[web] SkSemaphore is a no-op under -sWASM_WORKERS=1, racing FT_Face #193695

Summary

In #190048 (fixing #190039) and #191014, skwasm enabled SK_ENABLE_THREADLOCAL_STRIKECACHE because multithreaded skwasm and wimp are compiled with -sWASM_WORKERS=1 (engine/src/build/config/compiler/BUILD.gn:312) without -pthread, which links single-threaded Emscripten system libraries where POSIX synchronization primitives (sem_wait, pthread_cond_wait) are no-op stubs.

Making SkStrikeCache thread-local did not eliminate shared mutable state between main-thread $paragraph_layout and DedicatedWorker _surface_renderPicturesOnWorker:

• Both threads share the same SkFontMgr, FlutterFontCollection, and SkTypeface_FreeType (FT_Face / FT_Library) instances in shared WebAssembly.Memory (engine/src/flutter/skwasm/fonts.cc:38-42).
• Whenever the main thread's thread-local SkStrike (during $paragraph_layout) and the worker thread's thread-local SkStrike (during _surface_renderPicturesOnWorker) miss at the same time on the same typeface (for example, when animating layout or FittedBox scales), both threads concurrently enter SkScalerContext_FreeType::generateMetrics / generateImage -> FT_Load_Glyph -> TT_Load_Glyph on the same non-thread-safe FreeType FT_Face.
• In Skia (SkFontHost_FreeType.cpp), concurrent SkScalerContext_FreeType instances across threads rely on f_t_mutex() (SkMutex) and fFaceRec->fFTMutex (SkMutex) — backed by SkSemaphore (third_party/skia/src/base/SkSemaphore.cpp) — to serialize access to FT_Library and FT_Face.
• Because SkSemaphore::osWait() (sem_wait) compiles to a no-op stub (0 blocking or spinning instructions) in both skwasm.wasm (wasm-function[268]) and wimp.wasm (wasm-function[302]), f_t_mutex().acquire() returns immediately when contended (fCount.fetch_sub(1) <= 0), allowing both threads to mutate FT_Face simultaneously.

Steps to Reproduce

1. Build kevmoo/flutter-wasm-compare at commit 5c4e80e with flutter build web --wasm (3.49.0-1.0.pre-171, engine 1a3d11d5ab1).
2. Serve build/web with Cross-Origin-Opener-Policy: same-origin and Cross-Origin-Embedder-Policy: require-corp so skwasm_isMultiThreaded() == true.
3. Navigate to ?workload=grid&nodes=100&renderer=skwasm (or renderer=wimp, or nodes=1000), which animates 100 GridCell cards containing FittedBox + Text paragraphs every frame while the DedicatedWorker rasterizes the previous frame.

Observed Behavior

Across repeated runs on both macOS (Apple M4 Pro, Chrome 146) and Linux (x86_64, Headless Chrome 154), multi-threaded grid@100 and grid@1000 intermittently fail in one of two ways (0 failures in single-threaded skwasm or wimp across 100+ runs):

1. Uncaught RuntimeError: memory access out of bounds in $paragraph_layout -> FT_Load_Glyph -> TT_Load_Simple_Glyph:
   Instrumenting worker postMessage (cbId) and Runtime.exceptionThrown via CDP shows that while the DedicatedWorker is mid-flight executing _surface_renderPicturesOnWorker (cbId = 23, dispatched at t = 1721.6 ms, completed at t = 1753.1 ms), the main UI thread throws at t = 1731.0 ms:

RuntimeError: memory access out of bounds
    at skwasm.wasm:wasm-function[2107]:0x63e5b   // TT_Load_Simple_Glyph (FreeType ttgload.c)
    at skwasm.wasm:wasm-function[10931]:0x2863fd // load_truetype_glyph (FreeType ttgload.c)
    at skwasm.wasm:wasm-function[10942]:0x28832c // TT_Load_Glyph (FreeType ttgload.c)
    at skwasm.wasm:wasm-function[721]:0x1f3c1    // FT_Load_Glyph (FreeType ftobjs.c)
    at skwasm.wasm:wasm-function[11471]:0x2a7ee9 // SkScalerContext_FreeType::generateMetrics (calls f_t_mutex() func 452 -> func 268)
    at skwasm.wasm:wasm-function[6734]:0x185fda  // SkScalerContext::makeGlyph
    at skwasm.wasm:wasm-function[1776]:0x51f44   // SkStrike::digestFor
    at skwasm.wasm:wasm-function[2783]:0x8e456   // SkBulkGlyphMetrics::glyph
    at skwasm.wasm:wasm-function[6681]:0x182bb3  // SkBulkGlyphMetrics::glyphs
    at skwasm.wasm:wasm-function[2246]:0x6b813   // SkStrike::lock / glyph metrics
    ...
    at skwasm.wasm:wasm-function[9227]:0x227493  // skia::textlayout::ParagraphImpl::layout
    at $paragraph_layout (skwasm.wasm:0x26ee84)  // engine/src/flutter/skwasm/text/paragraph.cc:55

This uncaught trap aborts PipelineOwner.flushLayout() mid-frame and stops all subsequent scene production while requestAnimationFrame continues firing.

2. Main-thread infinite busy-spin in dlmalloc:
   If a thread traps while holding Emscripten's dlmalloc spinlock (wasm-function[276] / wasm-function[222], spinning on i32.atomic.rmw.xchg offset=603928) or concurrent FreeType FT_Memory allocations corrupt allocator state, the next main-thread allocation spins indefinitely in dlmalloc, hanging the tab.

Disassembling wasm-function[268] in skwasm.wasm (and identical wasm-function[302] in wimp.wasm) confirms that SkSemaphore::wait() returns immediately without waiting when fCount.fetch_sub(1) <= 0:

(func (;268;) (type 2) (param i32)
  block ;; label = @1
    block ;; label = @2
      local.get 0
      i32.const 1
      i32.atomic.rmw.sub
      i32.const 0
      i32.gt_s
      br_if 0 (;@2;) ;; Uncontended (old fCount > 0): lock acquired, return.
      block ;; label = @3
        block ;; label = @4
          local.get 0
          i32.atomic.load8_u offset=4 ;; SkOnce state (0=NotStarted, 1=Claimed, 2=Done)
          br_table 0 (;@4;) 1 (;@3;) 2 (;@2;) 1 (;@3;)
          ;; When SkOnce == 2 (Done), jumps directly to @2 (return) with zero wait instructions.
        end
        local.get 0
        i32.const 0
        i32.const 1
        i32.atomic.rmw8.cmpxchg_u offset=4
        i32.eqz
        br_if 2 (;@1;)
      end
      loop ;; label = @3
        local.get 0
        i32.atomic.load8_u offset=4
        i32.const 2
        i32.ne
        br_if 0 (;@3;)
      end
    end
    return
  end
  local.get 0
  i32.const 16
  call 224 ;; operator new(16) for OSSemaphore (sem_t)
  i32.store offset=8
  local.get 0
  i32.const 2
  i32.atomic.store8 offset=4
)

Expected Behavior

SkMutex / SkSemaphore (third_party/skia/src/base/SkSemaphore.cpp) must enforce mutual exclusion when compiled with -sWASM_WORKERS=1 (e.g., via Wasm atomics / spin-wait on the main browser thread and emscripten_futex_wait on workers), or SkTypeface_FreeType (FT_Face) must not be shared across threads without synchronization.

Environment / Target

• Target Commit / Version: Flutter 3.49.0-1.0.pre-171 (engine 1a3d11d5ab1, after [web] Use thread local strike caches in skwasm #190048 and Use officially supported compile time thread local strike cache flag. #191014)
• Runtime / Platform: WebAssembly (skwasm and wimp multi-threaded under COOP/COEP), Chrome 146 (macOS arm64) and Chrome 154 (Linux x86_64)