feat(dag): add --local-only to dag export and import

[ChayanDass]

Add --local-only to dag export and dag import

Summary

Adds support for partial CAR export and import: export only blocks that exist locally (skipping missing blocks), and import such partial CARs without pinning roots.

Changes

dag export

• --local-only (bool): When set, only blocks present in the local blockstore are exported; missing blocks are skipped (partial CAR). User must pass --offline explicitly for a local-only DAG walk; the command does not set offline automatically.

dag import

• --pin-roots (bool, default true): Pin optional roots listed in the CAR headers after importing.
• For partial CARs, user must pass --pin-roots=false and --local-only (not both with pin-roots true).

Usage

Export a partial CAR (local blocks only; user must pass both flags):

ipfs dag export --local-only --offline <root> > partial.car

Import that partial CAR without pinning roots:

ipfs dag import --pin-roots=false partial.car
# or
ipfs dag import --local-only --pin-roots=false partial.car

References

• Related: ipfs dag export --ignore-missing-blocks #10826, ipfs-webui#2380

[lidel]

Thank you @ChayanDass, quick first pass review below.

Also, we need to wait for go-car release with your fix before this can be merged.

[lidel]

Test guidance

@ChayanDass please add tests in test/cli, e.g. in dag_test.go. The existing tests there (TestDag, TestDagImportFastProvide) and the harness helpers IPFSDagExport/IPFSDagImport show the patterns to follow.

How to create partially local DAG

Perhaps useful: to test partial DAG export/import you need multi-block test data where some blocks are missing locally. The setup is:

1. ipfs add enough data (e.g. 300KB with --raw-leaves) to produce a root with multiple child blocks
2. Use ipfs refs <root> to list child block CIDs
3. Use ipfs block rm <child-cid> to remove one or more blocks from the local blockstore
4. Now the DAG is partially local -- dag export --offline should fail, but dag export --local-only --offline should succeed and produce a partial CAR

Specific tests to add

• TestDagExportLocalOnly -- set up a partial DAG (add content, remove a child block). Confirm dag export --offline fails. Confirm dag export --local-only --offline succeeds and the resulting CAR has fewer blocks than the full DAG (import on a second node with --pin-roots=false --stats to compare block counts).

• TestDagExportLocalOnlyRequiresOffline -- verify that --local-only without --offline either errors or behaves as documented. Currently there is no enforcement, and without --offline the node may fetch missing blocks from the network, making --local-only misleading.

• TestDagImportPartialCAR -- export a partial CAR with --local-only --offline, import it on a fresh node with --pin-roots=false, verify success.

• TestDagImportLocalOnlyPinRootsConflict -- verify dag import --local-only --pin-roots errors with the expected message.