Add Peer Model Support (#438)

This PR allows a single llama-swap to be the central proxy for models served by other inference servers. The peer servers can be another llama-swap or any API that supports the /v1/* inference endpoint.

Updates: #433, #299
Closes: #296

proxy/peerproxy.go

@@ -0,0 +1,127 @@
+package proxy
+
+import (
+	"fmt"
+	"net"
+	"net/http"
+	"net/http/httputil"
+	"runtime"
+	"sort"
+	"strings"
+	"time"
+
+	"github.com/mostlygeek/llama-swap/proxy/config"
+)
+
+type peerProxyMember struct {
+	peerID       string
+	reverseProxy *httputil.ReverseProxy
+	apiKey       string
+}
+
+type PeerProxy struct {
+	peers    config.PeerDictionaryConfig
+	proxyMap map[string]*peerProxyMember
+}
+
+func NewPeerProxy(peers config.PeerDictionaryConfig, proxyLogger *LogMonitor) (*PeerProxy, error) {
+	proxyMap := make(map[string]*peerProxyMember)
+
+	// Sort peer IDs for consistent iteration order
+	peerIDs := make([]string, 0, len(peers))
+	for peerID := range peers {
+		peerIDs = append(peerIDs, peerID)
+	}
+	sort.Strings(peerIDs)
+
+	// Create a shared transport with reasonable timeouts for peer connections
+	// these can be tuned with feedback later
+	peerTransport := &http.Transport{
+		DialContext: (&net.Dialer{
+			Timeout:   30 * time.Second, // Connection timeout
+			KeepAlive: 30 * time.Second,
+		}).DialContext,
+		TLSHandshakeTimeout:   10 * time.Second,
+		ResponseHeaderTimeout: 60 * time.Second, // Time to wait for response headers
+		ExpectContinueTimeout: 1 * time.Second,
+		MaxIdleConns:          100,
+		MaxIdleConnsPerHost:   10,
+		IdleConnTimeout:       90 * time.Second,
+	}
+
+	for _, peerID := range peerIDs {
+		peer := peers[peerID]
+		// Create reverse proxy for this peer
+		reverseProxy := httputil.NewSingleHostReverseProxy(peer.ProxyURL)
+		reverseProxy.Transport = peerTransport
+
+		// Wrap Director to set Host header for remote hosts (not localhost)
+		originalDirector := reverseProxy.Director
+		reverseProxy.Director = func(req *http.Request) {
+			originalDirector(req)
+			// Ensure Host header matches target URL for remote proxying
+			req.Host = req.URL.Host
+		}
+
+		reverseProxy.ModifyResponse = func(resp *http.Response) error {
+			if strings.Contains(strings.ToLower(resp.Header.Get("Content-Type")), "text/event-stream") {
+				resp.Header.Set("X-Accel-Buffering", "no")
+			}
+			return nil
+		}
+
+		reverseProxy.ErrorHandler = func(w http.ResponseWriter, r *http.Request, err error) {
+			proxyLogger.Warnf("peer %s: proxy error: %v", peerID, err)
+			errMsg := fmt.Sprintf("peer proxy error: %v", err)
+			if runtime.GOOS == "darwin" && strings.Contains(err.Error(), "connect: no route to host") {
+				errMsg += " (hint: on macOS, check System Settings > Privacy & Security > Local Network permissions)"
+			}
+			http.Error(w, errMsg, http.StatusBadGateway)
+		}
+
+		pp := &peerProxyMember{
+			peerID:       peerID,
+			reverseProxy: reverseProxy,
+			apiKey:       peer.ApiKey,
+		}
+
+		// Map each model to this peer's proxy
+		for _, modelID := range peer.Models {
+			if _, found := proxyMap[modelID]; found {
+				proxyLogger.Warnf("peer %s: model %s already mapped to another peer, skipping", peerID, modelID)
+				continue
+			}
+			proxyMap[modelID] = pp
+		}
+	}
+
+	return &PeerProxy{
+		peers:    peers,
+		proxyMap: proxyMap,
+	}, nil
+}
+
+func (p *PeerProxy) HasPeerModel(modelID string) bool {
+	_, found := p.proxyMap[modelID]
+	return found
+}
+
+func (p *PeerProxy) ListPeers() config.PeerDictionaryConfig {
+	return p.peers
+}
+
+func (p *PeerProxy) ProxyRequest(model_id string, writer http.ResponseWriter, request *http.Request) error {
+	pp, found := p.proxyMap[model_id]
+	if !found {
+		return fmt.Errorf("no peer proxy found for model %s", model_id)
+	}
+
+	// Inject API key if configured for this peer
+	if pp.apiKey != "" {
+		request.Header.Set("Authorization", "Bearer "+pp.apiKey)
+		request.Header.Set("x-api-key", pp.apiKey)
+	}
+
+	pp.reverseProxy.ServeHTTP(writer, request)
+	return nil
+}