lang/cargo#2044. Once
`RUST_LOG` was set to `cargo::ops::cargo_rustc::fingerprint=info`, the second
run of `./mach build` printed out:
```
precalculated components have changed:
1444364448.000000000s (/build/servo/components/script/dom/bindings/codegen/parser/WebIDL.pyc) !=
1444364235.000000000s (/build/servo/components/script/document_loader.rs)
```
Which should help easily diagnose these kinds of problems in the future!
After this pull request merged:
https://github.com/servo/servo/pull/7209
the 'script' component would never enter a 'built' state. In other
words, if one calls `mach build`, lets it complete, then calls `mach
build` again, the 'script' component would rebuild even though we
supposedly just built it. This was due to the `ParserResults.pkl`
getting placed in the `components/script` directory instead of the
output directory, causing cargo to think that there were unbuilt files.
Prior to this commit, the script codegen makefile relied on an
intermediary pythonpath.py file that handled python dependencies and
incorporated a couple hacks to get the codegen building working. This
commit removes that intermediary file and attempts to make the script
codegen build process cleaner.
This is necessary to ensure Cargo knows when to rebuild. Normally
.gitignore would be enough to exclude these from Cargo's freshness
calculation, but https://github.com/rust-lang/cargo/issues/1729 prevents
this currently. This is the new, correct way to do these thigns, just
like the style crate does.
