[tbb-bugs] #26475 [Applications/Tor Browser]: ESR60-based Tor Browser bundles are not built reproducibly with Stylo enabled using rustc > 1.25.0

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#26475: ESR60-based Tor Browser bundles are not built reproducibly with Stylo
enabled using rustc > 1.25.0
-------------------------------------------------+-------------------------
 Reporter:  gk                                   |          Owner:  tbb-
                                                 |  team
     Type:  defect                               |         Status:  new
 Priority:  Immediate                            |      Milestone:
Component:  Applications/Tor Browser             |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  tbb-rbm, TorBrowserTeam201810,       |  Actual Points:
  GeorgKoppen201810                              |
Parent ID:                                       |         Points:
 Reviewer:                                       |        Sponsor:
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Comment (by gk):

 Replying to [comment:33 gk]:
 > 2) There are additional pieces getting compiled in/used during the LLVM
 compilation done during the rust build that are causing the problem.

 That's what happening in this case. The relevant part is
 {{{
             if self.config.rust_codegen_units.is_none() &&
                self.build.is_rust_llvm(compiler.host) &&
                self.config.rust_thinlto {
                 cargo.env("RUSTC_THINLTO", "1");
             } else if self.config.rust_codegen_units.is_none() {
                 // Generally, if ThinLTO has been disabled for some
 reason, we
                 // want to set the codegen units to 1. However, we
 shouldn't do
                 // this if the option was specifically set by the user.
                 cargo.env("RUSTC_CODEGEN_UNITS", "1");
             }
 }}}
 in `builder.rs`.

 For some reason `RUSTC_THINLTO` is only set if one does not specify a LLVM
 with `--llvm-root` and that is the difference I hit. With it set the
 reproducibility problem emerges (even for LLVMs provided by `--llvm-root`
 if I rip out `self.build.is_rust_llvm(compiler.host) &&`) without it not.

 So, to sum up so far: Compiling with `-C lto` is fine unless
 `RUSTC_THINLTO` is used for compiling rust for macOS. Or to be more
 precise: unless `RUSTC_THINLTO` is used for the apple target libstd and
 related libraries. I could not pinpoint the exact lib that is causing this
 issue, though, yet (I can't easily replace them one by one as otherwise
 rustc is complaining about libstd being in need of recompilation).

 I guess more bisecting is next. :) Alex, do you think we could just avoid
 setting `RUSTC_THINLTO` for now when compiling the rust compiler? Or does
 that have any serious, known downsides?

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26475#comment:44>
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