[bitsandfoxes]

Problem

Empty attachments are supposed to be skipped, but an attachment backed by a non-seekable stream that turns out empty gets sent with length: 0, and Relay stores an empty file on the event.

• Envelope.AddEnvelopeItemFromAttachment only skips an attachment when stream.TryGetLength() returns 0.
• A non-seekable stream reports null (unknown length), so the item is kept.
• Its payload is only buffered when the envelope gets serialized, and the header ends up with length: 0.

sentry-unity hits this regularly: its screenshot attachment returns a non-seekable stream so that Envelope.FromEvent doesn't block waiting for the end-of-frame capture. When the screenshot is discarded (SetBeforeSendScreenshot returns null), times out or fails, the event shows an empty screenshot.jpg. See getsentry/sentry-unity#2835.

Fix

Serialization is the first point where the payload size is known, so the check happens there:

• Envelope.Serialize / SerializeAsync skip an attachment item when it came from an attachment stream of unknown length and its buffered payload is empty. The item header and its trailing newline are both left out.
• A warning gets logged, worded like the existing checks at envelope creation: Did not add '<file>' to envelope because the attachment was empty.

What stays the same

• Explicitly empty attachments: items with a known length: 0 are kept. The envelope spec's "Envelope with 2 empty attachments" example, which EnvelopeTests mirrors, serializes as before.
• Public API: EnvelopeItem.Serialize / SerializeAsync keep their signatures and always write. They delegate to new internal overloads that return whether the item was written, so Envelope can skip the trailing newline as well.
• Offline caching: the empty item is dropped before the envelope is written to disk. Items read back from the cache are never dropped. Deserialization strips length, so the check keys on the payload type (BufferedStreamSerializable) rather than the header, and a deliberately empty attachment survives the round trip, as it does on main.
• Client reports: none are recorded for the drop, matching the existing drops at envelope creation.

Edge case

An envelope whose only item gets dropped, e.g. an empty non-seekable attachment sent via Hub.CaptureAttachment, goes out as a header-only envelope. Relay accepts it and stores nothing: handle_managed_envelope returns early when envelope.is_empty(). The only cost is the request.

Tests

AttachmentSerializationTests:

• Serialization_EmptyNonSeekableStreamAttachment_DropsAttachment and Serialization_EmptyNonSeekableStreamAttachmentSynchronously_DropsAttachment: attach an empty non-seekable stream through Envelope.FromEvent, serialize async and sync, then deserialize again. Both failed before this change.
• Serialization_EmptyAttachmentOfKnownLengthAfterRoundTrip_KeepsAttachment: round-trips a known-length empty attachment through serialize → deserialize → serialize and checks it's kept.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.15%. Comparing base (ff0355e) to head (dac0cdd).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Sentry/Protocol/Envelopes/EnvelopeItem.cs	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5787      +/-   ##
==========================================
+ Coverage   76.13%   76.15%   +0.02%     
==========================================
  Files         528      528              
  Lines       19579    19590      +11     
  Branches     3829     3835       +6     
==========================================
+ Hits        14906    14919      +13     
+ Misses       3791     3790       -1     
+ Partials      882      881       -1     

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[ric-oliv]

One comment, but it looks good to me!