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fix(js): always fall back to non-streaming WebAssembly.instantiate #445

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Merged
sd2k merged 1 commit into from
Feb 24, 2026
Merged

fix(js): always fall back to non-streaming WebAssembly.instantiate #445

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75 changes: 75 additions & 0 deletions js/justfile
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Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,7 @@ build: \
(build-inner "seasons") \
(build-inner "transforms")
just fix-package-json
just fix-wasm-streaming-fallback
just copy-readme

build-inner target args='':
Expand All @@ -32,6 +33,80 @@ fix-package-json:
jq < augurs/package.json ". | .version = \"$VERSION\"" > augurs/package.json.tmp
mv augurs/package.json.tmp augurs/package.json

# Patch wasm-bindgen's generated __wbg_load to always fall back to the
# non-streaming WebAssembly.instantiate path. The generated code rethrows
# when instantiateStreaming fails and Content-Type is application/wasm,
# but this breaks in environments where fetch() returns a Proxy-wrapped
# Response (e.g. OpenTelemetry instrumentation-fetch) that passes JS
# `instanceof Response` but fails the browser's internal C++ slot check.
# Related: https://github.com/open-telemetry/opentelemetry-js/issues/5477
fix-wasm-streaming-fallback:
#!/usr/bin/env python3
import re, glob, sys
# wasm-bindgen 0.2.101-0.2.106: multi-line else block
CATCH_V1 = re.compile(
r'(\s*)\} catch \(e\) \{\s*\n'
r'\s*const validResponse.*?\n'
r'\s*\n'
r'\s*if \(validResponse.*?\n'
r'\s*console\.warn\(.*?\n'
r'\s*\n'
r'\s*\} else \{\s*\n'
r'\s*throw e;\s*\n'
r'\s*\}'
)
# wasm-bindgen 0.2.107+: single-line else block
CATCH_V2 = re.compile(
r'(\s*)\} catch \(e\) \{\s*\n'
r'\s*const validResponse.*?\n'
r'\s*\n'
r'\s*if \(validResponse.*?\n'
r'\s*console\.warn\(.*?\n'
r'\s*\n'
r'\s*\} else \{ throw e; \}'
)
CATCH_REPL = (
r'\1} catch (e) {\1 console.warn('
r'"`WebAssembly.instantiateStreaming` failed. Falling back to '
r'`WebAssembly.instantiate` which is slower. Original error:\\n", e);'
)
# Module-level Set (wasm-bindgen 0.2.101-0.2.106)
EXPECTED_TYPES_SET_RE = re.compile(r"const EXPECTED_RESPONSE_TYPES[^\n]*\n\n")
# Local function inside __wbg_load (wasm-bindgen 0.2.107+)
EXPECTED_TYPE_FN_RE = re.compile(
r"\n function expectedResponseType\(type\) \{"
r".*?return false;\n \}\n",
re.DOTALL,
)
errors = []
for f in sorted(glob.glob("augurs/*.js")):
text = open(f).read()
if "instantiateStreaming" not in text:
continue
# Replace the conditional rethrow with an unconditional warn + fallback.
# Try both patterns; only one will match depending on wasm-bindgen version.
text, n = CATCH_V2.subn(CATCH_REPL, text)
if n == 0:
text, n = CATCH_V1.subn(CATCH_REPL, text)
if n == 0:
errors.append(f"{f}: expected to patch __wbg_load but regex didn't match")
continue
Comment on lines +88 to +93
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@coderabbitai coderabbitai bot Feb 24, 2026

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⚠️ Potential issue | 🟡 Minor

Idempotency gap: running the recipe standalone on already-patched files produces a misleading error.

When both CATCH_V2.subn and CATCH_V1.subn return n == 0 on a previously-patched file, the error appended is "expected to patch __wbg_load but regex didn't match", and the final message printed is "The wasm-bindgen output format may have changed. Update the regex in js/justfile." A developer retrying without a clean rebuild (e.g., after a partial build failure) would chase a phantom wasm-bindgen format change. A simple content-based already-patched check avoids the false alarm:

♻️ Proposed fix — distinguish already-patched from genuinely unrecognised format 
      if n == 0:
-         errors.append(f"{f}: expected to patch __wbg_load but regex didn't match")
-         continue
+         # If neither pattern is present either, the file may already be patched.
+         if not (CATCH_V1.search(text) or CATCH_V2.search(text)):
+             print(f"  {f}: already patched or no matching pattern — skipping")
+             continue
+         errors.append(f"{f}: instantiateStreaming present but regex didn't match; "
+                        f"wasm-bindgen output format may have changed")
+         continue
🤖 Prompt for AI Agents 
Verify each finding against the current code and only fix it if needed.

In `@js/justfile` around lines 88 - 93, The current logic appends an error when
both CATCH_V2.subn and CATCH_V1.subn return n == 0 even if the file is already
patched; modify the branch after those two attempts to detect an already-patched
file (e.g., check if CATCH_REPL or a unique marker is present in text) and if so
skip adding the error and continue, otherwise keep appending the existing error;
adjust the code around the variables CATCH_V2, CATCH_V1, CATCH_REPL, text,
errors and f so the recipe is idempotent when run against already-patched files.

# Remove the now-unused response type check helper.
text = EXPECTED_TYPES_SET_RE.sub("", text)
text = EXPECTED_TYPE_FN_RE.sub("\n", text)
# Verify the original rethrow pattern is fully gone after patching.
if CATCH_V1.search(text) or CATCH_V2.search(text):
errors.append(f"{f}: patched but original rethrow pattern still present")
continue
open(f, "w").write(text)
print(f" patched {f} ({n} replacement(s))")
if errors:
print("ERROR: wasm-streaming-fallback patch failed:", file=sys.stderr)
for e in errors:
print(f" {e}", file=sys.stderr)
print("The wasm-bindgen output format may have changed. Update the regex in js/justfile.", file=sys.stderr)
sys.exit(1)

copy-readme:
cp README.md augurs/README.md

Expand Down