Add pure SSH LFS support (#31516)

Fixes #17554
/claim #17554

Docs PR https://gitea.com/gitea/docs/pulls/49

To test, run pushes like: `GIT_TRACE=1` git push. The trace output
should mention "pure SSH connection".

modules/lfstransfer/backend/util.go

@@ -0,0 +1,141 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package backend
+
+import (
+	"context"
+	"crypto/tls"
+	"fmt"
+	"net"
+	"net/http"
+	"time"
+
+	"code.gitea.io/gitea/modules/httplib"
+	"code.gitea.io/gitea/modules/proxyprotocol"
+	"code.gitea.io/gitea/modules/setting"
+
+	"github.com/charmbracelet/git-lfs-transfer/transfer"
+)
+
+// HTTP headers
+const (
+	headerAccept        = "Accept"
+	headerAuthorisation = "Authorization"
+	headerAuthX         = "X-Auth"
+	headerContentType   = "Content-Type"
+	headerContentLength = "Content-Length"
+)
+
+// MIME types
+const (
+	mimeGitLFS      = "application/vnd.git-lfs+json"
+	mimeOctetStream = "application/octet-stream"
+)
+
+// SSH protocol action keys
+const (
+	actionDownload = "download"
+	actionUpload   = "upload"
+	actionVerify   = "verify"
+)
+
+// SSH protocol argument keys
+const (
+	argCursor    = "cursor"
+	argExpiresAt = "expires-at"
+	argID        = "id"
+	argLimit     = "limit"
+	argPath      = "path"
+	argRefname   = "refname"
+	argToken     = "token"
+	argTransfer  = "transfer"
+)
+
+// Default username constants
+const (
+	userSelf    = "(self)"
+	userUnknown = "(unknown)"
+)
+
+// Operations enum
+const (
+	opNone = iota
+	opDownload
+	opUpload
+)
+
+var opMap = map[string]int{
+	"download": opDownload,
+	"upload":   opUpload,
+}
+
+var ErrMissingID = fmt.Errorf("%w: missing id arg", transfer.ErrMissingData)
+
+func statusCodeToErr(code int) error {
+	switch code {
+	case http.StatusBadRequest:
+		return transfer.ErrParseError
+	case http.StatusConflict:
+		return transfer.ErrConflict
+	case http.StatusForbidden:
+		return transfer.ErrForbidden
+	case http.StatusNotFound:
+		return transfer.ErrNotFound
+	case http.StatusUnauthorized:
+		return transfer.ErrUnauthorized
+	default:
+		return fmt.Errorf("server returned status %v: %v", code, http.StatusText(code))
+	}
+}
+
+func newInternalRequest(ctx context.Context, url, method string, headers map[string]string, body []byte) *httplib.Request {
+	req := httplib.NewRequest(url, method).
+		SetContext(ctx).
+		SetTimeout(10*time.Second, 60*time.Second).
+		SetTLSClientConfig(&tls.Config{
+			InsecureSkipVerify: true,
+		})
+
+	if setting.Protocol == setting.HTTPUnix {
+		req.SetTransport(&http.Transport{
+			DialContext: func(ctx context.Context, _, _ string) (net.Conn, error) {
+				var d net.Dialer
+				conn, err := d.DialContext(ctx, "unix", setting.HTTPAddr)
+				if err != nil {
+					return conn, err
+				}
+				if setting.LocalUseProxyProtocol {
+					if err = proxyprotocol.WriteLocalHeader(conn); err != nil {
+						_ = conn.Close()
+						return nil, err
+					}
+				}
+				return conn, err
+			},
+		})
+	} else if setting.LocalUseProxyProtocol {
+		req.SetTransport(&http.Transport{
+			DialContext: func(ctx context.Context, network, address string) (net.Conn, error) {
+				var d net.Dialer
+				conn, err := d.DialContext(ctx, network, address)
+				if err != nil {
+					return conn, err
+				}
+				if err = proxyprotocol.WriteLocalHeader(conn); err != nil {
+					_ = conn.Close()
+					return nil, err
+				}
+				return conn, err
+			},
+		})
+	}
+
+	for k, v := range headers {
+		req.Header(k, v)
+	}
+
+	req.Body(body)
+
+	return req
+}