Fix server errors when deleting static files or directories

Fixes #15174

hugolib/filesystems/basefs.go

@@ -426,16 +426,10 @@ func (d *SourceFilesystem) RealFilename(rel string) string {
 }
 
 // Contains returns whether the given filename is a member of the current filesystem.
+// It's stat-free, so it also works for deleted files.
 func (d *SourceFilesystem) Contains(filename string) bool {
-	for _, dir := range d.mounts() {
-		if !dir.IsDir() {
-			continue
-		}
-		if strings.HasPrefix(filename, dir.Meta().Filename) {
-			return true
-		}
-	}
-	return false
+	cps, _ := d.ReverseLookup(filename, false)
+	return len(cps) > 0
 }
 
 // RealDirs gets a list of absolute paths to directories starting from the given

hugolib/hugo_sites_build.go

@@ -1080,6 +1080,8 @@ func (h *HugoSites) processPartialFileEvents(ctx context.Context, l logg.LevelLo
 			changes = append(changes, identity.GenghisKhan)
 		case files.ComponentFolderArchetypes:
 			// Ignore for now.
+		case files.ComponentFolderStatic:
+			// Handled by the static file syncer.
 		default:
 			panic(fmt.Sprintf("unknown component: %q", pathInfo.Component()))
 		}

testscripts/server/server__delete_static_file_mount.txt

@@ -0,0 +1,32 @@
+# Test the hugo server command when removing a file mounted into static.
+# See issue 15174.
+
+hugo server &
+
+waitServer
+
+rm myfiles/robots.txt
+
+sleep 2
+
+stopServer
+! stderr .
+
+-- hugo.toml --
+title = "Hugo Server Test"
+baseURL = "https://example.org/"
+disableKinds = ["taxonomy", "term", "sitemap"]
+[[module.mounts]]
+source = "content"
+target = "content"
+[[module.mounts]]
+source = "layouts"
+target = "layouts"
+[[module.mounts]]
+source = "myfiles/robots.txt"
+target = "static/robots.txt"
+-- layouts/all.html --
+All.
+-- content/p1.md --
+-- myfiles/robots.txt --
+robots

testscripts/server/server__delete_static_mount_dir.txt

@@ -0,0 +1,32 @@
+# Test the hugo server command when removing a directory mounted into static.
+# See issue 15174.
+
+hugo server &
+
+waitServer
+
+rm mystatic/foo
+
+sleep 2
+
+stopServer
+! stderr .
+
+-- hugo.toml --
+title = "Hugo Server Test"
+baseURL = "https://example.org/"
+disableKinds = ["taxonomy", "term", "sitemap"]
+[[module.mounts]]
+source = "content"
+target = "content"
+[[module.mounts]]
+source = "layouts"
+target = "layouts"
+[[module.mounts]]
+source = "mystatic/foo"
+target = "static/foo"
+-- layouts/all.html --
+All.
+-- content/p1.md --
+-- mystatic/foo/static.txt --
+static