MultipartDeserialize: guard against nil part on malformed body

[SAY-5]

Summary

mime/multipart.Reader.NextPart can return a nil *Part alongside a non-EOF error when the body declares multipart/related but is not valid MIME multipart data — plain text, JSON, empty body, truncated part, or a mismatched boundary. The previous MultipartDeserialize loop treated any non-EOF error from NextPart as a silent "break and fall through", then dereferenced part.Header.Get("Content-Type") on line 611 and panicked.

Every NF using this helper (SMF on POST /nsmf-pdusession/v1/sm-contexts, UDM, AUSF, ...) returned HTTP 500 on a single crafted request, and Gin's recovery middleware was the only thing preventing the whole process from dying.

Fix

• Rewrite the NextPart branch so EOF breaks cleanly, any other error is wrapped and returned, and an unexpected nil part is rejected explicitly.
• Fix the inverted check on part.Read: the old code returned when err == nil, which dropped every successfully read part; replace with err != nil && err != io.EOF so only real failures abort the loop.
• Add TestMultipartDeserialize_MalformedBody covering plain-text / empty / JSON bodies. Pre-fix the plain-text case panicked on part.Header.Get; post-fix all three return an error with no panic.

Test

go test ./... -count=1   # all packages pass
go vet ./...             # clean
gofmt                    # clean

Fixes free5gc/free5gc#1026.

Signed-off-by: SAY-5 SAY-5@users.noreply.github.com

[Copilot]

Pull request overview

Hardens multipart/related deserialization to avoid panics and correctly surface errors when the request body is malformed (e.g., declared multipart but actually plain text/JSON/empty), addressing free5gc/free5gc#1026.

Changes:

• Update MultipartDeserialize to handle NextPart() EOF vs non-EOF errors correctly and explicitly reject unexpected nil parts.
• Fix inverted part.Read error handling so successful reads don’t abort the loop.
• Add a regression test ensuring malformed bodies return an error without panicking.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
client.go	Improves multipart parsing error handling to prevent nil deref panics and propagate malformed-body errors.
client_test.go	Adds coverage for malformed multipart bodies to ensure no panic and an error is returned.

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[Copilot]

part.Read(multipartBody) reads only once into a fixed 1400-byte buffer. If a multipart part exceeds 1400 bytes, Read can return n==len(buf) with err==nil, and the code will proceed with truncated data (e.g., incomplete JSON or binary). Consider reading the full part (e.g., io.ReadAll / io.ReadAll(io.LimitReader(...)) or a loop until io.EOF) so valid requests aren’t corrupted or rejected.

[d11nn]

@Alonza0314 LGTM