fix(bes): write the caller's --build_event_binary_file

.buildkite/pipeline.yaml

@@ -329,10 +329,12 @@ steps:
       aspect test --task:name test-bk
   # Smoke-tests the new test-task flags. `--target-pattern-file` is forwarded
   # to Bazel verbatim, so the assertion is just that aspect resolves the file
-  # and the run completes. The IDE/BSP step re-runs the same pattern file in
-  # the shape the IntelliJ Bazel plugin produces through the tools/bazel
-  # wrapper — Bazel's own flag spellings behind `--bazel-flag` — asserting
-  # that the composed command line is one Bazel accepts.
+  # and the run completes. The IDE/BSP steps re-run a build in the shape the
+  # IntelliJ Bazel plugin produces through the tools/bazel wrapper — Bazel's
+  # own flag spellings behind `--bazel-flag` — asserting that the composed
+  # command line is one Bazel accepts, and that the caller's
+  # `--build_event_binary_file` is actually written (the CLI appends its own
+  # last, and the Bazel option is single-valued).
   # `--coverage` is exercised against an sh_test rather
   # than a rust_test: `toolchains_llvm_bootstrapped` 0.5.2 (pinned in
   # MODULE.bazel) ships without `libclang_rt.profile.a`, so any rust_test under
@@ -367,6 +369,16 @@ steps:
         --bazel-flag=--tool_tag=bazelbsp:3.2.0
       rm -f $$PATTERNS
 
+      echo "--- :aspect: IDE/BSP shape — forwarded --build_event_binary_file"
+      BEP=$$(mktemp)
+      aspect build --task:name test-bk-ide-bep-passthrough \
+        --bazel-flag=--build_event_binary_file=$$BEP \
+        --bazel-flag=--build_event_binary_file_upload_mode=wait_for_upload_complete \
+        --bazel-flag=--tool_tag=bazelbsp:3.2.0 \
+        //examples/test_states:always_pass
+      test -s $$BEP || { echo "caller's --build_event_binary_file was not written"; exit 1; }
+      rm -f $$BEP
+
       echo "--- :aspect: aspect test --coverage (+ --coverage-report + --coverage-tool)"
       REPORT=$$(mktemp)
       aspect test --task:name test-bk-coverage \

.github/workflows/ci-workflows.yaml

@@ -613,10 +613,11 @@ jobs:
   # Smoke-tests the new test-task flags. Ported from the Buildkite
   # `test-flags-task` step. `--target-pattern-file` is forwarded to Bazel
   # verbatim, so the assertion is just that aspect resolves the file and the run
-  # completes. The IDE/BSP step re-runs the same pattern file in the shape the
-  # IntelliJ Bazel plugin produces through the tools/bazel wrapper — Bazel's
-  # own flag spellings behind `--bazel-flag` — asserting that the composed
-  # command line is one Bazel accepts.
+  # completes. The IDE/BSP step re-runs a build in the shape the IntelliJ
+  # Bazel plugin produces through the tools/bazel wrapper — Bazel's own flag
+  # spellings behind `--bazel-flag` — and asserts the caller's BEP file is
+  # actually written. The CLI appends its own `--build_event_binary_file`
+  # last, and the Bazel option is single-valued.
   # `--coverage` is exercised against an sh_test rather than a
   # rust_test: toolchains_llvm_bootstrapped 0.5.2 (pinned in MODULE.bazel) ships
   # without libclang_rt.profile.a, so any rust_test under --collect_code_coverage
@@ -647,6 +648,16 @@ jobs:
             --bazel-flag=--tool_tag=bazelbsp:3.2.0
           rm -f "$PATTERNS"
 
+          echo "--- IDE/BSP shape — forwarded --build_event_binary_file"
+          BEP=$(mktemp)
+          aspect build --task:name test-gha-ide-bep-passthrough \
+            --bazel-flag=--build_event_binary_file="$BEP" \
+            --bazel-flag=--build_event_binary_file_upload_mode=wait_for_upload_complete \
+            --bazel-flag=--tool_tag=bazelbsp:3.2.0 \
+            //examples/test_states:always_pass
+          test -s "$BEP" || { echo "caller's --build_event_binary_file was not written"; exit 1; }
+          rm -f "$BEP"
+
           echo "--- aspect test --coverage (+ --coverage-report + --coverage-tool)"
           REPORT=$(mktemp)
           aspect test --task:name test-gha-coverage \

crates/aspect-cli/src/builtins/aspect/bazel/build_events.axl

@@ -12,6 +12,11 @@ once rather than twice. Because that can leave no sink at all — and the Aspect
 Web UI link keys on an id only a sink mints — a task pairs the collect call with
 `bes_streamed_by_bazel`, which redirects the link to Bazel's invocation id.
 
+`collect_bes_sinks` also covers the one Bazel-facing BEP flag the CLI would
+otherwise clobber: a caller's `--build_event_binary_file` becomes a CLI file sink,
+because `Build::spawn` appends the CLI's own path to that single-valued option
+last and last wins. See `collect_bes_sinks` for what that changes.
+
 The Aspect login JWT is attached to an Aspect-owned backend's sink metadata when
 the user has not supplied their own `authorization` header — see
 `aspect_endpoint_auth.axl` for the host gate and best-effort credential
@@ -55,6 +60,22 @@ def bazel_bes_backend(rc, command: str) -> str:
         return ""
     return rc.flag_value("--bes_backend", command = command) or ""
 
+def bazel_bep_file(rc, command: str) -> str:
+    """The `--build_event_binary_file` path the caller asked Bazel to write, or `""`.
+
+    Reads through the run command, so it sees the flag wherever it came from —
+    `--bazel-flag=--build_event_binary_file=…`, a `.bazelrc`, or an expanded
+    `--config`. `rc` may be `None` for a caller with no run command to consult.
+
+    Nobody but the caller sets this: the CLI's own BEP file is appended inside
+    `Build::spawn`, after rc expansion, and never passes through the rc.
+
+    Public so `build_events_test.axl` can assert the resolution.
+    """
+    if rc == None:
+        return ""
+    return rc.flag_value("--build_event_binary_file", command = command) or ""
+
 def _drop_bazel_streamed(items: list, uri_of, bazel_backend: str) -> list:
     """`items` minus those whose `uri_of(item)` names the same endpoint as Bazel's
     own `bazel_backend`.
@@ -106,16 +127,38 @@ def collect_bes_sinks(ctx, bazel_trait, rc, command: str = "build", extra_backen
     Sinks to the endpoint Bazel's own `--bes_backend` uploads to are dropped from
     both groups so the invocation is streamed once — see `_drop_bazel_streamed`.
 
+    Last comes a file sink for the caller's own `--build_event_binary_file`, when
+    they asked for one. Bazel cannot write two: the option is single-valued and
+    `Build::spawn` appends the CLI's path last, so the caller's file would
+    silently never be written. Writing it as a sink instead re-creates it from
+    Bazel's own byte stream — the file sinks share the BES reader's raw-bytes
+    path, so the result is what Bazel would have written. This is what lets IDE /
+    BSP tooling (the IntelliJ Bazel plugin) drive builds through the
+    `tools/bazel` wrapper and still get its BEP back. Two consequences worth
+    knowing: `--build_event_binary_file_upload_mode` no longer governs that file
+    (the caller's `wait()` on the returned sinks is what completes it, before the
+    task concludes), and `--build_event_json_file` / `--build_event_text_file`
+    are untouched — different flags, so Bazel still writes those itself.
+
     The one call a bazel-spawning task makes; pair it with
     `bes_streamed_by_bazel` to keep the Aspect Web UI link resolvable. The caller
     `wait()`s the returned sinks after the build.
     """
     bazel_backend = bazel_bes_backend(rc, command)
-    return (
+    sinks = (
         collect_bes_from_args(ctx, extra_backends = extra_backends, bazel_backend = bazel_backend) +
         _drop_bazel_streamed(list(bazel_trait.build_event_sinks), lambda s: s.uri, bazel_backend)
     )
 
+    # Not routed through `_drop_bazel_streamed`: a file is a local dump, not a
+    # second upload to an endpoint Bazel already streams to.
+    bep_file = bazel_bep_file(rc, command)
+    if bep_file:
+        trace.event("bes.caller_bep_file", fields = {"path": bep_file, "command": command})
+        sinks.append(bazel.build_events.file(path = bep_file))
+
+    return sinks
+
 def dropped_bes_backends(ctx, bazel_trait, rc, command: str = "build", extra_backends = []) -> list[str]:
     """The BES endpoints `collect_bes_sinks` skipped because Bazel uploads to them
     itself, deduped. Same inputs as that call — pass the same arguments.

crates/aspect-cli/src/builtins/aspect/bazel/build_events_test.axl

@@ -1,13 +1,14 @@
 """Tests for `bazel/build_events.axl` — the BES upload summary line, the gRPC-sink
 filter shared by the announce/summary helpers, sink collection, and the
 duplicate-stream drop that keeps the CLI from streaming to an endpoint Bazel's
-own `--bes_backend` already uploads to.
+own `--bes_backend` already uploads to, and the file-sink tee that keeps a
+caller's `--build_event_binary_file` from being silently clobbered.
 
 Run with:
   aspect dev test-bes-sinks
 """
 
-load("@aspect//bazel/build_events.axl", "bazel_bes_backend", "bes_results_line", "bes_streamed_by_bazel", "bes_upload_line", "collect_bes_from_args", "collect_bes_sinks", "dropped_bes_backends", "grpc_backends")
+load("@aspect//bazel/build_events.axl", "bazel_bep_file", "bazel_bes_backend", "bes_results_line", "bes_streamed_by_bazel", "bes_upload_line", "collect_bes_from_args", "collect_bes_sinks", "dropped_bes_backends", "grpc_backends")
 
 def _eq(label, got, want):
     if got != want:
@@ -248,6 +249,44 @@ def _test_collect_bes_sinks(_):
         [None, "grpcs://bes.other.example.com"],
     )
 
+def _test_bazel_bep_file(_):
+    """`bazel_bep_file` reads the caller's `--build_event_binary_file` off the run
+    command, so it sees the flag from `--bazel-flag=`, a `.bazelrc`, or a
+    `--config` expansion alike."""
+    _eq("caller asked for one", bazel_bep_file(_fake_rc({"--build_event_binary_file": "/tmp/bep.binpb"}), "build"), "/tmp/bep.binpb")
+    _eq("flag unset", bazel_bep_file(_fake_rc({}), "build"), "")
+    _eq("no run command", bazel_bep_file(None, "build"), "")
+
+def _test_collect_bes_sinks_tees_caller_bep_file(_):
+    """A caller's `--build_event_binary_file` becomes a trailing file sink, so the
+    path they asked for is written from Bazel's own byte stream.
+
+    Without it their file is silently empty: the Bazel option is single-valued and
+    `Build::spawn` appends the CLI's own path last."""
+    trait = _fake_trait([struct(uri = "grpcs://bes.other.example.com")])
+
+    def uris(rc_values):
+        ctx = _fake_ctx(bes_backends = [])
+        return [s.uri for s in collect_bes_sinks(ctx, trait, _fake_rc(rc_values))]
+
+    _eq(
+        "file sink appended after the trait's own",
+        uris({"--build_event_binary_file": "/tmp/bep.binpb"}),
+        ["grpcs://bes.other.example.com", None],
+    )
+    _eq("no BEP flag, no extra sink", uris({}), ["grpcs://bes.other.example.com"])
+
+    # The tee is a local dump, so it survives the duplicate-stream drop that
+    # suppresses a CLI sink to the endpoint Bazel itself uploads to.
+    duplicate = "grpcs://bes.acme.aspect.build"
+    ctx = _fake_ctx(bes_backends = [duplicate])
+    rc = _fake_rc({"--bes_backend": duplicate, "--build_event_binary_file": "/tmp/bep.binpb"})
+    _eq(
+        "kept while every gRPC sink is suppressed",
+        [s.uri for s in collect_bes_sinks(ctx, _fake_trait([struct(uri = duplicate)]), rc)],
+        [None],
+    )
+
 def _test_bes_streamed_by_bazel(_):
     """True only when Bazel's `--bes_backend` is the runner's own Aspect BES
     backend — the case where the Web UI still holds the invocation (under
@@ -280,6 +319,8 @@ _UNIT_TESTS = [
     _test_bazel_bes_backend,
     _test_collect_drops_duplicate_backends,
     _test_collect_bes_sinks,
+    _test_bazel_bep_file,
+    _test_collect_bes_sinks_tees_caller_bep_file,
     _test_bes_streamed_by_bazel,
 ]