| 1 | // SPDX-License-Identifier: AGPL-3.0-or-later |
| 2 | |
| 3 | // Package protocol carries the smart-HTTP and (later) smart-SSH bits of |
| 4 | // the git wire protocol that we generate ourselves. The bulk of the |
| 5 | // protocol — capability negotiation, want/have, multi-ack, side-band — |
| 6 | // stays inside canonical `git`'s `upload-pack` and `receive-pack`. We |
| 7 | // only emit the framing we need to wrap their advertise-refs output for |
| 8 | // HTTP transport. |
| 9 | package protocol |
| 10 | |
| 11 | import ( |
| 12 | "fmt" |
| 13 | "io" |
| 14 | ) |
| 15 | |
| 16 | // MaxPktLine is git's hard limit for a single packet: 65520 bytes of |
| 17 | // payload + 4 bytes of length prefix. Anything longer is malformed. |
| 18 | const MaxPktLine = 65520 |
| 19 | |
| 20 | // FlushPkt is the literal pkt-line sequence that says "no more |
| 21 | // packets." git uses it to terminate sub-streams. |
| 22 | const FlushPkt = "0000" |
| 23 | |
| 24 | // WritePkt writes one pkt-line packet — a 4-hex-digit length prefix |
| 25 | // (covering payload + 4) followed by the payload. Exposed so the smart- |
| 26 | // HTTP info/refs handler can prepend its `# service=...` advertisement. |
| 27 | func WritePkt(w io.Writer, payload string) error { |
| 28 | if len(payload)+4 > MaxPktLine { |
| 29 | return fmt.Errorf("pkt-line: payload too long (%d > %d)", len(payload), MaxPktLine-4) |
| 30 | } |
| 31 | if _, err := fmt.Fprintf(w, "%04x%s", len(payload)+4, payload); err != nil { |
| 32 | return err |
| 33 | } |
| 34 | return nil |
| 35 | } |
| 36 | |
| 37 | // WriteFlush emits the flush packet (`0000`). Marks the end of a |
| 38 | // sub-stream like the service advertisement preamble. |
| 39 | func WriteFlush(w io.Writer) error { |
| 40 | _, err := io.WriteString(w, FlushPkt) |
| 41 | return err |
| 42 | } |
| 43 | |
| 44 | // WriteServiceAdvertisement writes the standard preamble that the smart- |
| 45 | // HTTP info/refs response uses to tell git which service is being |
| 46 | // advertised. The wire format is: |
| 47 | // |
| 48 | // 001e# service=git-upload-pack\n0000 |
| 49 | // └┬┘└──────────┬─────────────┘└─┬─┘ |
| 50 | // │ │ └ flush |
| 51 | // │ └ payload (trailing newline included) |
| 52 | // └ length prefix (hex) |
| 53 | // |
| 54 | // Then `git upload-pack --advertise-refs --stateless-rpc <repo>` writes |
| 55 | // its actual ref advertisement after this. |
| 56 | func WriteServiceAdvertisement(w io.Writer, service string) error { |
| 57 | if err := WritePkt(w, "# service="+service+"\n"); err != nil { |
| 58 | return err |
| 59 | } |
| 60 | return WriteFlush(w) |
| 61 | } |
| 62 |