[stanhu]

Implements the approach @vangent proposed in #3818, as an alternative to the URL-parameter PR #3819 (now closed).

What

Bucket.SetDefaultReaderOptions(ReaderOptions) error lets an application set bucket-wide default ReaderOptions once, instead of passing them on every Download/NewReader/NewRangeReader call. When a read method is called with nil opts, a copy of the default is used in place of an empty ReaderOptions.

b, _ := blob.OpenBucket(ctx, "s3://mybucket?region=us-west-2")
_ = b.SetDefaultReaderOptions(blob.ReaderOptions{AllowParallelDownload: true, BufferSize: 16 << 20, MaxConcurrency: 8})
// Now Download with nil opts uses parallel download by default.
_ = b.Download(ctx, key, f, nil)

How

• Adds a defaultReaderOptions *ReaderOptions field to Bucket (guarded by the existing b.mu).
• SetDefaultReaderOptions stores a copy. Only performance-related fields may be defaulted; setting BeforeRead returns an InvalidArgument error, since it's a per-call concern.
• A small readerOptionsOrDefault helper substitutes the default (a copy) when opts == nil, applied in Download and newRangeReader (the shared helper behind NewReader/NewRangeReader/ReadAll).

This lives entirely in the portable blob package and needs no driver changes — it just substitutes the ReaderOptions before the existing per-call plumbing (the AllowParallelDownload gate and the BufferSize/MaxConcurrency fields drivers already honor).

Semantics

A nil opts gets a copy of the default; a non-nil opts is used as-is (the default is not merged in), which avoids any unset-vs-zero ambiguity. Explicit per-call options therefore always win.

Tests

TestSetDefaultReaderOptions covers: the BeforeRead allowlist error; a default enabling the parallel path for a nil-opts Download; part-size/concurrency defaults flowing to the driver; fallback to the reader with no default; and that an explicit opts does not inherit the default. go vet/gofmt clean; full blob + s3blob/gcsblob/azureblob suites pass (no driver changes).

Follow-up

SetDefaultWriterOptions (with a smaller allowlist) is a natural follow-up.

[vangent]

Nit: it should be OK to set b.defaultReaderOptions = &optshere, right?opts` is already a copy.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.14%. Comparing base (18e4aa5) to head (8b6b2cc).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3820      +/-   ##
==========================================
- Coverage   80.15%   80.14%   -0.01%     
==========================================
  Files         104      104              
  Lines       12454    12464      +10     
==========================================
+ Hits         9982     9989       +7     
- Misses       2471     2474       +3     
  Partials        1        1              

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