main: fold the two infohash-dedup paths into one

dedupeTorrents and markInfoHash were the same map-of-infohash logic, one
batch and one incremental, with the same hazard explained in prose four
times. Both now go through a single seenInfoHash helper.

main.go

@@ -187,13 +187,27 @@ func (f *failedDownload) Stat() download.Stat {
 	return download.Stat{Name: f.name, Status: download.Errored, Total: -1}
 }
 
-// dedupeTorrents finds torrent/magnet sources that name the same torrent as an
-// earlier source (same infohash), returning each duplicate URI mapped to the
-// earlier source it repeats. HTTP(S) URLs and any source whose infohash can't be
-// read without a network fetch are never duplicates here. The shared client keys
-// torrents by infohash and does not refcount, so build() turns each duplicate
-// into a failed download (duplicate infohash) rather than letting two jobs
-// share — and Drop — one torrent.
+// seenInfoHash records source under its torrent infohash in seen and reports
+// whether that infohash was already recorded, returning the source that first
+// claimed it. The shared client keys torrents by infohash and does not refcount,
+// so two sources naming one torrent would have the first to finish Drop it out
+// from under the other; callers fail the duplicate instead. A source whose
+// infohash can't be known without a network fetch (an HTTP URL), or that is
+// v2-only or unreadable, is never a duplicate: it isn't recorded, so it runs.
+func seenInfoHash(seen map[metainfo.Hash]string, source string, isFile bool) (first string, dup bool) {
+	h, ok := bt.SourceInfoHash(source, isFile)
+	if !ok {
+		return "", false
+	}
+	if first, dup = seen[h]; dup {
+		return first, true
+	}
+	seen[h] = source
+	return "", false
+}
+
+// dedupeTorrents finds torrent/magnet sources that repeat an earlier source's
+// infohash, returning each duplicate URI mapped to the source it repeats.
 func dedupeTorrents(uris []string, follow bool) map[string]string {
 	seen := map[metainfo.Hash]string{}
 	dup := map[string]string{}
@@ -202,37 +216,13 @@ func dedupeTorrents(uris []string, follow bool) map[string]string {
 		if k != kindMagnet && k != kindTorrent {
 			continue
 		}
-		h, ok := bt.SourceInfoHash(u, k == kindTorrent)
-		if !ok {
-			continue
-		}
-		if first, isDup := seen[h]; isDup {
+		if first, isDup := seenInfoHash(seen, u, k == kindTorrent); isDup {
 			dup[u] = first
-		} else {
-			seen[h] = u
 		}
 	}
 	return dup
 }
 
-// markInfoHash records the v1 infohash of the .torrent at path in seen and
-// reports whether it repeats one already seen, returning the path first recorded
-// under that infohash. Callers fail the duplicate rather than let two jobs share
-// one torrent on the client (see bt.SourceInfoHash for why that aliasing is
-// unsafe). A v2-only or unreadable infohash is never a duplicate — the dedup
-// can't see it, so it runs.
-func markInfoHash(seen map[metainfo.Hash]string, path string) (first string, dup bool) {
-	h, ok := bt.SourceInfoHash(path, true)
-	if !ok {
-		return "", false
-	}
-	if first, dup = seen[h]; dup {
-		return first, true
-	}
-	seen[h] = path
-	return "", false
-}
-
 func build(opts *cli.Options, targets []target) ([]job, *torrent.Client, error) {
 	flat := allURIs(targets)
 	// Two sources naming the same torrent (a magnet and its own .torrent, say)
@@ -383,11 +373,9 @@ func followUps(jobs []job, client *torrent.Client, bo bt.Options) []job {
 		if _, err := bt.Files(path); err != nil {
 			continue // downloaded file is not a valid torrent
 		}
-		// Two fetched .torrent files that name the same torrent would share one
-		// torrent on the refcount-less client: the first to finish Drops it out
-		// from under the other, stranding it at 0%. build() dedupes pass-1 sources
-		// but cannot reach a not-yet-fetched HTTP .torrent, so dedupe here too.
-		if first, dup := markInfoHash(seen, path); dup {
+		// build() dedupes pass-1 sources but cannot reach a not-yet-fetched HTTP
+		// .torrent, so dedupe the fetched ones here too against the same hazard.
+		if first, dup := seenInfoHash(seen, path, true); dup {
 			out = append(out, job{source: path, dl: &failedDownload{
 				name: path,
 				err:  fmt.Errorf("%w: same torrent as %s", errDuplicate, first),