copy: Add text-based progress output for non-TTY environments (skopeo#658)

[oshe-gi]

Relates-to: containers/skopeo#658

Problem

When copying images in non-TTY environments (CI/CD pipelines, redirected output, piped commands), the visual mpb progress bars are discarded via io.Discard, leaving users with no visibility into transfer progress. This makes it difficult to:

• Detect stalled transfers in CI/CD pipelines
• Monitor progress of long-running copies
• Distinguish between a slow transfer and a hung process

Currently, only a single "Copying blob..." message is printed via printCopyInfo() when non-TTY, with no subsequent progress updates.

Solution

This change automatically enables text-based progress output for non-TTY environments by leveraging the existing Options.Progress channel mechanism.

Design decisions

1. Automatic for non-TTY: When output is not a TTY and the caller hasn't already provided a buffered Progress channel, we automatically set up text-based progress output. No opt-in flag needed.

2. Aggregate progress instead of per-blob: Rather than printing a line for each blob (which would be verbose for multi-layer images), we track total bytes across all blobs and print a single aggregate progress line. This keeps CI logs clean while still providing visibility.

3. Reuse existing Progress channel: The progressReader in blob.go already sends ProgressEventNewArtifact, ProgressEventRead, and ProgressEventDone events. We simply consume these events and format them as text output.

4. Buffered channel: We create a buffered channel to prevent blocking senders during parallel blob downloads. Callers who need custom consumption should provide a properly buffered channel.

5. Sensible interval defaults: If ProgressInterval is not set, we default to 500ms. If the caller sets a larger interval, we respect that. The interval is clamped to a minimum of 500ms to prevent log spam.

Output example

Progress: 13.1 MiB / 52.3 MiB
Progress: 26.2 MiB / 52.3 MiB
Progress: 52.3 MiB / 52.3 MiB

Files changed

• copy/progress_nontty.go - New file with nonTTYProgressWriter and setupNonTTYProgressWriter
• copy/progress_nontty_test.go - Table-driven tests for writer and setup logic
• copy/copy.go - Call setupNonTTYProgressWriter in non-TTY mode

Testing

Build and unit tests:

go build ./copy/...  # ✓
go test ./copy/...   # ✓ all existing tests pass

Manual testing with skopeo (pipe to cat to force non-TTY):

go run ./cmd/skopeo \
  --policy ./default-policy.json \
  --override-os linux \
  --override-arch amd64 \
  copy \
  docker://docker.io/library/alpine:latest \
  dir:/tmp/alpine-copy 2>&1 | cat

Signed-off-by: Oleksandr Shestopal ar.shestopal@gmail.com

[github-actions]

This repository has been migrated to https://github.com/containers/container-libs. Please open your PR there.