sdk: honor TraceConfig.compression_type on the in-process backend

[dreveman]

Mirror what src/traced/service/service.cc does for the system backend: when the SDK is built with enable_perfetto_zlib=true, construct the in-process backend's TracingService with init_opts.compressor_fn = &ZlibCompressFn so that TraceConfig.compression_type = COMPRESSION_TYPE_DEFLATE actually takes effect. Without this, the in-process service is created with default init_opts (compressor_fn == nullptr) and the compression_type field on the TraceConfig is silently ignored — embedders writing a local .pftrace via TracingSession::Setup(cfg, fd) got an uncompressed file regardless of what they set in the config.

The compressor is wired only when PERFETTO_ZLIB is enabled, matching the existing pattern in service.cc:136-139. No new public API; the behavior is opt-in via the TraceConfig field that already exists.

[github-actions]

🎨 Perfetto UI Builds

• 567cf38 (2026-05-13 22:43 UTC): https://storage.googleapis.com/perfetto-ci-artifacts/gh-25830597570-1-ui/ui/index.html
• 4addb6f (2026-05-14 01:25 UTC): https://storage.googleapis.com/perfetto-ci-artifacts/gh-25836068083-1-ui/ui/index.html

[LalitMaganti]

I think this is not as simple as this: we also need to do what I said in #3118 - basically define some new #defines you can add to the SDK enabling ZLIB in the amalgmated SDK (but you are responsible for making zlib available to us).

[dreveman]

I think this is not as simple as this: we also need to do what I said in #3118 - basically define some new #defines you can add to the SDK enabling ZLIB in the amalgmated SDK (but you are responsible for making zlib available to us).

Works fine for me as long as I build the SDK with the following and add zlib dependency to my target:

tools/gen_amalgamated \
     --sdk cpp \
     --gn_args 'enable_perfetto_ipc=true enable_perfetto_zlib=true enable_perfetto_re2=false enable_perfetto_pcre2=false is_debug=false is_perfetto_build_generator=true is_perfetto_embedder=true perfetto_enable_git_rev_version_header=true use_custom_libcxx=false' \
     --output /tmp/perfetto-sdk/perfetto \
     //:libperfetto_client_experimental \
     //include/perfetto/protozero:protozero \
     //protos/perfetto/config:zero \
     //protos/perfetto/trace:zero \
     //src/tracing/service:zlib_compressor

I can work on adding a simpler flag to gen_amalgamated for this as a follow up if that's something we want? Either way, it would be nice to make this change so someone (like me) who is OK with maintaining a custom generated version of the SDK doesn't have to manually patch the code.

[LalitMaganti]

Hmmmmm I thought the amalgamator stripped out the buildflags (at least that's what I say in the issue). I'm quite confused by this, will have to look into this a bit.