[ffumero2003]

Problem

When a large-file upload fails permanently, io.Copy(w, r) into a *b2.Writer returned context canceled instead of the upload error. io.Copy uses Writer.ReadFrom, which copies under the Writer's context. A part failure calls Writer.setErr, which records the error and cancels that context; copyContext then saw ctx.Done() and returned ctx.Err(), masking the real failure. Close() returned the right error, but most callers check io.Copy's.

Root cause

The Writer couldn't tell "the Writer cancelled itself because an upload failed" from "the caller cancelled". A first fix that compared error types was wrong: base.makeNetRequest flattens most transport errors (a dial or I/O error caused by the caller's cancellation) into a plain b2err, so a caller's cancellation could come back as an opaque transport error.

Fix

• The Writer's context is created with context.WithCancelCause (b2/b2.go).
• setErr cancels it with the first recorded error, wrapped in an unexported writerCause tag.
• ReadFrom replaces a context error only when context.Cause unwraps to a writerCause, and returns the original upload error (so errors.Is / errors.As match it, as they do for Close). Anything else, including a caller's own cause from WithCancelCause / WithTimeoutCause, leaves context.Canceled / context.DeadlineExceeded unchanged.
• ReadFrom godoc states both outcomes for every path (non-seekable, Resume, streaming).
• Changelog: Fixed.

Verification

Tests in b2/writer_readfrom_error_test.go and the tightened TestWriterReturnsError (errors.Is on the injected error) cover:

• upload failure on the non-seekable, Resume and streaming paths;
• caller cancellation and deadline with a flattened transport error recorded;
• caller cancellation and deadline with a caller-supplied cause.

The deadline case starts its timer only after the controlled part upload is reached, so it doesn't race setup.

Break	Failing output
setErr cancels without the tag	io.Copy error = context canceled, want the permanent upload-part failure
ReadFrom substitutes any cause	io.Copy error = caller-provided cancellation cause, want caller's context canceled (and deadline exceeded)
ReadFrom always returns the recorded error	io.Copy error = flattened transport error, want caller's context canceled (and deadline exceeded)
ReadFrom never substitutes	io.Copy error = context canceled, want the permanent upload-part failure

With no B2 credentials set: go test -race -count=50 -run ReadFrom ./b2 passes, as do #236's cancellation tests at -race -count=30; go test -race -count=5 -shuffle=on ./... and go vet ./... pass. The SDK panel reviewed two rounds; its findings (the caller-cause leak, godoc identity, deadline-test timing) are addressed.

Also: fixes the b2 test build on main

#247 removed the objectForDeleteOptionTest helper from b2/b2_test.go, while #249's TestObjectDeleteRejectsFolderStatus (b2/core_edges_test.go) still used it, so the b2 tests stopped compiling on main once both merged. Commit 9d869bb inlines the same two setup lines TestObjectDeleteAllowsCopyStatus uses. Test-only; merging this PR makes main green again.

Merge order

#236 has merged; this PR is now based on main and synced with it.

Closes #245