wasm: add experimental browser runtime and SDK

[edsiper]

Fluent Bit currently depends on native facilities that are unavailable in a browser. This adds an experimental Emscripten build profile and browser SDK that run the engine in workers, ingest application records, and export through browser Fetch.

The SDK supports start/stop/restart, bounded command admission, configuration assets, opt-in IndexedDB storage, namespace locking, checkpoint recovery, and status counters. push() acknowledges input acceptance, not remote delivery. Persistent checkpoints require a stopped engine; failed checkpoints block restart until recovery succeeds.

Scope

• Pin Emscripten 6.0.9 and build selected browser-compatible plugins with Lua 5.4, OpenSSL helpers, and virtual filesystem support.
• Adapt engine lifecycle and legacy/NG HTTP clients to Fetch, including cancellation and rejection of unsupported browser modes.
• Add the SDK, TypeScript declarations, browser demo, startup/storage fault tests, and release/ASan CI jobs.
• Reject dedicated threaded inputs, cap the SDK pthread pool, and use cooperative child shutdown on startup failure. Repeated testing showed sequential startup alone was insufficient.
• Include Lua sparse-table and SQL string-boundary fixes found during validation.

Bundled changes are isolated in library commits: lib/flb_libco (fiber backend), lib/monkey (protocol/event/timer portability), and lib/chunkio (virtual filesystem/storage support). These updates are required, not optional: flb_libco #14, Monkey #448, and ChunkIO #116 have all merged upstream. Their bundled changes must accompany the Fluent Bit integration. The browser build additionally cross-compiles SHA-256-pinned OpenSSL 3.5.8, libyaml 0.2.5, and portable Lua 5.4.9 with Emscripten 6.0.9. The installed SDK is not patched.

Validation — refreshed 2026-09-19

Fresh, isolated source worktrees on Linux; official Emscripten 6.0.9, Node 24.19.0, Chromium 151, Valgrind 3.23.0. No unrelated files from the development checkout are included.

Clean-checkout CI exposed a missing Lua test harness header: previous local native builds had generated it in the source tree. The WASM Lua target now generates its own build-tree header. This refresh changes only that test-build interface and prerequisite documentation; runtime and bundled-library sources are unchanged.

Completed locally:

• Fresh optimized and ASan builds: 17/17 and 19/19 CTests passed, without a native-generated Lua harness.
• Real Chromium SDK, pthread-startup fault, and full HTTPS demo suites passed in both configurations, including HTTP/Loki/OTLP, Lua, persistence/recovery, cancellation, and teardown.
• 54 pthread-startup fault cases passed (27 per configuration).
• 20 Node contract tests; TypeScript 5.9.3 strict consumer check; 15 Python browser-harness tests plus 6 subtests passed.
• Fresh native build: 9/9 focused Lua/SQL, routing/chunk, YAML, backlog, and output CTests passed.
• Native integration: 99 passed normally (including six startup-helper tests).
• Lua and SQL unit tests passed strict Valgrind: zero errors, no remaining allocations.

Pending: the fresh full native strict-Valgrind integration run and hosted CI on the updated head. Previous validation passed focused native integrations under strict Valgrind; this section will be updated with the fresh result rather than treating old results as a new pass.

Focused native commands (set FLUENT_BIT_BINARY when using an external build):

tests/integration/.venv/bin/python -m pytest tests/integration/test_fluent_bit_manager_startup.py tests/integration/scenarios/filter_lua tests/integration/scenarios/out_http tests/integration/scenarios/out_stdout tests/integration/scenarios/in_storage_backlog tests/integration/scenarios/in_http tests/integration/scenarios/internal_http_server -q
VALGRIND=1 VALGRIND_STRICT=1 tests/integration/.venv/bin/python -m pytest tests/integration/scenarios/filter_lua tests/integration/scenarios/out_http tests/integration/scenarios/out_stdout tests/integration/scenarios/in_storage_backlog tests/integration/scenarios/in_http tests/integration/scenarios/internal_http_server -q

Full PR-range commit-prefix lint and whitespace checks pass, all new commits carry DCO sign-offs, and a merge check against fetched master is clean. This update is a normal fast-forward push; no published history was rewritten.

The previous hosted native matrix also failed flb-rt-out_s3 in ordered_shared_upload_retries_safely with SIGSEGV. The focused local rerun passed, but that does not establish a fix; no unrelated S3 change is included. Hosted CI on this head must still be checked before merge.

Build and browser reproduction commands are in the browser documentation.

Draft / production gates

The API remains experimental. It requires a secure, cross-origin-isolated browser context and trusted application configuration. Native builds retain their existing runtime model.

Outstanding work includes end-to-end backlog budgets, engine delivery/drop observability, crash-safe delivery semantics, real quota/eviction/corruption/upgrade testing, Firefox/WebKit/mobile and production CSP/bundler qualification, long-running lifecycle/pressure tests, and reproducible release/security processes. Default ASan passes; optional stack-use-after-return mode still has the documented toolchain cleanup limitation. This draft does not claim production readiness.

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