- SetZlibProvider now panics on a nil provider rather than storing it
  and deferring the failure to the first pool Get (a confusing nil
  dereference deep in call sites).

- The zlib reader pool's New func panics with context if the provider
  returns an error constructing the seed reader. A conforming provider
  must accept the minimal valid stream used to seed the pool, so an
  error here is a programmer error that should fail loudly.

- ZlibReader and ZlibWriter godoc now spell out the behavioral
  contract go-git relies on: Close must not close the underlying
  writer, Reset after Close is required (packfile.Encoder reuses the
  writer across entries), and Reset on a pool-rented reader reseeds
  it for the next rental.

- The EXTENDING.md klauspost example snippet now imports "io" so the
  copy/paste compiles as written.

- Added a test covering the nil-provider panic.

Assisted-by: Claude Opus 4.7 (1M context) <[email protected]>

- Export the stdlib provider as StdlibZlibProvider (previously the
  unexported stdlibZlibProvider). The zlib_test helper stdlibWrapper
  was a verbatim re-implementation of the unexported type; using the
  exported default deletes the duplication and gives downstream users
  a supported delegation target.

- Handle the zlib.Resetter type assertion in the stdlib provider with
  comma-ok and a descriptive error instead of an implicit panic. The
  klauspost snippet in EXTENDING.md is updated to the same safer
  pattern.

- Prune the method-level godoc on ZlibReader.Reset and ZlibWriter's
  Reset/Flush that restated the method signatures; fold the contract
  into the type-level comment block. Add a note that Reset runs on
  the per-object hot path during packfile scan/encode and should be
  O(1) so custom providers do not regress throughput.

- Move the v6 breaking-change note above the example in EXTENDING.md
  so readers upgrading see it before the tutorial code.

- Add paralleltest-compliant annotations to the zlib tests:
  t.Parallel() on the two that do not touch global state;
  //nolint:paralleltest // modifies global zlib provider state on
  the three that swap the provider (matching the convention used in
  x/plugin/plugin_test.go).

Assisted-by: Claude Opus 4.7 (1M context) <[email protected]>

TestDefaultProviderRoundTripWithStdlib was marked t.Parallel() on the
reasoning that every tracking provider in the suite wraps stdlib, so
the round-trip is stdlib-compatible regardless of what a concurrent
test has installed. That is correctness-by-coincidence: the test name
promises a default-provider check, but under parallelism with another
test swapping the provider the assertion is really about whichever
provider happens to be active.

Swap to an explicit save-and-restore of StdlibZlibProvider and drop
t.Parallel() (with the existing //nolint:paralleltest rationale) so
the test always exercises the intended path and does not become
flaky if a future test installs a non-stdlib-compatible provider.

Assisted-by: Claude Opus 4.7 (1M context) <[email protected]>

Addresses PR feedback from @pjbgf to align zlib pluggability with
the x/plugin registration pattern already used by ObjectSigner and
ConfigLoader. Replaces the bespoke SetZlibProvider / atomic.Pointer
mechanism in utils/sync with plugin.Register(plugin.Zlib(), factory).

The Reader, Writer, and Provider interfaces live in x/plugin/zlib
(the leaf package) and are aliased from x/plugin, which avoids an
import cycle and lets *xzlib.Stdlib satisfy plugin.ZlibProvider
directly without an adapter. utils/sync resolves the provider lazily
via sync.OnceValue on the first pool miss.

Removes SetZlibProvider, StdlibZlibProvider, and activeZlibProvider
from utils/sync. utils/sync.ZlibReader and ZlibWriter remain as type
aliases so callers in packfile/encoder.go and objfile/writer.go need
no changes.

Assisted-by: Claude Opus 4.7 (1M context) <[email protected]>

golangci-lint v2.11.4 (CI-pinned) flags the //nolint:revive directive
on Zlib() as unused, failing validate-lint. Unlike ConfigLoader() and
ObjectSigner() where revive still fires on the unexported key return,
ZlibProvider is a type alias so revive resolves through it and never
complains — leaving the directive unused.

Assisted-by: Claude Opus 4.7 (1M context) <[email protected]>

Name the concrete interface in the type-assertion failure message
(plugin.ZlibReader) rather than the stdlib Resetter shorthand, since
that is what the assertion actually checks against. Also handle
plugin.Register's error by panicking in init — silent _ = could hide
a frozen registry or nil factory misuse, which is hard to debug
post-hoc.

Assisted-by: Claude Opus 4.7 (1M context) <[email protected]>

Signed-off-by: Paulo Gomes <[email protected]>

Signed-off-by: Paulo Gomes <[email protected]>