ci: update cargo fuzz command to use nightly toolchain explicitly (#9298)

* ci: update cargo fuzz command to use nightly toolchain explicitly

* ci: honor RUSTUP_TOOLCHAIN pin in fuzz orchestration script

An explicit `+toolchain` argument overrides the RUSTUP_TOOLCHAIN env var
in rustup precedence, so the hard-coded `cargo +nightly` in
run-fuzz-targets.sh bypassed the pinned FUZZ_RUST_TOOLCHAIN
(nightly-2026-03-21) configured in the workflow.

- Invoke `cargo +"${RUSTUP_TOOLCHAIN:-nightly}" fuzz run` in the script
  so CI uses the pinned toolchain and local runs fall back to the
  floating nightly
- Pass RUSTUP_TOOLCHAIN through to all four fuzz-test action invocations,
  covering the no-prebuilt-binaries path and making the reproduce command
  in the summary print the exact pinned toolchain
- Add test_rustup_toolchain_env_is_honored covering the pinned-env
  scenario for both the cargo invocation args and the summary text

Addresses #9298 (review).

Signed-off-by: Ning Sun <sunning@greptime.com>

---------

Signed-off-by: Ning Sun <sunning@greptime.com>

.github/scripts/run-fuzz-targets-test.sh

@@ -31,7 +31,7 @@ new_fixture() {
 #!/usr/bin/env bash
 set -euo pipefail
 printf '%s\t%s\t%s\n' "${GT_FUZZ_DUMP_DIR}" "$*" "${MOCK_CARGO_MARKER:-}" >>"${MOCK_CARGO_LOG}"
-target="$3"
+target="$4"
 case " ${MOCK_FAIL_TARGETS:-} " in
   *" ${target} "*) exit 17 ;;
 esac
@@ -117,6 +117,20 @@ EOF
   set -e
 }
 
+test_rustup_toolchain_env_is_honored() {
+  new_fixture
+  export RUSTUP_TOOLCHAIN=nightly-2026-03-21
+  run_fixture $'fuzz_create_table' true true "fuzz_create_table"
+  unset RUSTUP_TOOLCHAIN
+
+  assert_eq 1 "${fixture_status}" "pinned toolchain run status"
+  grep -q -- '+nightly-2026-03-21 fuzz run fuzz_create_table' "${fixture}/cargo.log" || \
+    fail "pinned toolchain missing from cargo invocation"
+  grep -q 'cargo +nightly-2026-03-21 fuzz run fuzz_create_table' "${fixture}/artifacts/summary.md" || \
+    fail "pinned toolchain missing from reproduce command"
+  cleanup_fixture
+}
+
 test_successful_targets_run_in_order() {
   new_fixture
   run_fixture $'fuzz_create_table\nfuzz_insert' false true ""
@@ -125,7 +139,7 @@ test_successful_targets_run_in_order() {
   assert_eq 2 "$(wc -l <"${fixture}/cargo.log" | tr -d ' ')" "cargo invocation count"
   assert_eq \
     $'fuzz_create_table\nfuzz_insert' \
-    "$(awk -F '\t' '{print $2}' "${fixture}/cargo.log" | sed -E 's/^fuzz run ([^ ]+).*/\1/')" \
+    "$(awk -F '\t' '{print $2}' "${fixture}/cargo.log" | sed -E 's/^\+nightly fuzz run ([^ ]+).*/\1/')" \
     "target order"
   grep -q -- '--features=unstable' "${fixture}/cargo.log" || fail "unstable feature missing"
   grep -q -- '-max_total_time=120' "${fixture}/cargo.log" || fail "fuzz time missing"
@@ -175,7 +189,7 @@ test_fail_fast_stops_after_first_failure() {
     fail "skipped target annotation missing"
   grep -q '### Reproduce failed targets' "${fixture}/artifacts/summary.md" || \
     fail "reproduction section missing"
-  grep -q 'cargo fuzz run fuzz_insert' "${fixture}/artifacts/summary.md" || \
+  grep -q 'cargo +nightly fuzz run fuzz_insert' "${fixture}/artifacts/summary.md" || \
     fail "reproduction command missing"
   cleanup_fixture
 }
@@ -367,5 +381,6 @@ test_collector_keeps_target_scopes_separate
 test_cluster_collector_honors_target_scope_and_namespace
 test_setup_failure_writes_artifact_contract
 test_setup_failure_keeps_manifest_when_collection_fails
+test_rustup_toolchain_env_is_honored
 
 printf 'All fuzz orchestration script tests passed.\n'

.github/scripts/run-fuzz-targets.sh

@@ -104,7 +104,7 @@ write_summary() {
             reproduce_args+=("GT_FUZZ_INSTANCE_ROOT_DIR=${GT_FUZZ_INSTANCE_ROOT_DIR}")
           fi
         fi
-        reproduce_args+=(cargo fuzz run "${target}" --fuzz-dir tests-fuzz -D -s none)
+        reproduce_args+=(cargo +"${RUSTUP_TOOLCHAIN:-nightly}" fuzz run "${target}" --fuzz-dir tests-fuzz -D -s none)
         if [[ "${fuzz_unstable}" == true ]]; then
           reproduce_args+=(--features=unstable)
         fi
@@ -195,7 +195,7 @@ for ((index = 0; index < ${#targets[@]}; index++)); do
     fi
     run_args=("${fuzz_binary}" "-max_total_time=${FUZZ_MAX_TOTAL_TIME}" "-artifact_prefix=${target_dir}/libfuzzer/")
   else
-    run_args=(cargo fuzz run "${target}" --fuzz-dir tests-fuzz -D -s none)
+    run_args=(cargo +"${RUSTUP_TOOLCHAIN:-nightly}" fuzz run "${target}" --fuzz-dir tests-fuzz -D -s none)
     if [[ "${fuzz_unstable}" == true ]]; then
       run_args+=(--features=unstable)
     fi