Declarative Apply: grow, create, and shrink partitions; remove Run

[eriknordmark]

Reconcile a GPT disk to a desired set of partitions in a single, idempotent,
restart-safe pass. Apply grows partitions, creates absent ones (with an
empty filesystem), and optionally shrinks one to free space.

This began as "create a partition as part of a resize" (#22) and, following
review, folds the remaining Run capabilities into Apply and removes Run,
so there is a single entry point that can do it all.

Behavior

• desired is monotonically non-destructive: absent ⇒ create, smaller than its
  size ⇒ grow (relocate + copy, partition number preserved), already ≥ size ⇒
  no-op (never shrunk).
• Matching: a PartitionSpec locates an existing partition to grow by
  Match (name/label/uuid); without Match it is keyed by GUID, and an absent
  GUID is created (the GUID becomes the created partition's identity). A matched
  partition's type/label are asserted so it never operates on an unexpected disk.
• Shrink is the only reducing operation and is optional: to an explicit
  size, or — when the size is 0 — shrink-to-fit (only as much as the grows and
  creates need). When nil, nothing can be shrunk by accident.
• Disk discovery: with no disk given, the disk is discovered from the
  identifiers; by-name (e.g. sda3) identifiers are resolved to UUIDs via sysfs.
• A created partition's GPT entry is published only by the single final table
  write, so "entry present ⇒ filesystem present" and an interrupted create
  converges on re-run.

Go API

Apply(disk, desired []PartitionSpec, shrink *ShrinkSpec, fixErrors, dryRun) —
the signature is unchanged, so existing callers (e.g. EVE's storage-resizer)
need no code change. PartitionSpec gains an optional Match PartitionIdentifier;
ShrinkSpec.Size == 0 requests shrink-to-fit. Run, PartitionChange, and
NewPartitionChange are removed.

CLI

The resize/apply subcommands collapse into a single bare command:

# before
resizer resize --grow-partition name:sda1:20G --shrink-partition name:sda3 /dev/sda
# after
resizer --partition match=name:sda1,minsize=20G --shrink name:sda3 /dev/sda

--partition takes guid= or match=<id> plus minsize= (and optional
label=,type=,index=,fs=); --shrink takes identifier:value[:size] (size
optional for to-fit); the disk is an optional positional argument. --preserve-numbers
is dropped — partition numbers are always preserved.

Tests

Suites ported from Run to Apply, plus new coverage for by-name/label
matching, shrink-to-fit, disk discovery, and name resolution. The full
end-to-end suite, the opt-in SIGKILL chaos soak, and golangci-lint all pass.

Commits

Two commits: the declarative Apply feature (grow/create/shrink, remove Run,
single bare resizer CLI, tests + README) and a separate go-diskfs re-pin to
current master.

[eriknordmark]

@deitch ready for review. Note the open question in the description: this moves the existing grow/shrink CLI under a resizer resize subcommand to make room for the new resizer apply (create + declarative reconcile) — the Go API stays additive (Run unchanged, Apply new). Worth your take on whether to keep both or standardize on apply. Thanks!

[deitch]

It is a good open question. I do think Apply is better than Run (and the CLI equivalents), and as you said, it is almost a superset. Keeping both also can create confusion. On the other hand, we lose the "by name/label" options and the shrink capabilities. Should we move those into Apply and then a single one can do it all?

[eriknordmark]

@deitch done — folded Run's remaining capabilities into Apply and removed Run, so there's a single entry point that does it all:

• By name/label/uuid matching: a PartitionSpec can set Match (name/label/uuid) to locate an existing partition to grow; without it the spec is keyed by GUID (match-or-create, as before). The shrink already took an identifier.
• Shrink-to-fit: ShrinkSpec.Size == 0 shrinks only as much as the grows/creates need (the old auto-fit); a positive size still shrinks to exactly that.
• Auto-discovery + by-name: Apply with an empty disk discovers it from the identifiers, and by-name (sda3) identifiers resolve to UUIDs via sysfs.

Apply's Go signature is unchanged, so existing callers (EVE's storage-resizer) need no code change. On the CLI the resize/apply subcommands collapse into a single bare resizer [disk] (--partition gains match=; --shrink's size is optional for to-fit).

Tests were ported from Run to Apply, with new coverage for matching, shrink-to-fit, discovery, and name resolution; the full suite, the SIGKILL chaos soak, and golangci-lint are all green.

One thing I dropped: the --preserve-numbers toggle — Apply always preserves partition numbers (the safe default the consumer relies on). Happy to re-add it as an option if you'd prefer.

[eriknordmark]

@deitch follow-up now that this is polished for review: restructured into two commits — the declarative Apply feature (grow/create/shrink, Run removed, single bare resizer CLI, tests + README) and a separate go-diskfs re-pin to master — rebased onto main. Full end-to-end suite, the SIGKILL chaos soak, and golangci-lint v2.6 all pass, and EVE's storage-resizer compiles unchanged against the new API. Retitled accordingly; ready for your review.