first commit

httpdl/control.go

@@ -0,0 +1,79 @@
+package httpdl
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"sync/atomic"
+)
+
+// control is the on-disk resume state, a small JSON sidecar next to the output
+// file (<out>.got). It is a Go-simple binary-free control file: enough to skip
+// finished segments and resume partial ones, plus validators (Total +
+// ETag/LastModified) so we never trust a stale file.
+type control struct {
+	URL          string     `json:"url"`
+	Total        int64      `json:"total"`
+	ETag         string     `json:"etag,omitempty"`
+	LastModified string     `json:"last_modified,omitempty"`
+	Segs         []segState `json:"segs"`
+}
+
+type segState struct {
+	Start   int64 `json:"start"`
+	End     int64 `json:"end"`
+	Written int64 `json:"written"`
+}
+
+func controlPath(out string) string { return out + ".got" }
+
+// snapshot builds a control record from the live segments.
+func snapshot(url string, total int64, etag, lastmod string, segs []seg) control {
+	c := control{URL: url, Total: total, ETag: etag, LastModified: lastmod, Segs: make([]segState, len(segs))}
+	for i := range segs {
+		c.Segs[i] = segState{segs[i].start, segs[i].end, atomic.LoadInt64(&segs[i].written)}
+	}
+	return c
+}
+
+// save writes the control file atomically (temp + rename).
+func (c control) save(out string) error {
+	data, err := json.Marshal(c)
+	if err != nil {
+		return err
+	}
+	if err := os.MkdirAll(filepath.Dir(out), 0o755); err != nil {
+		return err
+	}
+	tmp := controlPath(out) + ".tmp"
+	if err := os.WriteFile(tmp, data, 0o644); err != nil {
+		return err
+	}
+	return os.Rename(tmp, controlPath(out))
+}
+
+// loadControl reads a control file if it is present and still matches the
+// download (same URL, length and validator). It returns nil when there is
+// nothing trustworthy to resume from.
+func loadControl(out, url string, total int64, etag, lastmod string) *control {
+	data, err := os.ReadFile(controlPath(out))
+	if err != nil {
+		return nil
+	}
+	var c control
+	if json.Unmarshal(data, &c) != nil {
+		return nil
+	}
+	if c.URL != url || c.Total != total {
+		return nil
+	}
+	if (etag != "" || c.ETag != "") && c.ETag != etag {
+		return nil
+	}
+	if (lastmod != "" || c.LastModified != "") && c.LastModified != lastmod {
+		return nil
+	}
+	return &c
+}
+
+func removeControl(out string) { os.Remove(controlPath(out)) }