build: Tweak the organisation of the build config

The goal is enable us to add distro-toolchain specific compile time
options trivially, whilst still having reasonably conservative onboarding
defaults, such that users can successfully run `just get-started` on a
variety of distributions.

By setting os_release_id="(...)" in build.rs in boulder/ and moss/, we
can now add specific flags known to work on platforms where we control
the toolchain build-time options.

As of now, the logic is working well enough that compilation doesn't stop
working if you're not on Solus. The current solution is meant as starting
point for a potentially more refined future solution if necessary.

Tested on fedora 39 and Solus.

Signed-off-by: Rune Morling <[email protected]>

.cargo/config.toml

@@ -1,22 +1,42 @@
-[target."x86_64-unknown-linux-gnu"]
-# - On systems that do not use lld as the system linker (such as Solus) using lld directly saves about a second 
-# of build time for incremental compiles for building boulder (from 2.191s to 1.198s on my machine).
-# - Compressing debug symbols with zstd shrinks the dev profile boulder binary from 206.03MB to 81.44MB, a 124.59MB
-# or ~60% savings. It doesn't affect the binary size for packaging builds since we strip those, but the debug symbols
-# are reduced in size from 113.16MB to 34.63MB. It adds about ~152ms to the build times which is less than we gained 
-# by switching to lld
-# - The new symbol mangling format (https://doc.rust-lang.org/rustc/symbol-mangling/v0.html) improves the backtrace
-# shown by RUST_BACKTRACE=1 and other debug utilities. It should also be helpful once we have ABI reports. Upstream
-# hasn't switched to it yet by default due to stable distros not having new enough tools, but that doesn't matter for us
+# Having a way to detect on which system we are compiled means we can get
+# away with adding rustflags here that we know are present in the system
+# toolchain builds.
+#
+# We can set these extra flags via matching on a target cfg() expression.
+#
+# - On systems that do not use lld as the system linker (such as Solus) using
+#   lld directly saves about a second  of build time for incremental compiles
+#   for building boulder (from 2.191s to 1.198s on Reilly's machine).
+#
+# - In testing, compression of debug symbols with zstd shrinks the dev profile
+#   boulder binary from 206.03MB to 81.44MB, a 124.59MB or ~60% savings.
+#   It doesn't affect the binary size for packaging builds since we strip those,
+#   but the debug symbols are reduced in size from 113.16MB to 34.63MB.
+#   It adds about ~152ms to the build times which is less than we gained by
+#   switching to lld. This feature requires a compiler compiled with support
+#   for zstd debug symbols.
+#
+# - The new symbol mangling format[1] improves the backtrace shown by
+#   RUST_BACKTRACE=1 and other debug utilities. It should also be helpful once
+#   we have ABI reports. Upstream hasn't switched to it yet by default due to
+#   stable distros not having new enough tools, but that doesn't matter for us
+#   [1]:  https://doc.rust-lang.org/rustc/symbol-mangling/v0.html
+#
+
+# NB: os_release patterns need to be added to both target configs for this to
+#     work...
+#
+# The Solus toolchain supports zstd debug sections currently (Serpent doesn't)
+[target.'cfg(any(os_release_id = "solus"))']
 rustflags = [
     "-Clink-arg=-fuse-ld=lld",
     "-Clink-arg=-Wl,--compress-debug-sections=zstd",
     "-Csymbol-mangling-version=v0",
 ]
 
-[target."aarch64-unknown-linux-gnu"]
+# Default flags
+[target.'cfg(not(any(os_release_id = "solus")))']
 rustflags = [
     "-Clink-arg=-fuse-ld=lld",
-    "-Clink-arg=-Wl,--compress-debug-sections=zstd",
     "-Csymbol-mangling-version=v0",
 ]

Cargo.toml

@@ -27,6 +27,7 @@ diesel = { version = "2.2.1", features = ["sqlite", "returning_clauses_for_sqlit
 diesel_migrations = "2.2.0"
 dirs = "5.0.1"
 elf = "0.7.4"
+etc-os-release = "0.1.0"
 indicatif = "0.17.8"
 itertools = "0.13.0"
 futures = "0.3.30"
@@ -54,6 +55,19 @@ url = { version = "2.5.2", features = ["serde"] }
 xxhash-rust = { version = "0.8.11", features = ["xxh3"] }
 zstd = { version = "0.13.2", features = ["zstdmt"] }
 
+# We want people who use the onboarding steps to get a nice compromise
+# between fast compilation and fast runtime, but with checks in place
+# and full backtraces. Hyperfine tests shows opt-level = 1 to be a good
+# compromise between compile speed and runtime speed.
+# During testing, opt-level = 2 caused a non-trivial slowdown in compilation
+# iteration speed, but also sped up execution times commensurably. /ermo
+[profile.onboarding]
+inherits = "dev"
+opt-level = 1
+lto = "thin"
+debug = true
+strip = "none"
+
 [profile.release]
 lto = "thin"
 
@@ -66,13 +80,4 @@ opt-level = 3
 strip = "none"
 debug = true
 
-# We want people who use the onboarding steps to get a nice compromise
-# between fast compilation and fast runtime, but with checks in place
-# and full backtraces. Hyperfine tests shows opt-level = 1 to be a good
-# compromise between compile speed and runtime speed.
-[profile.onboarding]
-inherits = "dev"
-opt-level = 1
-lto = "thin"
-debug = true
-strip = "none"
+# NB: Consult .cargo/config.toml to read more about the conditional [target] rustflags we use!

boulder/Cargo.toml

@@ -43,3 +43,6 @@ strum.workspace = true
 thiserror.workspace = true
 tokio.workspace = true
 url.workspace = true
+
+[build-dependencies]
+etc-os-release.workspace = true

boulder/build.rs

@@ -0,0 +1,15 @@
+use etc_os_release::OsRelease;
+use std::error::Error;
+
+/// Set cargo::rustc-cfg=os_release_id="whatever" when /etc/os-release warrants it.
+/// The intent is to enable trivial conditional compilation via [target.'cfg(...)']
+/// stanzas.
+fn main() -> Result<(), Box<dyn Error>> {
+    // only recompile when necessary
+    println!("cargo::rerun-if-changed=./build.rs");
+    // if /etc/os-release doesn't exist, we have a problem big enough that it's OK to crash
+    let os_release = OsRelease::open()?;
+    println!("cargo::rustc-cfg=os_release_id=\"{}\"", os_release.id());
+
+    Ok(())
+}

moss/Cargo.toml

@@ -21,10 +21,10 @@ clap.workspace = true
 derive_more.workspace = true
 diesel.workspace = true
 diesel_migrations.workspace = true
-itertools.workspace = true
 fnmatch = { path = "../crates/fnmatch" }
 futures.workspace = true
 hex.workspace = true
+itertools.workspace = true
 libsqlite3-sys.workspace = true
 log.workspace = true
 nix.workspace = true
@@ -39,6 +39,9 @@ thiserror.workspace = true
 url.workspace = true
 xxhash-rust.workspace = true
 
+[build-dependencies]
+etc-os-release.workspace = true
+
 [package.metadata.cargo-machete]
 # Needed for unixepoch() in src/db/state/migrations/2024-03-04-201550_init/up.sql
 ignored = ["libsqlite3-sys"]

moss/build.rs

@@ -0,0 +1,15 @@
+use etc_os_release::OsRelease;
+use std::error::Error;
+
+/// Set cargo::rustc-cfg=os_release_id="whatever" when /etc/os-release warrants it.
+/// The intent is to enable trivial conditional compilation via [target.'cfg(...)']
+/// stanzas.
+fn main() -> Result<(), Box<dyn Error>> {
+    // only recompile when necessary
+    println!("cargo::rerun-if-changed=./build.rs");
+    // if /etc/os-release doesn't exist, we have a problem big enough that it's OK to crash
+    let os_release = OsRelease::open()?;
+    println!("cargo::rustc-cfg=os_release_id=\"{}\"", os_release.id());
+
+    Ok(())
+}