util/http: lazily init the global HTTP client to fix admin metrics nil panic (#10044)

util/http: lazily init the global HTTP client

GetGlobalHttpClient returned a nil client until InitGlobalHttpClient ran,
which only happens in weed.go's main. Anything that starts a command
in-process bypasses that: the admin server's metrics goroutine seeds a
dashboard sample on startup, reaching fetchPublicUrlMap -> GetGlobalHttpClient().Do,
and nil-derefs the receiver in GetHttpScheme.

Init the client on first Get via sync.Once so it is never nil regardless of
the startup path. InitGlobalHttpClient keeps its eager-init role through the
same Once.

weed/util/http/http_global_client_init.go

@@ -1,27 +1,37 @@
 package http
 
 import (
+	"sync"
+
 	"github.com/seaweedfs/seaweedfs/weed/glog"
 	util_http_client "github.com/seaweedfs/seaweedfs/weed/util/http/client"
 )
 
 var (
-	globalHttpClient *util_http_client.HTTPClient
+	globalHttpClient     *util_http_client.HTTPClient
+	globalHttpClientOnce sync.Once
 )
 
 func NewGlobalHttpClient(opt ...util_http_client.HttpClientOpt) (*util_http_client.HTTPClient, error) {
 	return util_http_client.NewHttpClient(util_http_client.Client, opt...)
 }
 
+// GetGlobalHttpClient returns the process-wide HTTP client, initializing it on
+// first use. Lazy init keeps callers that bypass weed.go's main (in-process
+// test harnesses, libraries) from dereferencing a nil client.
 func GetGlobalHttpClient() *util_http_client.HTTPClient {
+	globalHttpClientOnce.Do(initGlobalHttpClient)
 	return globalHttpClient
 }
 
 func InitGlobalHttpClient() {
-	var err error
+	globalHttpClientOnce.Do(initGlobalHttpClient)
+}
 
-	globalHttpClient, err = NewGlobalHttpClient()
+func initGlobalHttpClient() {
+	client, err := NewGlobalHttpClient()
 	if err != nil {
 		glog.Fatalf("error init global http client: %v", err)
 	}
+	globalHttpClient = client
 }