Handle truncated protobuf responses by bjtitus · Pull Request #4078 · Automattic/pocket-casts-ios

bjtitus · 2026-03-18T03:00:12Z

📘 Part of: Watch + Up Next Sync

There are indications from our Up Next Sentry logging that users are receiving truncated protobuf responses. Theoretically, this could lead to "empty" Up Next responses where the response truncated is exactly between the serverModified date and the list of episodes.

The changes here verify that we received the full response by checking the content-length header and verifying the byte count we received.

Two things limit the scope of these changes:

The detectTruncatedBackgroundSyncDownloads Feature Flag
The Watch App is the only one using BackgroundSyncManager.performBackgroundRefresh

To test

This is difficult to test because Watch ↔ iPhone syncing doesn't work well in the simulators and I cannot figure out how to proxy the watch connection to rewrite the response.

Overall, this can be smoke tested by running the Watch app in Watch mode for a while and making sure new episodes are synced.

Checklist

I have considered if this change warrants user-facing release notes and have added them to CHANGELOG.md if necessary.
I have considered adding unit tests for my changes.
I have updated (or requested that someone edit) the spreadsheet to reflect any new or changed analytics.

Copilot

Pull request overview

Adds detection for potentially truncated background-sync protobuf downloads (primarily Watch background refresh) by comparing the downloaded byte count against the response’s expected content length, with unit tests for the completeness check.

Changes:

Introduce BackgroundSyncManager.isDownloadComplete(receivedBytes:expectedContentLength:) helper for validating download completeness.
Add a feature-flagged check in the URLSession download delegate to drop data when a Content-Length mismatch is detected (and log it).
Add unit tests covering common completeness/mismatch scenarios.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
Modules/Server/Sources/PocketCastsServer/Public/Sync/BackgroundSyncManager.swift	Adds the download completeness helper used by background sync.
Modules/Server/Sources/PocketCastsServer/Public/Sync/BackgroundSyncManager+URLSession.swift	Applies the completeness check after download completion, behind a feature flag, and logs/drops mismatching payloads.
Modules/Server/Tests/PocketCastsServerTests/BackgroundSyncManagerTests.swift	Adds unit tests for the completeness helper across expected/unknown/mismatch cases.

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bjtitus · 2026-03-19T03:30:54Z

Modules/Server/Sources/PocketCastsServer/Public/Sync/BackgroundSyncManager+URLSession.swift

+        if FeatureFlag.detectTruncatedBackgroundSyncDownloads.enabled {
+            let expectedLength = downloadTask.response?.expectedContentLength ?? -1
+            if let receivedData = data, !BackgroundSyncManager.isDownloadComplete(receivedBytes: receivedData.count, expectedContentLength: expectedLength) {
+                FileLog.shared.addMessage("Background sync data truncated for task \(downloadTask.taskDescription ?? "unknown"): received \(receivedData.count) bytes, expected \(expectedLength)")


I don't think this is likely and we have no evidence. In theory, the URLSession background task is downloading an incomplete response (due to disconnect or something). It shouldn't receive a response greater than expected.

Modules/Server/Tests/PocketCastsServerTests/BackgroundSyncManagerTests.swift

Modules/Server/Sources/PocketCastsServer/Public/Sync/BackgroundSyncManager+URLSession.swift

Modules/Server/Sources/PocketCastsServer/Public/Sync/BackgroundSyncManager.swift

NSURLResponseUnknownLength is not available from Swift

Copilot

Pull request overview

Adds detection of potentially truncated protobuf downloads during background sync by comparing received bytes against the HTTP Content-Length, gated behind a feature flag.

Changes:

Introduces detectTruncatedBackgroundSyncDownloads feature flag.
Adds byte-count vs expectedContentLength validation helper in BackgroundSyncManager.
Adds unit tests for the completeness-check helper.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
Modules/Utils/Sources/PocketCastsUtils/Feature Flags/FeatureFlag.swift	Adds a new feature flag and sets its default enabled behavior.
Modules/Server/Sources/PocketCastsServer/Public/Sync/BackgroundSyncManager.swift	Introduces `isDownloadComplete` and an “unknown length” constant used for validation.
Modules/Server/Sources/PocketCastsServer/Public/Sync/BackgroundSyncManager+URLSession.swift	Applies the truncation detection during background sync processing and drops data when mismatched.
Modules/Server/Tests/PocketCastsServerTests/BackgroundSyncManagerTests.swift	Adds unit tests for the byte-count validation helper.

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Modules/Utils/Sources/PocketCastsUtils/Feature Flags/FeatureFlag.swift

            true
        case .cleanUpTmpFiles:
            true
+        case .detectTruncatedBackgroundSyncDownloads:
+            true


SergioEstevao · 2026-03-19T14:31:29Z

Modules/Server/Sources/PocketCastsServer/Public/Sync/BackgroundSyncManager.swift

+    /// Returns `false` if the received byte count doesn't match the expected Content-Length,
+    /// indicating the download was truncated or corrupt.
+    /// Value returned by `URLResponse.expectedContentLength` when the length is unknown.
+    static let unknownContentLength: Int64 = -1


This looks like a good suggestion.

Unfortunately, NSURLResponseUnknownLength was not available in Swift. It's an Objective-C macro instead of a constant. This commit message mentions it.

Modules/Server/Sources/PocketCastsServer/Public/Sync/BackgroundSyncManager+URLSession.swift

+        // Verify the download completed fully by comparing received bytes to Content-Length.
+        // A truncated download can produce valid-but-incomplete protobuf that silently drops fields.
+        if FeatureFlag.detectTruncatedBackgroundSyncDownloads.enabled {
+            let expectedLength = downloadTask.response?.expectedContentLength ?? BackgroundSyncManager.unknownContentLength
+            if let receivedData = data, !BackgroundSyncManager.isDownloadComplete(receivedBytes: receivedData.count, expectedContentLength: expectedLength) {
+                FileLog.shared.addMessage("Background sync data truncated for task \(downloadTask.taskDescription ?? "unknown"): received \(receivedData.count) bytes, expected \(expectedLength)")
+                data = nil
+            }
+        }


SergioEstevao

Looking good, I tested this on my watch and phone and all worked correctly.

There is very small nitpick comment regarding the unknow size constant that I think it's a good suggestion, but up to you.

SergioEstevao · 2026-03-19T14:31:29Z

Modules/Server/Sources/PocketCastsServer/Public/Sync/BackgroundSyncManager.swift

+    /// Returns `false` if the received byte count doesn't match the expected Content-Length,
+    /// indicating the download was truncated or corrupt.
+    /// Value returned by `URLResponse.expectedContentLength` when the length is unknown.
+    static let unknownContentLength: Int64 = -1


This looks like a good suggestion.

bjtitus added 3 commits March 17, 2026 18:53

Add feature flag for truncated background sync detection

3cb3996

Bail when full response is not received in background sync

6deee56

Add unit tests for background sync truncated response

6f05b0e

bjtitus added the [Project] Watch + Up Next Sync Improvements for Watch + Up Next Sync issues label Mar 18, 2026

Copilot AI review requested due to automatic review settings March 18, 2026 03:00

Copilot started reviewing on behalf of bjtitus March 18, 2026 03:00 View session

Copilot AI reviewed Mar 18, 2026

View reviewed changes

Add constant for unknown content length

f1fdcf2

NSURLResponseUnknownLength is not available from Swift

bjtitus marked this pull request as ready for review March 19, 2026 03:41

bjtitus requested a review from a team as a code owner March 19, 2026 03:41

bjtitus requested review from SergioEstevao and Copilot and removed request for a team March 19, 2026 03:41

Copilot AI reviewed Mar 19, 2026

View reviewed changes

bjtitus added this to the 8.9 milestone Mar 19, 2026

SergioEstevao approved these changes Mar 19, 2026

View reviewed changes

bjtitus merged commit a8fab5c into trunk Mar 19, 2026
10 of 12 checks passed

bjtitus deleted the bjtitus/handle-truncated-protobuf-responses branch March 19, 2026 18:45

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Handle truncated protobuf responses#4078

Handle truncated protobuf responses#4078
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bjtitus/handle-truncated-protobuf-responses

bjtitus commented Mar 18, 2026 •

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bjtitus commented Mar 18, 2026 •

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