vergen: idempotent builds from CI only, I guess?

.github/buildomat/jobs/image.sh

@@ -31,6 +31,20 @@ ptime -m ./tools/install_builder_prerequisites.sh -y
 
 banner build
 
+# We collect some build envrionment information with vergen to augment
+# development builds, but this would otherwise put extra build-env info into
+# the production (and test) binaries built here. While we don't test or rely on
+# deterministic rebuilds of propolis-server, set Vergen up to use idempotent
+# environment variables so we're not needlessly introducing build-time
+# variance.
+#
+# "idempotent" still lets us collect the current relevant git info, so CI
+# builds of propolis-server will still be able to self-report what commit they
+# came from. See
+# https://docs.rs/vergen/10.0.3/vergen/struct.Emitter.html#method.idempotent
+# for more.
+export VERGEN_IDEMPOTENT="true"
+
 # Enable the "omicron-build" feature to indicate this is an artifact destined
 # for production use on an appropriately configured Oxide machine
 #

.github/buildomat/jobs/phd-build.sh

@@ -39,6 +39,10 @@ ptime -m ./tools/install_builder_prerequisites.sh -y
 # should fire during tests.
 banner build-propolis
 
+# The same reasoning as in `image.sh` applies here, where we'd prefer more
+# deterministic CI builds over less deterministic.
+export VERGEN_IDEMPOTENT="true"
+
 # We'll do a few cargo builds, keeping features the same means we reuse build
 # artifacts from crates these configure.
 TEST_FEATURES="omicron-build,failure-injection"

lib/propolis/build.rs

@@ -9,10 +9,29 @@ fn main() -> anyhow::Result<()> {
         .dirty(true)
         .sha(true)
         .build();
-    vergen_git2::Emitter::default()
-        .idempotent()
-        .add_instructions(&git2)?
-        .emit()?;
+    vergen_git2::Emitter::default().add_instructions(&git2)?.emit()?;
+    // Apologies for the broad rerun-if-changed, but stick with me: build.rs
+    // *must* rerun when source files are changed so that a version string's
+    // VERGEN_GIT_DIRTY is accurate. If we only rerun-on-changed for changes in
+    // the bin crates, that almost works, but means a source-only change in
+    // a crate in this tree that a bin depends on directly (say,
+    // propolis-api-types) wouldn't cause this build.rs to re-evaluate and get
+    // the right dirty bit in the version string.
+    //
+    // We could do the VERGEN_GIT_DIRTY work in the binary crates too, and plumb
+    // that back to propolis-lib version's string, but we'd want that to be
+    // broad to cover crates too. This takes the other approach: rerun
+    // propolis-lib build.rs if any files here have changed, which will cause
+    // propolis dependents to rebuild, and get updated version strings. This
+    // comes at the cost of recompiling the propolis-lib crate more often than
+    // strictly necessary during development, but that crate on its own is
+    // *much* faster to build than any of the bins being linked afterwards!
+    //
+    // This also means we can keep all of the versioning fiddliness here instead
+    // of subjecting every (e.g. both) user to thinking about this stuff.
+    println!("cargo:rerun-if-changed=../lib");
+    println!("cargo:rerun-if-changed=../bin");
+    println!("cargo:rerun-if-changed=../crates");
 
     Ok(())
 }