goget/docs/download-pipeline.md

# Download Pipeline

This document explains how goget processes a download request from start to finish — the phase structure, parallel chunking, resume logic, and output writing.

## High-Level Flow

```mermaid
flowchart TD
    Start([User invokes goget])
    ParseFlags["Parse CLI flags"]
    LoadConfig["Load config + merge overrides"]
    ResolveProtocol["Resolve protocol handler\nfrom URL scheme"]
    CreateRequest["Build core.DownloadRequest"]
    CheckResume{"Resume metadata\nexists?"}
    LoadResume["Load .goget.meta\n(resume info + chunk map)"]
    SetupOutput["Setup output writer\n(atomic temp file + resume)"]
    CreateDirs{"--create-dirs\n(parent missing)?"}
    MkdirAll["os.MkdirAll → create\nparent directories"]
    CheckParallel{"File > 100 MB\nor --parallel set?"}
    SequentialDownload["Sequential download\n(single connection)"]
    ParallelDownload["Parallel chunked download\n(multiple Range requests)"]
    Decompress{"Auto-decompress\nenabled?"}
    Decompression["Decompress response\n(gzip, deflate, bzip2, zlib)"]
    VerifyChecksum{"Checksum\nspecified?"}
    ChecksumVerify["Verify checksum\n(SHA-256, SHA-512, etc.)"]
    PGPVerify{"PGP verify/\ndecrypt?"}
    PGPProcess["Verify signature /\ndecrypt file"]
    SaveHSTS["Save HSTS cache\n(RFC 6797)"]
    AtomicRename["Atomic rename\ntemp → final"]
    SigInt{"SIGINT received?"}
    SaveSidecar["Save .goget.meta\nfor resume"]
    Exit130["Exit 130"]
    RunHook{"--on-complete\nset?"}
    PostHook["Run post-download command\n(GOGET_OUTPUT, GOGET_SIZE, GOGET_URL)"]
    Done([Done])

    Start --> ParseFlags --> LoadConfig --> ResolveProtocol
    ResolveProtocol --> CreateRequest --> CheckResume
    CheckResume -->|Yes| LoadResume --> SetupOutput
    CheckResume -->|No| SetupOutput
    SetupOutput --> CreateDirs
    CreateDirs -->|Yes| MkdirAll --> CheckParallel
    CreateDirs -->|No| CheckParallel
    CheckParallel -->|Yes| ParallelDownload
    CheckParallel -->|No| SequentialDownload
    SequentialDownload --> Decompress
    ParallelDownload --> Decompress
    Decompress -->|Yes| Decompression --> VerifyChecksum
    Decompress -->|No| VerifyChecksum
    VerifyChecksum -->|Yes| ChecksumVerify -->|pass| PGPVerify
    ChecksumVerify -->|fail| Done
    VerifyChecksum -->|No| PGPVerify
    PGPVerify -->|Yes| PGPProcess --> SaveHSTS
    PGPVerify -->|No| SaveHSTS
    SaveHSTS --> AtomicRename
    AtomicRename --> SigInt
    SigInt -->|Yes| SaveSidecar --> Exit130
    SigInt -->|No| RunHook
    RunHook -->|Yes| PostHook --> Done
    RunHook -->|No| Done
```

## Phase 1: Request Construction

A `core.DownloadRequest` is built from CLI flags and config:

```go
req := &core.DownloadRequest{
    URL:              parsedURL,
    Output:           outputPath,
    Resume:           resumeEnabled,
    Timeout:          effectiveTimeout,
    Verbose:          verbose,
    Headers:          customHeaders,
    Proxy:            proxyURL,
    Checksum:         expectedChecksum,
    Parallel:         parallelConfig,
    Recursive:        recursiveEnabled,
    MaxDepth:         maxDepth,
    ProgressCallback: progressFn,
    Ctx:              ctx,
}
```

## Phase 2: Protocol Resolution

The protocol registry resolves the handler by URL scheme:

```mermaid
flowchart LR
    URL["https://example.com"]
    Parse["Parse scheme\n→ https"]
    Normalized["Normalize\nhttps → http"]
    Registry["Registry lookup\nprotocols[http]"]
    Handler["HTTP protocol\nhandler"]

    URL --> Parse --> Normalized --> Registry --> Handler
```

Scheme normalization maps:
- `https://` → `http` (same handler)
- `ftps://` → `ftp` (same handler)
- `webdavs://` → `webdav` (same handler)

## Phase 3: Output Setup

The `internal/output` package creates an atomic writer:

1. **Temp file** — Writes to `<filename>.goget.tmp` during download
2. **Parent directories** — If `--create-dirs` is enabled (default: `true`), missing parent directories are created via `os.MkdirAll` before opening the output file. Pass `--create-dirs=false` to restore strict behaviour
3. **Resume metadata** — Reads `<filename>.goget.meta` if resuming
4. **Progress callback** — Hooks into the writer for real-time speed/ETA

```go
writer, err := output.NewWriter(&output.WriterConfig{
    Output:           outputFile,
    Atomic:           true,
    Resume:           resumeEnabled,
    CreateDirs:       true,  // auto-create parent dirs (curl --create-dirs)
    ProgressCallback: progressFunc,
})
```

### Resume Metadata Format

```json
{
    "downloaded_bytes": 524288000,
    "etag": "\"abc123\"",
    "last_modified": "Mon, 01 Jun 2026 12:00:00 GMT",
    "url": "https://example.com/file.zip",
    "last_write": "2026-06-01T12:05:00Z",
    "chunks": {
        "0": 131072000,
        "1": 131072000,
        "2": 131072000,
        "3": 131072000
    }
}
```

For parallel downloads, each chunk's progress is individually tracked.

## Phase 4: Download Strategy

### Sequential Download

Used when the file is under 100 MB or `--parallel 1` is set:

```go
resp, _ := client.Do(request)
io.Copy(writer, resp.Body)
```

### Parallel Chunked Download

Triggered automatically for files over 100 MB, or explicitly with `--parallel N`:

```mermaid
sequenceDiagram
    participant Main
    participant Chunk1
    participant Chunk2
    participant Chunk3
    participant Chunk4
    participant Writer as Atomic Writer

    Main->>Main: HEAD request → get file size
    Main->>Main: Split into N equal chunks
    Main->>Chunk1: Start: bytes 0-13107199
    Main->>Chunk2: Start: bytes 13107200-26214399
    Main->>Chunk3: Start: bytes 26214400-39321599
    Main->>Chunk4: Start: bytes 39321600-52428799
    Chunk1->>Writer: Write bytes to temp/chunk_0
    Chunk2->>Writer: Write bytes to temp/chunk_1
    Chunk3->>Writer: Write bytes to temp/chunk_2
    Chunk4->>Writer: Write bytes to temp/chunk_3
    Main->>Writer: Merge chunks → final file
```

Each chunk downloads via a separate HTTP `Range` request:

```http
GET /large.iso HTTP/1.1
Host: example.com
Range: bytes=13107200-26214399
```

### Concurrency Control

- Max 4 parallel connections by default
- Customizable via `--parallel N` or config `parallel` key
- Min chunk size: 1 MB
- Max chunk size: 50 MB

## Phase 5: Post-Processing

### Decompression

If `auto_decompress` is enabled (default) and the server sends compressed content, the response body is transparently decompressed:

| Content-Encoding | Handler |
|---|---|
| `gzip` | `compress/gzip` |
| `deflate` | `compress/flate` |
| `zlib` | Internal |
| `bzip2` | Internal |
| `lzw` | Internal |

Use `--no-decompress` to preserve the compressed response.

### Checksum Verification

If `--checksum` or `--checksum-file` is provided, the downloaded file is hashed and compared:

```
Expected: a1b2c3d4...
Actual:   a1b2c3d4...
Checksum OK
```

Supported algorithms: SHA-256, SHA-512, SHA3-256, SHA3-512, BLAKE2b, MD5.

### Atomic Rename

On successful completion, the temp file is atomically renamed:

```go
os.Rename("file.zip.goget.tmp", "file.zip")
```

If the download fails or is interrupted, the temp file remains for resume.

### PGP Verification and Decryption

If `--pgp-verify` or `--pgp-decrypt` is set, the downloaded file is processed via `golang.org/x/crypto/openpgp`:

- **Detached signature verification** — `--pgp-sig file.sig --pgp-key public.key`
- **Decryption** — `--pgp-decrypt --pgp-key private.key --pgp-passphrase "secret"`
- Auto-detect signature files: `.asc`, `.sig`

The decrypted file replaces the encrypted one (`.gpg` → stripped extension, or `.decrypted` suffix).

### HSTS Cache

After every successful HTTPS connection, the HSTS cache (`~/.config/goget/hsts`) is updated per RFC 6797. Expired entries are pruned on load. This ensures that future `http://` requests to known hosts are automatically upgraded to `https://`.

### Graceful Shutdown (SIGINT)

When the user sends Ctrl+C (SIGINT) during a download, goget persists partial progress to the `.goget.meta` sidecar for **all protocols** (HTTP, FTP, SFTP, WebDAV) and exits with code 130. A subsequent `goget --resume` picks up where it left off. The signal handler uses `signal.NotifyContext` and propagates cancellation through the download pipeline via `ctx.Err()` checks after every read.

## Smart Timeout

When no explicit `--timeout` is set, goget calculates a timeout based on file size:

```
timeout = (fileSize / minSpeed) × safetyFactor
```

Where:
- `minSpeed` = 10 KB/s (conservative minimum)
- `safetyFactor` = 3.0
- `minTimeout` = 30 seconds
- `maxTimeout` = 24 hours

A 1 GB file at 10 KB/s → 34 hours (capped to 24 hours max).
A 10 MB file at 10 KB/s → 51 minutes.

## Rate Limiting

`--rate-limit` uses a token bucket algorithm:

```go
bucket := transport.NewTokenBucket(rate)
// For each read:
bucket.Wait(n)
```

The token bucket allows short bursts above the limit while maintaining the average rate.

### Speed Format

```
--rate-limit 1MB/s    # 1,000,000 bytes/sec
--rate-limit 500KB/s  # 500,000 bytes/sec
--rate-limit 1GB/s    # 1,000,000,000 bytes/sec
--rate-limit 100      # 100 bytes/sec (plain number)
```

## Retry Logic

When a download fails, goget retries with exponential backoff:

```
Attempt 1: wait 1s, retry
Attempt 2: wait 2s, retry
Attempt 3: wait 4s, retry
...
Attempt N: wait min(2^(N-1) × 1s, 30s), retry
```

Max retries default to 3. Use `--max-retries N` to increase. Use `--retry-all-errors` to retry on HTTP 4xx/5xx (by default, only 5xx server errors trigger retries).