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| 1 | package web |
| 2 | |
| 3 | // Byte ranges for a download. |
| 4 | // |
| 5 | // A blob is streamed from git, which cannot seek, so a range is served by |
| 6 | // reading and discarding the bytes before it and copying only the requested |
| 7 | // part. The win is what does not cross the network: a player asking for the |
| 8 | // first few KiB of a 100 MiB asset costs one copy of the skipped bytes on the |
| 9 | // server, not 100 MiB of transfer to the client. |
| 10 | |
| 11 | import ( |
| 12 | "strconv" |
| 13 | "strings" |
| 14 | ) |
| 15 | |
| 16 | // byteRange is one satisfiable range of a representation. |
| 17 | type byteRange struct { |
| 18 | start int64 |
| 19 | length int64 |
| 20 | } |
| 21 | |
| 22 | // parseByteRange reads a Range header against a representation of size bytes. |
| 23 | // |
| 24 | // It answers one of three ways: a range to serve (partial), a range that cannot |
| 25 | // be satisfied (unsatisfiable), or neither — no header, an unparsable one, or |
| 26 | // more than one range. The last group means "serve the whole file", which |
| 27 | // RFC 9110 allows; answering multiple ranges with a single full response keeps |
| 28 | // the reader honest rather than silently dropping the ones it did not implement. |
| 29 | func parseByteRange(header string, size int64) (br byteRange, partial, unsatisfiable bool) { |
| 30 | header = strings.TrimSpace(header) |
| 31 | const prefix = "bytes=" |
| 32 | if !strings.HasPrefix(header, prefix) { |
| 33 | return byteRange{}, false, false |
| 34 | } |
| 35 | spec := strings.TrimSpace(strings.TrimPrefix(header, prefix)) |
| 36 | if spec == "" || strings.ContainsRune(spec, ',') { |
| 37 | return byteRange{}, false, false |
| 38 | } |
| 39 | first, last, ok := strings.Cut(spec, "-") |
| 40 | if !ok { |
| 41 | return byteRange{}, false, false |
| 42 | } |
| 43 | first, last = strings.TrimSpace(first), strings.TrimSpace(last) |
| 44 | |
| 45 | if first == "" { |
| 46 | // A suffix range: the last n bytes. |
| 47 | n, ok := parseRangeInt(last) |
| 48 | if !ok { |
| 49 | return byteRange{}, false, false |
| 50 | } |
| 51 | if n == 0 || size == 0 { |
| 52 | return byteRange{}, false, true |
| 53 | } |
| 54 | if n > size { |
| 55 | n = size |
| 56 | } |
| 57 | return byteRange{start: size - n, length: n}, true, false |
| 58 | } |
| 59 | start, ok := parseRangeInt(first) |
| 60 | if !ok { |
| 61 | return byteRange{}, false, false |
| 62 | } |
| 63 | if start >= size { |
| 64 | return byteRange{}, false, true |
| 65 | } |
| 66 | if last == "" { |
| 67 | return byteRange{start: start, length: size - start}, true, false |
| 68 | } |
| 69 | end, ok := parseRangeInt(last) |
| 70 | if !ok || end < start { |
| 71 | return byteRange{}, false, false |
| 72 | } |
| 73 | if end >= size { |
| 74 | end = size - 1 |
| 75 | } |
| 76 | return byteRange{start: start, length: end - start + 1}, true, false |
| 77 | } |
| 78 | |
| 79 | // parseRangeInt reads the digits of one range bound. A sign, a space, or an |
| 80 | // overflow is not a bound, so the header is ignored rather than guessed at. |
| 81 | func parseRangeInt(s string) (int64, bool) { |
| 82 | if s == "" { |
| 83 | return 0, false |
| 84 | } |
| 85 | for _, r := range s { |
| 86 | if r < '0' || r > '9' { |
| 87 | return 0, false |
| 88 | } |
| 89 | } |
| 90 | n, err := strconv.ParseInt(s, 10, 64) |
| 91 | return n, err == nil |
| 92 | } |
| 93 |