perf: pre-size fileSource reads (close the InitFrom io.ReadAll churn)

[reflog]

Follow-up to #9.

Gap

#9 made webSource (HTTP Content-Length) and tarGzSource (archive entry size) size-aware, so readAll pre-sizes those reads. But fileSource.Fetch still returned a bare *os.File, which exposes no size — so readAll's knownSize() returns false for file reads and falls back to io.ReadAll's doubling growth.

getlantern/geo loads its cached ~75MB MaxMind mmdb from disk via Runner.InitFrom(FromFile(path)) at startup, so that read still churns through the 75→60→48→38→30→24MB realloc staircase on every boot.

Confirmed via alloc_space pprof on a production lantern-box v0.0.91 box (already carrying #9):

io.ReadAll  373.82MB  (under)
  keepcurrent.readAll
  keepcurrent.(*Runner).syncOnce
  keepcurrent.(*Runner).InitFrom
  geo.FromWeb

Live/inuse heap was already clean (the web-sync path from #9 pre-sizes correctly); this is the remaining transient spike, and it lands at startup on the smallest (1GB) boxes.

Fix

fileSource.Fetch now returns sizedReadCloser{f, fi.Size()} when there's no preprocessor, so the read is a single pre-sized allocation. The preprocessor path keeps the io.ReadAll fallback (a preprocessor can change the byte count, so the file size isn't the payload size). Also closes the file on the early-return (ErrUnmodified) and error paths, which previously leaked the fd.

go build/vet/test green; adds TestFileSourceIsSizeAware.

Rollout

Downstream bumps follow (http-proxy-lantern, lantern-box) once this merges.

[Copilot]

Pull request overview

This PR is a follow-up performance improvement to reduce transient allocation churn when initializing/syncing from disk-backed sources by making FromFile(...).Fetch(...) size-aware (so readAll can pre-size) and tightening file-handle cleanup paths.

Changes:

• Update fileSource.Fetch to surface file size via sizedReadCloser when no preprocessor is used.
• Close the opened file on early-return (ErrUnmodified) and error paths.
• Add a unit test asserting FromFile produces a size-aware reader and that readAll performs a single pre-sized allocation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
source.go	Makes FromFile fetches size-aware and adds explicit file closes on error / unmodified paths.
read_test.go	Adds a test to validate file sources report size and that readAll pre-sizes the buffer.

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