Improve Concurrency and Parallel Request Handling (#19)

Rewrite the swap behaviour so that in-flight requests block process swapping until they are completed. 

Additionally: 

- add tests for parallel requests with proxy.ProxyManager and proxy.Process
- improve Process startup behaviour and simplified the code 
- stopping of processes are sent SIGTERM and have 5 seconds to terminate, before they are killed

proxy/proxymanager_test.go

@@ -5,7 +5,9 @@ import (
 	"fmt"
 	"net/http"
 	"net/http/httptest"
+	"sync"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -72,5 +74,60 @@ func TestProxyManager_SwapMultiProcess(t *testing.T) {
 
 	_, exists = proxy.currentProcesses["test/model2"]
 	assert.True(t, exists, "expected test/model2 key in currentProcesses")
-
+}
+
+// When a request for a different model comes in ProxyManager should wait until
+// the first request is complete before swapping. Both requests should complete
+func TestProxyManager_SwapMultiProcessParallelRequests(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping slow test")
+	}
+
+	config := &Config{
+		HealthCheckTimeout: 15,
+		Models: map[string]ModelConfig{
+			"model1": getTestSimpleResponderConfig("model1"),
+			"model2": getTestSimpleResponderConfig("model2"),
+			"model3": getTestSimpleResponderConfig("model3"),
+		},
+	}
+
+	proxy := New(config)
+	defer proxy.StopProcesses()
+
+	results := map[string]string{}
+
+	var wg sync.WaitGroup
+	var mu sync.Mutex
+
+	for key := range config.Models {
+		wg.Add(1)
+		go func(key string) {
+			defer wg.Done()
+
+			reqBody := fmt.Sprintf(`{"model":"%s"}`, key)
+			req := httptest.NewRequest("POST", "/v1/chat/completions?wait=1000ms", bytes.NewBufferString(reqBody))
+			w := httptest.NewRecorder()
+
+			proxy.HandlerFunc(w, req)
+
+			if w.Code != http.StatusOK {
+				t.Errorf("Expected status OK, got %d for key %s", w.Code, key)
+			}
+
+			mu.Lock()
+
+			results[key] = w.Body.String()
+			mu.Unlock()
+		}(key)
+
+		<-time.After(time.Millisecond)
+	}
+
+	wg.Wait()
+	assert.Len(t, results, len(config.Models))
+
+	for key, result := range results {
+		assert.Equal(t, key, result)
+	}
 }