[bitsandfoxes]

An attachment whose content is a non-seekable stream was sent with length: 0 when the stream turned out empty. Relay then stored an empty file on the event.

Envelope.AddEnvelopeItemFromAttachment skips empty attachments only when stream.TryGetLength() returns 0. A non-seekable stream reports null, so the item was kept, and its header got length: 0 once the payload was buffered at serialization. sentry-unity hits this on purpose: its screenshot content returns a non-seekable stream so 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).

The fix acts at serialization, the first point where the payload size is known. Envelope.Serialize/SerializeAsync skip an attachment item, header and trailing newline included, when it was built from an attachment stream of unknown length and its buffered payload is empty. A warning is logged in the same wording as the checks at envelope creation.

• Items with an explicit length: 0 are left alone. The envelope spec's framing example "Envelope with 2 empty attachments", which EnvelopeTests mirrors, keeps serializing as before.
• The public EnvelopeItem.Serialize/SerializeAsync keep their signatures and behaviour (they always write). They delegate to new internal overloads that return whether the item was written, so Envelope can skip the item's trailing newline too. No public API change.
• With offline caching on, the item is dropped before it's written to disk. Items read back from the cache are never dropped. Deserialization strips length, so the check keys on the payload type, not the header, and a deliberately empty attachment survives the round trip as on main.

An envelope whose only item is dropped, e.g. an empty non-seekable attachment sent via Hub.CaptureAttachment, goes out as a header-only envelope. Relay parses that and returns 200 without storing anything (handle_managed_envelope returns early when envelope.is_empty()), so the only cost is the request.

No client report is recorded for the drop, matching the existing drops at creation.

Tests: AttachmentSerializationTests gets a non-seekable empty stream attached through Envelope.FromEvent, serialized sync and async and deserialized again. Both tests failed before the change. A third test round-trips a known-length empty attachment through serialize, deserialize and serialize, and checks that 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 76.16%. Comparing base (ff0355e) to head (3c27ce8).
⚠️ Report is 1 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%   76.16%   +0.02%     
==========================================
  Files         528      528              
  Lines       19579    19590      +11     
  Branches     3829     3835       +6     
==========================================
+ Hits        14906    14920      +14     
+ Misses       3791     3789       -2     
+ Partials      882      881       -1     

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