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). Related upstream work: flb_libco #14, Monkey #448, ChunkIO #116.

Validation

Local validation passed on Linux with Emscripten 6.0.9 and Chromium 151:

• Release and ASan builds; 17 release and 19 ASan CTests.
• Real-browser SDK and full demo suites in both configurations, including HTTP/Loki/OTLP, Lua, persistent replay, injected checkpoint failure/recovery, and teardown.
• 108 startup-fault cases across repeated release/ASan runs after cooperative cleanup was added.
• 20 Node contract tests, TypeScript checking, and 13 Python browser-harness tests.
• Seven focused native input/routing CTests.
• Lua/HTTP integration: 36 cases normally and with strict Valgrind. Native stdout/input-routing integration: 14 cases normally and with strict Valgrind.

Focused integration commands:

tests/integration/.venv/bin/python -m pytest tests/integration/scenarios/filter_lua tests/integration/scenarios/out_http -q
VALGRIND=1 VALGRIND_STRICT=1 tests/integration/.venv/bin/python -m pytest tests/integration/scenarios/filter_lua tests/integration/scenarios/out_http -q
tests/integration/.venv/bin/python -m pytest tests/integration/scenarios/out_stdout -q
VALGRIND=1 VALGRIND_STRICT=1 tests/integration/.venv/bin/python -m pytest tests/integration/scenarios/out_stdout -q

Full PR-range commit-prefix lint and a local merge check against fetched master pass. Hosted CI results are pending. 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.

[coderabbitai]

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