WIP: Add explicit `--fetch` flag to bst commands

Description

This introduces a new flag --fetch that was mentioned in the mailing list.

Proposed changes

To put simply, this changes the following commands' behaviour so they'd not try to implicitly fetch junctions and elements (offline-first?).

• bst {pull,push,track,checkout} element(s) command

  • Before: Fetches the junction(s) if they are used and not cached yet.
  • After: Don't do junction fetch implicitly. In case one is needed, error out by saying need to fetch junction. --fetch is supplied to fetch_subprojects.

• bst source-bundle element command

  • Before: (Possibly track and) fetch implicitly.
  • After: Now we have 4 combinations in total (--fetch and --track)
    • Only --track: Attempt track elements, error out if there is a new ref (since no --fetch supplied, continue if there is nothing new.
    • Only --fetch: Fetch current refs for elements (including related junctions) - they might be cached already.
    • --fetch and --track: Previous behaviour of the command with --track, track for new refs if possible and fetch them.
    • No flags: error out if the current refs are not cached.

• bst {workspace-open,workspace-reset} element.bst .. We are interested in the cases where no_checkout is NOT used.

  • Before: Error out if junction(s) are required but not cached yet. Fetch for element is always done. Track before fetch can be requested using --track.
  • After: Now we have 4 combinations in total (--fetch and --track)
    • Only --track: Attempt track elements, error out if there is a new ref (since no --fetch supplied, continue if there is nothing new.
    • Only --fetch: Fetch current refs for elements (including related junctions) - they might be cached already.
    • --fetch and --track: Previous behaviour of the command with --track, track for new refs if possible and fetch them.
    • No flags: error out if the current refs are not cached.

You can notice that the original behaviour of workspace commands are almost equivalent to source-bundle, except the latter implicitly fetches junctions where the former complains and terminates.

This merge request, when approved, will close: N/A at the moment.

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added 98 commits

• 7951f7f6...c7ac7e7d - 91 commits from branch master
• 2975e27f - Add explicit fetch flag for 'bst pull' command
• 0d96fbca - Add explicit fetch flag for 'bst push' command
• 366844b8 - Add explicit fetch flag for 'bst track' command
• c21603e3 - Add explicit fetch flag for 'bst checkout' command
• 4807fc66 - Add explicit fetch flag for 'bst source-bundle'
• 5fb2ee60 - Add explicit fetch flag for 'bst workspace-open'
• 618a6cfe - Add explicit fetch flag for 'bst workspace-reset'

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• ca3b745b - fixup! Add explicit fetch flag for 'bst workspace-reset'

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• 83a7c2cf - fixup! fixup! Add explicit fetch flag for 'bst workspace-reset'
• d368a236 - fixup! Add explicit fetch flag for 'bst workspace-reset'

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• d368a236...7f79b9ce - 8 commits from branch master
• 3f15801c - Add explicit fetch flag for 'bst pull' command
• 28f53e33 - Add explicit fetch flag for 'bst push' command
• efff5259 - Add explicit fetch flag for 'bst track' command
• 781017b6 - Add explicit fetch flag for 'bst checkout' command
• 31f47d1f - Add explicit fetch flag for 'bst source-bundle'
• e35d54be - Add explicit fetch flag for 'bst workspace-open'
• 25578cec - Add explicit fetch flag for 'bst workspace-reset'

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• 6cc4bfcb - fixup! Add explicit fetch flag for 'bst workspace-open'

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To put simply, this changes the following commands' behaviour so they'd not try to implicitly fetch junctions and elements (offline-first?).

I think we want to be online-first, and have offline be the exception. We should be able to modify the behavior (through cli and configuration), but the defaults should work best for the common use case.

See https://mail.gnome.org/archives/buildstream-list/2018-October/msg00074.html for a bit more discussion. Maybe we need to expand there about implicit/explicit behavior in general.

added 132 commits

• 6cc4bfcb...abef70fe - 125 commits from branch master
• 92067452 - Add no-fetch flag for 'bst pull' command
• bf827ed2 - Add no-fetch flag for 'bst push' command
• 834dc160 - Add no-fetch flag for 'bst track' command
• e00c5bc1 - Add no-fetch flag for 'bst checkout' command
• a45a40ff - Add no-fetch flag for 'bst source-bundle'
• dacfba35 - Add no-fetch flag for 'bst workspace-open'
• 1c45f727 - Add no-fetch flag for 'bst workspace-reset'

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Thank you @sstriker , I opened a --no-fetch version of the same changes athttps://gitlab.com/BuildStream/buildstream/merge_requests/970 and will close this now.

closed