80 lines
2.2 KiB
Go
80 lines
2.2 KiB
Go
package httpdl
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import (
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"encoding/json"
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"os"
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"path/filepath"
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"sync/atomic"
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)
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// control is the on-disk resume state, a small JSON sidecar next to the output
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// file (<out>.got). It is a Go-simple binary-free control file: enough to skip
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// finished segments and resume partial ones, plus validators (Total +
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// ETag/LastModified) so we never trust a stale file.
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type control struct {
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URL string `json:"url"`
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Total int64 `json:"total"`
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ETag string `json:"etag,omitempty"`
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LastModified string `json:"last_modified,omitempty"`
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Segs []segState `json:"segs"`
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}
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type segState struct {
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Start int64 `json:"start"`
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End int64 `json:"end"`
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Written int64 `json:"written"`
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}
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func controlPath(out string) string { return out + ".got" }
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// snapshot builds a control record from the live segments.
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func snapshot(url string, total int64, etag, lastmod string, segs []seg) control {
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c := control{URL: url, Total: total, ETag: etag, LastModified: lastmod, Segs: make([]segState, len(segs))}
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for i := range segs {
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c.Segs[i] = segState{segs[i].start, segs[i].end, atomic.LoadInt64(&segs[i].written)}
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}
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return c
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}
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// save writes the control file atomically (temp + rename).
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func (c control) save(out string) error {
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data, err := json.Marshal(c)
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if err != nil {
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return err
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}
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if err := os.MkdirAll(filepath.Dir(out), 0o755); err != nil {
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return err
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}
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tmp := controlPath(out) + ".tmp"
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if err := os.WriteFile(tmp, data, 0o644); err != nil {
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return err
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}
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return os.Rename(tmp, controlPath(out))
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}
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// loadControl reads a control file if it is present and still matches the
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// download (same URL, length and validator). It returns nil when there is
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// nothing trustworthy to resume from.
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func loadControl(out, url string, total int64, etag, lastmod string) *control {
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data, err := os.ReadFile(controlPath(out))
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if err != nil {
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return nil
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}
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var c control
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if json.Unmarshal(data, &c) != nil {
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return nil
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}
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if c.URL != url || c.Total != total {
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return nil
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}
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if (etag != "" || c.ETag != "") && c.ETag != etag {
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return nil
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}
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if (lastmod != "" || c.LastModified != "") && c.LastModified != lastmod {
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return nil
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}
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return &c
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}
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func removeControl(out string) { os.Remove(controlPath(out)) }
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