Patching Native Modules Skill

Patching Native Modules

Follow this workflow to analyze crash logs, fix native module bugs, and generate patches.

Workflow Overview

1. Analyze Crash Log → 2. Locate Bug → 3. Fix Code → 4. Clean Build Artifacts → 5. Generate Patch → 6. Commit & PR

Step 1: Analyze Crash Log

iOS Crash (EXC_BAD_ACCESS / KERN_INVALID_ADDRESS)

Key information to extract:

Exception type: EXC_BAD_ACCESS, SIGABRT, etc.
Stack trace: Identify the crashing function
Memory address: Helps identify nil pointer issues
Library: Which native module is crashing

Example crash pattern:

EXC_BAD_ACCESS: KERN_INVALID_ADDRESS at 0x3c61c1a3b0d0
 objc_msgSend in unknown file
 -[SDWebImageManager cacheKeyForURL:context:]  ← Crashing function
 -[SDWebImageManager loadImageWithURL:options:context:progress:completed:]

Android Crash (NullPointerException / OOM)

Look for:

Exception class: NullPointerException, OutOfMemoryError
Stack trace: Java/Kotlin method chain
Thread info: Main thread vs background

Step 2: Locate the Bug

Find native module source

# iOS (Swift/Objective-C)
ls node_modules/<package>/ios/

# Android (Kotlin/Java)
ls node_modules/<package>/android/src/main/java/

Common crash causes

| Crash Type | Common Cause | Fix Pattern | |------------|--------------|-------------| | EXC_BAD_ACCESS | Nil pointer dereference | Add guard let check | | KERN_INVALID_ADDRESS | Accessing deallocated memory | Use weak references | | NullPointerException | Null object access | Add null check | | OutOfMemoryError | Large image/data processing | Add size limits |

Step 3: Fix the Code

iOS (Swift) - Nil Check Pattern

// Before (crashes when uri is nil)
imageManager.loadImage(with: source.uri, ...)

// After (safe)
guard let sourceUri = source.uri, !sourceUri.absoluteString.isEmpty else {
  onError(["error": "Image source URI is nil or empty"])
  return
}
imageManager.loadImage(with: sourceUri, ...)

Android (Kotlin) - Null Check Pattern

// Before
val uri = source.uri
loadImage(uri)

// After
val uri = source.uri ?: return
if (uri.toString().isEmpty()) return
loadImage(uri)

Step 4: Clean Build Artifacts (CRITICAL)

Before generating patch, MUST clean Android build cache:

# Remove Android build artifacts to avoid polluting the patch
rm -rf node_modules/<package>/android/build

# For expo-image specifically:
rm -rf node_modules/expo-image/android/build

Why this matters:

Android build generates .class, .jar, binary files
These pollute the patch file (can grow to 5000+ lines)
patch-package will include these unwanted files

Step 5: Generate Patch

# Generate patch file
npx patch-package <package-name>

# Example:
npx patch-package expo-image

Patch file location: patches/<package-name>+<version>.patch

Verify patch content

# Check patch doesn't include unwanted files
grep -c "android/build" patches/<package-name>*.patch
# Should return 0

# View actual changes
head -100 patches/<package-name>*.patch

Step 6: Commit & Create PR

# Stage patch file
git add patches/<package-name>*.patch

# Commit with descriptive message
git commit -m "fix(ios): prevent EXC_BAD_ACCESS crash in <package> when <condition>

Add guard checks in <package> native layer to prevent crash when <scenario>.

Fixes Sentry issue #XXXXX"

# Create PR
gh pr create --title "fix(ios): <description>" --base x

Common Packages & Their Native Locations

| Package | iOS Source | Android Source | |---------|------------|----------------| | expo-image | node_modules/expo-image/ios/ | node_modules/expo-image/android/src/ | | react-native | node_modules/react-native/React/ | node_modules/react-native/ReactAndroid/ | | @react-native-async-storage/async-storage | node_modules/@react-native-async-storage/async-storage/ios/ | ...android/src/ | | react-native-reanimated | node_modules/react-native-reanimated/ios/ | ...android/src/ |

Existing Patches Reference

Check existing patches for patterns:

ls patches/
cat patches/expo-image+3.0.10.patch

Troubleshooting

Patch file too large

# Clean all build artifacts
rm -rf node_modules/<package>/android/build
rm -rf node_modules/<package>/ios/build
rm -rf node_modules/<package>/.gradle

# Regenerate
npx patch-package <package>

Patch not applying

# Check package version matches
cat node_modules/<package>/package.json | grep version

# Rename patch if version changed
mv patches/<package>+old.patch patches/<package>+new.patch

Swift/Kotlin syntax help

Swift guard let:

guard let value = optionalValue else {
  return  // Must exit scope
}
// value is now non-optional

Kotlin null check:

val value = nullableValue ?: return
// value is now non-null

Related Files

Patches directory: patches/
expo-image iOS: node_modules/expo-image/ios/ImageView.swift
expo-image Android: node_modules/expo-image/android/src/main/java/expo/modules/image/

Agent Skills: Patching Native Modules

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