better error handling and fix remoteAttachment tests

[mchenani]

closes #467

[claude]

Claude finished @mchenani's task —— View job

---

PR Review: Better error handling for remote attachments

I've completed my review of this PR that addresses issue #467. Here's my comprehensive feedback:

Todo List:

• Read all changed files to understand modifications
• Review RemoteAttachmentCodec.kt for error handling and optional fields
• Review Dm.kt and Group.kt for exception handling
• Review TestHelpers.kt for test utilities
• Review RemoteAttachmentTest.kt for test coverage
• Assess Kotlin/Android best practices
• Check null safety and coroutine usage
• Evaluate performance implications for mobile
• Review security concerns for messaging/crypto operations
• Provide comprehensive feedback

Summary

This PR improves error handling for remote attachments and reorganizes tests from unit tests to instrumented tests. The changes make contentLength and filename optional fields and enhance error messages by preserving exception causes. Overall, the changes are well-implemented with good attention to null safety and proper coroutine usage.

🚨 Issues to Address

1. Redundant Variable Assignments in RemoteAttachmentTest.kt

Lines 76-77: These assignments appear redundant since the RemoteAttachment.from() method no longer takes these parameters:

remoteAttachment.contentLength = attachment.data.size()
remoteAttachment.filename = attachment.filename

The from() method (line 118-131 in RemoteAttachmentCodec.kt) only takes url and encryptedEncodedContent, so these fields are set to null by default. If these fields are needed, they should be set in the from() method or documented as required post-construction steps. Consider either:

• Updating the from() method to accept these optional parameters
• Removing these assignments if they're not needed
• Or adding a comment explaining why they're set after construction

2. API Breaking Change

The RemoteAttachment.from() method signature changed from taking 4 parameters to 2 parameters. While this simplifies the API, it's a breaking change that could affect existing client code. Consider:

• Documenting this as a breaking change in release notes
• Or adding a deprecated overloaded method for backward compatibility

✅ Positive Changes

1. Improved Error Handling with Exception Chaining

Dm.kt:129 and Group.kt:170: Excellent improvement to preserve the original exception as the cause:

throw XMTPException("Failed to encode content: ${e.message}", e)

This maintains the full stack trace, making debugging significantly easier. This is a Kotlin/Android best practice.

2. Flexible Optional Field Handling

RemoteAttachmentCodec.kt:165-171: Making contentLength and filename optional is a pragmatic improvement:

content.contentLength?.let {
    parametersMap["contentLength"] = it.toString()
}

content.filename?.let {
    parametersMap["filename"] = it
}

The codec gracefully handles missing fields during both encoding (lines 165-171) and decoding (lines 186-187). This follows Kotlin's idiomatic null safety patterns.

3. Test Infrastructure Improvements

TestHelpers.kt:117-121: The new TestFetcher class provides a clean way to test remote attachments locally:

class TestFetcher(val filesDir: File) : Fetcher {
    override fun fetch(url: URL): ByteArray {
        return File(filesDir, url.toString().replace("https://", "")).readBytes()
    }
}

This is well-designed for testing purposes. Moving RemoteAttachmentTest from unit tests to instrumented tests is also appropriate since it uses Android-specific features like Context.filesDir.

4. Proper Test Setup with @before

RemoteAttachmentTest.kt:26-31: Good use of @Before annotation for test setup, following JUnit best practices.

🔍 Code Quality & Kotlin/Android Best Practices

✅ Excellent Practices Observed

1. Null Safety: Proper use of nullable types (Int?, String?) and safe calls (?.let)
2. Exception Chaining: Preserves debugging context by passing original exceptions
3. Coroutine Context Switching: Both Dm.kt and Group.kt consistently use withContext(Dispatchers.IO) for all suspend functions performing IO operations
4. Immutability: Most fields use val appropriately
5. Clean Separation: Test utilities properly separated in TestHelpers.kt
6. Proper Resource Management: Tests correctly initialize fixtures in @Before method

⚠️ Minor Concerns

1. Mutable Fields in RemoteAttachment (RemoteAttachmentCodec.kt:33-35):

var contentLength: Int? = null,
var filename: String? = null,
var fetcher: Fetcher = HTTPFetcher(),

These var fields could potentially cause thread safety issues in concurrent scenarios. Consider making them val if possible, or document thread safety requirements.

2. String Interpolation in Fallback (RemoteAttachmentCodec.kt:203):

return "Can't display \"${content.filename}". This app doesn't support attachments."

There's a missing closing quote after ${content.filename}. This appears to be a syntax error that should be:

return "Can't display \"${content.filename}\". This app doesn't support attachments."

🔒 Security Analysis

✅ Security Strengths

1. HTTPS Enforcement (RemoteAttachmentCodec.kt:119-121): Properly validates that URLs use HTTPS:

if (URI(url.toString()).scheme != "https") {
    throw XMTPException("scheme must be https://")
}

2. Content Integrity Validation (RemoteAttachmentCodec.kt:61-63): Validates content digest to ensure attachment hasn't been tampered with:

if (Hash.sha256(encrypted.payload.toByteArray()).toHex() != encrypted.contentDigest) {
    throw XMTPException("contentDigest does not match")
}

3. Secure Random Generation (RemoteAttachmentCodec.kt:81, 102): Uses SecureRandom for cryptographic key generation, which is the correct choice for security-sensitive operations.

4. AES-256-GCM Encryption: The code uses AES-256-GCM with HKDF-SHA256 for encryption, which is a strong, authenticated encryption scheme.

💡 Security Recommendations

1. URL Validation: Consider adding more robust URL format validation to prevent malformed URLs or potential injection attacks.

2. Content Length Limits: Consider adding a maximum content length check to prevent DoS attacks from extremely large attachments.

3. Path Traversal in TestFetcher (TestHelpers.kt:119): While this is test-only code, the replace("https://", "") could potentially be vulnerable to path traversal if used with untrusted input. For production code, you'd want proper path sanitization.

🎯 Performance Considerations

✅ No Performance Regressions

1. Error Handling Path: The changes mainly affect error handling paths, which are not performance-critical.
2. Optional Field Overhead: The ?.let checks add minimal overhead and are appropriate.
3. Proper Threading: withContext(Dispatchers.IO) ensures IO operations don't block the main thread on Android.
4. Efficient Test Scope: Test utilities are appropriately scoped to test environment only.

Mobile-Specific Considerations

• The code properly uses coroutines with appropriate dispatchers for IO operations
• No obvious memory leaks or retention issues
• File operations in tests are appropriately scoped to test context

🧪 Test Coverage

✅ Good Coverage

The test suite covers key scenarios:

1. Encryption/Decryption (testEncryptedContentShouldBeDecryptable): Validates the core encryption functionality
2. End-to-End Codec Usage (testCanUseRemoteAttachmentCodec): Tests the full codec flow including sending and receiving
3. HTTPS Enforcement (testCannotUseNonHTTPSURL): Validates security requirement
4. Content Integrity (testEnsuresContentDigestMatches): Tests tamper detection

📝 Test Coverage Suggestions

1. Error Message Tests: Add tests specifically for the new error messages in encodeContent to ensure they're helpful
2. Null Handling: Add tests for contentLength and filename being null
3. Edge Cases: Test extremely large content lengths, empty filenames, special characters in filenames
4. Concurrent Access: If the mutable fields are intended for concurrent use, add thread safety tests

🎨 Null Safety & Coroutine Usage

✅ Excellent Null Safety

• Proper use of nullable types throughout (Int?, String?)
• Safe calls with ?.let pattern for optional processing
• No obvious NPE risks identified
• toIntOrNull() used appropriately for safe parsing (RemoteAttachmentCodec.kt:186)

✅ Excellent Coroutine Usage

• Consistent use of withContext(Dispatchers.IO) for all IO operations in Dm.kt and Group.kt
• Proper use of runBlocking in tests (appropriate for test context)
• No blocking calls in suspend functions
• Proper use of suspend functions for async operations

📋 Recommendations Summary

1. Critical: Fix the string interpolation syntax error in fallback() method (RemoteAttachmentCodec.kt:203)
2. Important: Remove or justify the redundant assignments in RemoteAttachmentTest.kt (lines 76-77)
3. Important: Document the breaking change to RemoteAttachment.from() method in release notes
4. Consider: Making RemoteAttachment fields immutable (val) for better thread safety
5. Consider: Adding more input validation for URLs and content lengths
6. Nice to Have: Add tests for the new error messages and edge cases

Overall Assessment

This is a well-implemented PR that improves error handling and test organization. The code follows Kotlin and Android best practices, demonstrates excellent null safety and coroutine usage, and maintains good security practices. The identified issues are minor and can be easily addressed. The changes make the SDK more robust and easier to debug.

Recommendation: Approve with minor fixes (the syntax error and redundant assignments).

---

[Copilot]

Pull Request Overview

This PR improves error handling across the XMTP Android library and fixes issues in the remote attachment tests. The primary focus is on providing more descriptive error messages and ensuring the remote attachment functionality works properly with the updated API.

• Better error handling with more informative exception messages that include the underlying cause
• Fixed remote attachment codec to properly handle filename and contentLength parameters with null checks
• Updated remote attachment tests to work with current test infrastructure and removed flaky test annotations

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
RemoteAttachmentCodec.kt	Enhanced parameter validation and error handling for remote attachment encoding
Group.kt	Improved error message to include underlying exception details
Dm.kt	Improved error message to include underlying exception details
TestHelpers.kt	Added TestFetcher class for testing remote attachment functionality
RemoteAttachmentTest.kt	Fixed test setup, updated method calls, and removed flaky test annotations
GroupTest.kt	Added comprehensive test for group membership and activity state after removal and restart

---

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[Copilot]

This import appears to be unused and unrelated to the functionality. The @Length annotation from checker framework is for unit checking of length measurements, which doesn't seem relevant to this remote attachment codec.

Suggested change

import org.checkerframework.checker.units.qual.Length

[Copilot]

[nitpick] There's an extra blank line at line 173 that should be removed to maintain consistent spacing.

Suggested change

	content.contentLength?.let { contentLength ->
	parametersMap["contentLength"] = contentLength.toString()
	} ?: throw XMTPException("missing contentLength")
	content.filename?.let { fileName ->
	parametersMap["filename"] = fileName
	} ?: throw XMTPException("missing filename")

[Copilot]

These assignments are redundant since the RemoteAttachment.from() method now takes filename and contentLength as parameters and sets them during construction. These lines should be removed.

[Copilot]

Commented-out code should be removed rather than left in the codebase. If sync is not needed, remove this line entirely.

Suggested change

// runBlocking { caroGroupAfterRestart!!.sync() }

[macroscopeapp]

Preserve exception causes in Dm and Group content-encoding errors and fix RemoteAttachment instrumentation tests to use local files and expect 2 messages

• Update RemoteAttachmentCodec encoding and decoding in RemoteAttachmentCodec.kt to omit contentLength and filename when absent and decode them as optional without throwing, passing contentLength as nullable.
• Change error handling in Dm.kt and Group.kt to throw XMTPException with the original exception message and cause.
• Modify instrumentation tests in RemoteAttachmentTest.kt to initialize fixtures via @Before, remove @Ignore, write to filesDir, use a TestFetcher, and assert 2 messages per conversation.
• Add TestFetcher in TestHelpers.kt implementing Fetcher to load data from filesDir.

📍Where to Start

Start with the decode and encode methods in RemoteAttachmentCodec.kt.

---

Macroscope summarized 9a98923.

[claude]

Claude finished @mchenani's task —— View job

---

PR Review: Better error handling for remote attachments

I've completed my review of this PR that addresses issue #467. Here's my comprehensive feedback:

Todo List:

• Read all changed files to understand the modifications
• Review RemoteAttachmentCodec.kt for error handling improvements
• Review Dm.kt and Group.kt for exception handling changes
• Review TestHelpers.kt for new testing utilities
• Review RemoteAttachmentTest.kt for test fixes and coverage
• Check for Kotlin/Android best practices
• Assess null safety and potential NPEs
• Evaluate performance implications
• Review security considerations for messaging operations
• Provide comprehensive feedback

Summary

This PR makes error handling and test organization improvements, but there are several issues that should be addressed before merging:

🚨 Critical Issues

1. Unused Import in RemoteAttachmentCodec.kt

Line 6: The import org.checkerframework.checker.units.qual.Length appears to be unused and should be removed.

2. Redundant Variable Assignments in RemoteAttachmentTest.kt

Lines 76-78: These assignments are redundant since RemoteAttachment.from() no longer takes these parameters:

remoteAttachment.contentLength = attachment.data.size()  // Redundant
remoteAttachment.filename = attachment.filename          // Redundant

These fields should either be set in the from() method or the assignments should be removed if not needed.

3. Commented-Out Code in GroupTest.kt

Line 504: There's commented-out code that should be removed or uncommented if needed:

// runBlocking { caroGroupAfterRestart!!.sync() }

⚠️ Code Quality Issues

1. API Breaking Change

The change to RemoteAttachment.from() method signature is a breaking change. The method now has fewer parameters:

• Old: from(url, encryptedEncodedContent, filename, contentLength)
• New: from(url, encryptedEncodedContent)

This could break existing client code. Consider:

• Adding a deprecated overloaded method for backward compatibility
• Or clearly documenting this as a breaking change in the release notes

2. Inconsistent Error Handling Pattern

The codec now makes contentLength and filename optional during encoding but the error messages in the previous review suggest they were required. The current implementation:

content.contentLength?.let {
    parametersMap["contentLength"] = it.toString()
}

content.filename?.let {
    parametersMap["filename"] = it
}

This is inconsistent with the commit message which claims to require these fields.

3. Improved Error Messages in Dm.kt and Group.kt

The error handling improvement is good:

throw XMTPException("Failed to encode content: ${e.message}", e)

This preserves the original exception as the cause, which helps with debugging.

✅ Positive Changes

1. Better Error Context: Including the original exception cause in encodeContent methods
2. Test Organization: Moving RemoteAttachmentTest to instrumented tests is appropriate since it uses Android-specific features
3. TestFetcher Utility: Good test utility for simulating remote fetches locally
4. Making Fields Optional: The decision to make contentLength and filename optional is pragmatic

🔒 Security Considerations

1. HTTPS Enforcement: ✅ Good - The code correctly enforces HTTPS for remote attachment URLs
2. Content Integrity: ✅ Good - Content digest validation ensures attachment integrity
3. Missing Validations: ⚠️ Consider adding:
   • URL format validation to prevent malformed URLs
   • Maximum content length check to prevent DoS attacks
   • Content digest format validation (hex string of specific length)

🎯 Performance Considerations

No performance regressions identified. The changes mainly affect error handling paths which are not in the hot path.

🧪 Test Coverage

The tests are comprehensive but could be improved:

1. Missing edge cases: No tests for extremely large content lengths
2. Missing error path tests: Should test the new error messages in encodeContent
3. Good coverage: Tests for HTTPS enforcement, content digest validation, and basic functionality

📋 Recommendations

1. Remove unused import in RemoteAttachmentCodec.kt
2. Remove redundant assignments in RemoteAttachmentTest.kt or update the from() method
3. Remove commented code in GroupTest.kt
4. Consider backward compatibility for the from() method signature change
5. Add input validation for security (URL format, max content length)
6. Document breaking changes if the API change is intentional

Kotlin/Android Best Practices

The code generally follows good Kotlin practices:

• ✅ Proper use of nullable types
• ✅ Exception handling with cause preservation
• ✅ Appropriate use of let for nullable handling
• ⚠️ Could improve null safety by using requireNotNull in some places

Overall, this PR improves the codebase's error handling, but the identified issues should be addressed before merging.

[claude]

Claude finished @mchenani's task —— View job

---

PR Review: Better error handling for remote attachments

I've completed my review of this PR that addresses issue #467. Here's my comprehensive feedback:

Todo List:

• Read all changed files to understand the modifications
• Review RemoteAttachmentCodec.kt for error handling improvements
• Review Dm.kt and Group.kt for exception handling changes
• Review TestHelpers.kt for new testing utilities
• Review RemoteAttachmentTest.kt for test fixes and coverage
• Check for Kotlin/Android best practices
• Assess null safety and potential NPEs
• Evaluate performance implications
• Review security considerations for messaging operations
• Provide comprehensive feedback

Summary

This PR improves error handling for remote attachments and test infrastructure. The changes are generally good, making contentLength and filename optional fields and improving error messages. However, there are a few issues that should be addressed.

🚨 Issues to Address

1. Redundant Variable Assignments in RemoteAttachmentTest.kt

Lines 76-77: These assignments appear redundant since RemoteAttachment.from() no longer takes these parameters as arguments:

remoteAttachment.contentLength = attachment.data.size()  // Line 76
remoteAttachment.filename = attachment.filename          // Line 77

These assignments are setting fields that should already be handled in the creation process. Consider removing these lines if they're not needed.

2. API Breaking Change

The change to RemoteAttachment.from() method signature is a breaking change:

• Old: from(url, encryptedEncodedContent, filename, contentLength)
• New: from(url, encryptedEncodedContent)

This could break existing client code. Consider documenting this as a breaking change in the release notes.

✅ Positive Changes

1. Better Error Messages

The improved error handling in Dm.kt and Group.kt now preserves the original exception as the cause:

throw XMTPException("Failed to encode content: ${e.message}", e)

This makes debugging much easier by maintaining the full stack trace.

2. Flexible Parameter Handling

Making contentLength and filename optional in RemoteAttachmentCodec is a pragmatic improvement:

content.contentLength?.let {
    parametersMap["contentLength"] = it.toString()
}

content.filename?.let {
    parametersMap["filename"] = it
}

The codec gracefully handles missing fields during decoding as well.

3. Test Infrastructure Improvements

• Moving RemoteAttachmentTest to instrumented tests is appropriate since it uses Android-specific features
• The new TestFetcher class provides a clean way to test remote attachments locally:

class TestFetcher(val filesDir: File) : Fetcher {
    override fun fetch(url: URL): ByteArray {
        return File(filesDir, url.toString().replace("https://", "")).readBytes()
    }
}

🔍 Code Quality & Kotlin/Android Best Practices

✅ Good Practices

• Proper use of Kotlin's null safety features with ?.let
• Exception chaining maintains debugging context
• Clear separation of test utilities in TestHelpers.kt
• Appropriate use of @Before annotation in test setup

⚠️ Areas for Improvement

1. Consider using require or check for validation instead of throwing exceptions directly in some cases
2. The mutable var fields in RemoteAttachment (lines 33-34) could potentially cause thread safety issues in concurrent scenarios

🔒 Security Considerations

✅ Security Strengths

1. HTTPS enforcement: The code correctly validates HTTPS scheme (line 116 in RemoteAttachmentCodec.kt)
2. Content integrity: Content digest validation ensures attachment hasn't been tampered with
3. Secure random generation: Uses SecureRandom for key generation

⚠️ Security Recommendations

1. Input validation: Consider adding URL format validation to prevent malformed URLs
2. Resource limits: Consider adding a maximum content length check to prevent DoS attacks
3. Path traversal: The TestFetcher could potentially be vulnerable to path traversal if used with untrusted input (though it's test-only code)

🎯 Performance Considerations

No performance regressions identified:

• The changes mainly affect error handling paths which are not performance-critical
• Optional field handling adds minimal overhead
• Test utilities are appropriately scoped to test environment only

🧪 Test Coverage

✅ Good Coverage

• Tests for HTTPS enforcement (testCannotUseNonHTTPSURL)
• Tests for content digest validation (testEnsuresContentDigestMatches)
• Tests for basic codec functionality (testCanUseRemoteAttachmentCodec)
• Tests for encryption/decryption (testEncryptedContentShouldBeDecryptable)

📝 Suggestions

1. Consider adding tests for the new error messages in encodeContent
2. Add edge case tests for extremely large content lengths
3. Consider adding tests for concurrent access scenarios

🎨 Null Safety & Coroutine Usage

✅ Null Safety

• Proper use of nullable types and safe calls throughout
• No obvious NPE risks identified
• Appropriate use of Kotlin's ?.let pattern

✅ Coroutine Usage

• Proper use of runBlocking in tests for coroutine contexts
• No blocking calls in suspend functions

📋 Recommendations

1. Remove or justify the redundant assignments in RemoteAttachmentTest.kt (lines 76-77)
2. Document the breaking change to RemoteAttachment.from() method
3. Consider adding input validation for URLs and content lengths
4. Consider making RemoteAttachment fields immutable if possible to improve thread safety

Overall, this PR improves the codebase's error handling and makes the remote attachment system more flexible. The identified issues are minor and can be easily addressed.

[claude]

Claude finished @mchenani's task —— View job

---

PR Review: Better error handling for remote attachments

I've completed my review of this PR that addresses issue #467. Here's my comprehensive feedback:

Todo List:

• Read all changed files to understand the modifications
• Review RemoteAttachmentCodec.kt for error handling improvements
• Review Dm.kt and Group.kt for exception handling changes
• Review TestHelpers.kt for new testing utilities
• Review RemoteAttachmentTest.kt for test fixes and coverage
• Check for Kotlin/Android best practices
• Assess null safety and potential NPEs
• Evaluate performance implications
• Review security considerations for messaging operations
• Provide comprehensive feedback

Summary

This PR improves error handling for remote attachments and reorganizes tests. The changes are generally good, making contentLength and filename optional fields and improving error messages. However, there are a few issues that should be addressed before merging.

🚨 Critical Issues

1. Redundant Variable Assignments in RemoteAttachmentTest.kt

Lines 76-77: These assignments appear redundant since RemoteAttachment.from() no longer takes these parameters as arguments:

remoteAttachment.contentLength = attachment.data.size()  // Line 76
remoteAttachment.filename = attachment.filename          // Line 77

These should be removed if they're not needed, or the from() method should be updated to properly set these fields.

2. API Breaking Change

The change to RemoteAttachment.from() method signature is a breaking change:

• Old: from(url, encryptedEncodedContent, filename, contentLength)
• New: from(url, encryptedEncodedContent)

This could break existing client code. Consider documenting this as a breaking change in the release notes.

✅ Positive Changes

1. Better Error Messages

The improved error handling in Dm.kt and Group.kt now preserves the original exception as the cause:

throw XMTPException("Failed to encode content: ${e.message}", e)

This makes debugging much easier by maintaining the full stack trace.

2. Flexible Parameter Handling

Making contentLength and filename optional in RemoteAttachmentCodec is a pragmatic improvement:

content.contentLength?.let {
    parametersMap["contentLength"] = it.toString()
}

content.filename?.let {
    parametersMap["filename"] = it
}

The codec gracefully handles missing fields during decoding as well.

3. Test Infrastructure Improvements

• Moving RemoteAttachmentTest to instrumented tests is appropriate since it uses Android-specific features
• The new TestFetcher class provides a clean way to test remote attachments locally:

class TestFetcher(val filesDir: File) : Fetcher {
    override fun fetch(url: URL): ByteArray {
        return File(filesDir, url.toString().replace("https://", "")).readBytes()
    }
}

4. Coroutine Best Practices

The addition of withContext(Dispatchers.IO) to all suspension functions in Dm.kt and Group.kt is excellent for proper thread management.

🔍 Code Quality & Kotlin/Android Best Practices

✅ Good Practices

• Proper use of Kotlin's null safety features with ?.let
• Exception chaining maintains debugging context
• Clear separation of test utilities in TestHelpers.kt
• Appropriate use of @Before annotation in test setup
• Consistent use of withContext(Dispatchers.IO) for IO operations

⚠️ Areas for Improvement

1. Consider using require or check for validation instead of throwing exceptions directly in some cases
2. The mutable var fields in RemoteAttachment (lines 33-34) could potentially cause thread safety issues in concurrent scenarios

🔒 Security Considerations

✅ Security Strengths

1. HTTPS enforcement: The code correctly validates HTTPS scheme (line 119 in RemoteAttachmentCodec.kt)
2. Content integrity: Content digest validation ensures attachment hasn't been tampered with
3. Secure random generation: Uses SecureRandom for key generation

⚠️ Security Recommendations

1. Input validation: Consider adding URL format validation to prevent malformed URLs
2. Resource limits: Consider adding a maximum content length check to prevent DoS attacks
3. Path traversal: The TestFetcher could potentially be vulnerable to path traversal if used with untrusted input (though it's test-only code)

🎯 Performance Considerations

No performance regressions identified:

• The changes mainly affect error handling paths which are not performance-critical
• Optional field handling adds minimal overhead
• Test utilities are appropriately scoped to test environment only
• The addition of withContext(Dispatchers.IO) ensures IO operations don't block the main thread

🧪 Test Coverage

✅ Good Coverage

• Tests for HTTPS enforcement (testCannotUseNonHTTPSURL)
• Tests for content digest validation (testEnsuresContentDigestMatches)
• Tests for basic codec functionality (testCanUseRemoteAttachmentCodec)
• Tests for encryption/decryption (testEncryptedContentShouldBeDecryptable)

📝 Suggestions

1. Consider adding tests for the new error messages in encodeContent
2. Add edge case tests for extremely large content lengths
3. Consider adding tests for concurrent access scenarios

🎨 Null Safety & Coroutine Usage

✅ Null Safety

• Proper use of nullable types and safe calls throughout
• No obvious NPE risks identified
• Appropriate use of Kotlin's ?.let pattern

✅ Coroutine Usage

• Excellent addition of withContext(Dispatchers.IO) to all suspend functions
• Proper use of runBlocking in tests for coroutine contexts
• No blocking calls in suspend functions

📋 Recommendations

1. Remove or justify the redundant assignments in RemoteAttachmentTest.kt (lines 76-77)
2. Document the breaking change to RemoteAttachment.from() method
3. Consider adding input validation for URLs and content lengths
4. Consider making RemoteAttachment fields immutable if possible to improve thread safety
5. Add deprecation notices where appropriate for the API changes

Additional Notes

The extensive changes to Dm.kt and Group.kt that add proper coroutine context switching and deprecation warnings are excellent improvements to the SDK's architecture. These changes ensure proper thread management for IO operations and provide a clear migration path for SDK users.

Overall, this PR improves the codebase's error handling and makes the remote attachment system more flexible. The identified issues are minor and can be easily addressed.