fix(build): production flamegraphs are useless

[problame]

Problem

Running a flamegraph on a pageserver build today yields a flamegraph with sort-of flattened stacks:

Analysis

We use mold to link the production binaries.

It seems that, similar to when using lld, it's necessary to set --no-rosegment , as is also recommended by the flamegraph-rs tools.

Setting it fixes the issue on my workstation.
I don't use mold -run there, but instead use mold by setting default rustflags in my ~/.cargo/config.toml:

# repeat / adjust for your target arch
[target.x86_64-unknown-linux-gnu]
linker = "clang"
rustflags = ["-C", "link-arg=-fuse-ld=/usr/local/bin/mold", "-Clink-arg=-Wl,--no-rosegment"]

Solution For Production Binaries

Our production binaries are the ones built inside docker, see ./Dockerfile.

It would be nice if we could just make the setting once, somewhere inside $checkout/Cargo.toml or $checkout/.cargo/config.toml.
That way, all developers, _and the artifact builds, would automatically get the correct flags.

Sadly, the various ways to configure the rust compiler flags in cargo don't compose well.
I.e., there's a strict priority order and one replaces the other.

So, this PR sets the right rust compiler flags through theRUSTFLAGS env var, which is otherwise unset in the Dockerfile, so, we don't have to worry about losing any flag overrides.

This PR also augments the README with a hint on how to get proper flamegraphs on developer builds.

[problame]

Requesting review from @abhigets and @bayandin mostly to raise awareness.

@jcsp might be generally interested in this, and @koivunej and @conradludgate are probably the best to judge whether my solution follows best practices with Cargo.

[conradludgate]

I have no thoughts or concerns about the flags here

[github-actions]

2436 tests run: 2318 passed, 0 failed, 118 skipped (full report)

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Flaky tests (3)

Postgres 16

• test_create_snapshot: release

Postgres 15

• test_crafted_wal_end[last_wal_record_crossing_segment]: debug
• test_timeline_deletion_with_files_stuck_in_upload_queue: debug

Code coverage (full report)

• functions: 56.0% (12860 of 22974 functions)
• lines: 82.6% (69522 of 84170 lines)

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_{The comment gets automatically updated with the latest test results
aca8877 at 2024-02-14T18:13:22.591Z :recycle:}

[koivunej]

If android is using this, it should be safe for us as well. This is what I've been using locally, never tried to produce flamegraphs with production binaries.