[Fizz] declare bootstrap script preloads to be fetchPriority: 'low' (#27189)

Generally scripts should not be preloaded before images but if they
arrive earlier than image preloads (or images) the network (or server)
may be saturated responding to inflight script preloads and not
sufficiently prioritize images arriving later. This change marks the
preloaded bootstrap script with a `low` fetch priority to signal to
supporting browsers that the request should be deprioritized. This
should make the preload operate similar to async script fetch priority
which is low by default according to https://web.dev/fetch-priority/

Additionally the bootstrap script preloads will emit before
preinitialized scripts do. Normal script preloads will continue to be
prioritized after stylesheets

This change can land separatrely but is part of a larger effort to
implement elevating image loading and making script loading less
blocking. Later changes will emit used suspensey images earlier in the
queue and will stop favoring scripts over images that are explicitly
preloaded

DiffTrain build for [9edf470d6e](https://github.com/facebook/react/commit/9edf470d6ed1f3ac12c23a248e64293f367c1797)

compiled/facebook-www/REVISION

@@ -1 +1 @@
-ea17cc18f458010c89c1bf494be08bb782d034af
+9edf470d6ed1f3ac12c23a248e64293f367c1797

compiled/facebook-www/ReactDOMServer-dev.classic.js

@@ -19,7 +19,7 @@ if (__DEV__) {
 var React = require("react");
 var ReactDOM = require("react-dom");
 
-var ReactVersion = "18.3.0-www-classic-8651ccc3";
+var ReactVersion = "18.3.0-www-classic-728bbc0a";
 
 // This refers to a WWW module.
 var warningWWW = require("warning");
@@ -6282,6 +6282,7 @@ function writePreamble(
   resources.fontPreloads.clear(); // Flush unblocked stylesheets by precedence
 
   resources.precedences.forEach(flushAllStylesInPreamble, destination);
+  resources.bootstrapScripts.forEach(flushResourceInPreamble, destination);
   resources.scripts.forEach(flushResourceInPreamble, destination);
   resources.scripts.clear();
   resources.explicitStylesheetPreloads.forEach(
@@ -6346,7 +6347,9 @@ function writeHoistables(destination, resources, responseState) {
   resources.fontPreloads.clear(); // Preload any stylesheets. these will emit in a render instruction that follows this
   // but we want to kick off preloading as soon as possible
 
-  resources.precedences.forEach(preloadLateStyles, destination);
+  resources.precedences.forEach(preloadLateStyles, destination); // bootstrap scripts should flush above script priority but these can only flush in the preamble
+  // so we elide the code here for performance
+
   resources.scripts.forEach(flushResourceLate, destination);
   resources.scripts.clear();
   resources.explicitStylesheetPreloads.forEach(flushResourceLate, destination);
@@ -6836,6 +6839,7 @@ function createResources() {
     // usedImagePreloads: new Set(),
     precedences: new Map(),
     stylePrecedences: new Map(),
+    bootstrapScripts: new Set(),
     scripts: new Set(),
     explicitStylesheetPreloads: new Set(),
     // explicitImagePreloads: new Set(),
@@ -7482,6 +7486,7 @@ function preloadBootstrapScript(resources, src, nonce, integrity, crossOrigin) {
     rel: "preload",
     href: src,
     as: "script",
+    fetchPriority: "low",
     nonce: nonce,
     integrity: integrity,
     crossOrigin: crossOrigin
@@ -7493,7 +7498,7 @@ function preloadBootstrapScript(resources, src, nonce, integrity, crossOrigin) {
     props: props
   };
   resources.preloadsMap.set(key, resource);
-  resources.explicitScriptPreloads.add(resource);
+  resources.bootstrapScripts.add(resource);
   pushLinkImpl(resource.chunks, props);
 } // This function is only safe to call at Request start time since it assumes
 // that each module has not already been preloaded. If we find a need to preload
@@ -7518,6 +7523,7 @@ function preloadBootstrapModule(resources, src, nonce, integrity, crossOrigin) {
   var props = {
     rel: "modulepreload",
     href: src,
+    fetchPriority: "low",
     nonce: nonce,
     integrity: integrity,
     crossOrigin: crossOrigin
@@ -7529,7 +7535,7 @@ function preloadBootstrapModule(resources, src, nonce, integrity, crossOrigin) {
     props: props
   };
   resources.preloadsMap.set(key, resource);
-  resources.explicitScriptPreloads.add(resource);
+  resources.bootstrapScripts.add(resource);
   pushLinkImpl(resource.chunks, props);
   return;
 }

compiled/facebook-www/ReactDOMServer-dev.modern.js

@@ -19,7 +19,7 @@ if (__DEV__) {
 var React = require("react");
 var ReactDOM = require("react-dom");
 
-var ReactVersion = "18.3.0-www-modern-f39e0868";
+var ReactVersion = "18.3.0-www-modern-9727d465";
 
 // This refers to a WWW module.
 var warningWWW = require("warning");
@@ -6282,6 +6282,7 @@ function writePreamble(
   resources.fontPreloads.clear(); // Flush unblocked stylesheets by precedence
 
   resources.precedences.forEach(flushAllStylesInPreamble, destination);
+  resources.bootstrapScripts.forEach(flushResourceInPreamble, destination);
   resources.scripts.forEach(flushResourceInPreamble, destination);
   resources.scripts.clear();
   resources.explicitStylesheetPreloads.forEach(
@@ -6346,7 +6347,9 @@ function writeHoistables(destination, resources, responseState) {
   resources.fontPreloads.clear(); // Preload any stylesheets. these will emit in a render instruction that follows this
   // but we want to kick off preloading as soon as possible
 
-  resources.precedences.forEach(preloadLateStyles, destination);
+  resources.precedences.forEach(preloadLateStyles, destination); // bootstrap scripts should flush above script priority but these can only flush in the preamble
+  // so we elide the code here for performance
+
   resources.scripts.forEach(flushResourceLate, destination);
   resources.scripts.clear();
   resources.explicitStylesheetPreloads.forEach(flushResourceLate, destination);
@@ -6836,6 +6839,7 @@ function createResources() {
     // usedImagePreloads: new Set(),
     precedences: new Map(),
     stylePrecedences: new Map(),
+    bootstrapScripts: new Set(),
     scripts: new Set(),
     explicitStylesheetPreloads: new Set(),
     // explicitImagePreloads: new Set(),
@@ -7482,6 +7486,7 @@ function preloadBootstrapScript(resources, src, nonce, integrity, crossOrigin) {
     rel: "preload",
     href: src,
     as: "script",
+    fetchPriority: "low",
     nonce: nonce,
     integrity: integrity,
     crossOrigin: crossOrigin
@@ -7493,7 +7498,7 @@ function preloadBootstrapScript(resources, src, nonce, integrity, crossOrigin) {
     props: props
   };
   resources.preloadsMap.set(key, resource);
-  resources.explicitScriptPreloads.add(resource);
+  resources.bootstrapScripts.add(resource);
   pushLinkImpl(resource.chunks, props);
 } // This function is only safe to call at Request start time since it assumes
 // that each module has not already been preloaded. If we find a need to preload
@@ -7518,6 +7523,7 @@ function preloadBootstrapModule(resources, src, nonce, integrity, crossOrigin) {
   var props = {
     rel: "modulepreload",
     href: src,
+    fetchPriority: "low",
     nonce: nonce,
     integrity: integrity,
     crossOrigin: crossOrigin
@@ -7529,7 +7535,7 @@ function preloadBootstrapModule(resources, src, nonce, integrity, crossOrigin) {
     props: props
   };
   resources.preloadsMap.set(key, resource);
-  resources.explicitScriptPreloads.add(resource);
+  resources.bootstrapScripts.add(resource);
   pushLinkImpl(resource.chunks, props);
   return;
 }

compiled/facebook-www/ReactDOMServer-prod.classic.js

@@ -3649,6 +3649,10 @@ function flushCompletedQueues(request, destination) {
         resources.fontPreloads.forEach(flushResourceInPreamble, destination);
         resources.fontPreloads.clear();
         resources.precedences.forEach(flushAllStylesInPreamble, destination);
+        resources.bootstrapScripts.forEach(
+          flushResourceInPreamble,
+          destination
+        );
         resources.scripts.forEach(flushResourceInPreamble, destination);
         resources.scripts.clear();
         resources.explicitStylesheetPreloads.forEach(
@@ -3949,6 +3953,7 @@ function renderToStringImpl(
       fontPreloads: new Set(),
       precedences: new Map(),
       stylePrecedences: new Map(),
+      bootstrapScripts: new Set(),
       scripts: new Set(),
       explicitStylesheetPreloads: new Set(),
       explicitScriptPreloads: new Set(),
@@ -4014,4 +4019,4 @@ exports.renderToString = function (children, options) {
     'The server used "renderToString" which does not support Suspense. If you intended for this Suspense boundary to render the fallback content on the server consider throwing an Error somewhere within the Suspense boundary. If you intended to have the server wait for the suspended component please switch to "renderToReadableStream" which supports Suspense on the server'
   );
 };
-exports.version = "18.3.0-www-classic-849751e4";
+exports.version = "18.3.0-www-classic-e05f0ecc";

compiled/facebook-www/ReactDOMServer-prod.modern.js

@@ -3547,6 +3547,10 @@ function flushCompletedQueues(request, destination) {
         resources.fontPreloads.forEach(flushResourceInPreamble, destination);
         resources.fontPreloads.clear();
         resources.precedences.forEach(flushAllStylesInPreamble, destination);
+        resources.bootstrapScripts.forEach(
+          flushResourceInPreamble,
+          destination
+        );
         resources.scripts.forEach(flushResourceInPreamble, destination);
         resources.scripts.clear();
         resources.explicitStylesheetPreloads.forEach(
@@ -3847,6 +3851,7 @@ function renderToStringImpl(
       fontPreloads: new Set(),
       precedences: new Map(),
       stylePrecedences: new Map(),
+      bootstrapScripts: new Set(),
       scripts: new Set(),
       explicitStylesheetPreloads: new Set(),
       explicitScriptPreloads: new Set(),
@@ -3912,4 +3917,4 @@ exports.renderToString = function (children, options) {
     'The server used "renderToString" which does not support Suspense. If you intended for this Suspense boundary to render the fallback content on the server consider throwing an Error somewhere within the Suspense boundary. If you intended to have the server wait for the suspended component please switch to "renderToReadableStream" which supports Suspense on the server'
   );
 };
-exports.version = "18.3.0-www-modern-a94e9f5e";
+exports.version = "18.3.0-www-modern-432efc32";

compiled/facebook-www/ReactDOMServerStreaming-dev.modern.js

@@ -6289,6 +6289,7 @@ function writePreamble(
   resources.fontPreloads.clear(); // Flush unblocked stylesheets by precedence
 
   resources.precedences.forEach(flushAllStylesInPreamble, destination);
+  resources.bootstrapScripts.forEach(flushResourceInPreamble, destination);
   resources.scripts.forEach(flushResourceInPreamble, destination);
   resources.scripts.clear();
   resources.explicitStylesheetPreloads.forEach(
@@ -6353,7 +6354,9 @@ function writeHoistables(destination, resources, responseState) {
   resources.fontPreloads.clear(); // Preload any stylesheets. these will emit in a render instruction that follows this
   // but we want to kick off preloading as soon as possible
 
-  resources.precedences.forEach(preloadLateStyles, destination);
+  resources.precedences.forEach(preloadLateStyles, destination); // bootstrap scripts should flush above script priority but these can only flush in the preamble
+  // so we elide the code here for performance
+
   resources.scripts.forEach(flushResourceLate, destination);
   resources.scripts.clear();
   resources.explicitStylesheetPreloads.forEach(flushResourceLate, destination);
@@ -6843,6 +6846,7 @@ function createResources() {
     // usedImagePreloads: new Set(),
     precedences: new Map(),
     stylePrecedences: new Map(),
+    bootstrapScripts: new Set(),
     scripts: new Set(),
     explicitStylesheetPreloads: new Set(),
     // explicitImagePreloads: new Set(),
@@ -7489,6 +7493,7 @@ function preloadBootstrapScript(resources, src, nonce, integrity, crossOrigin) {
     rel: "preload",
     href: src,
     as: "script",
+    fetchPriority: "low",
     nonce: nonce,
     integrity: integrity,
     crossOrigin: crossOrigin
@@ -7500,7 +7505,7 @@ function preloadBootstrapScript(resources, src, nonce, integrity, crossOrigin) {
     props: props
   };
   resources.preloadsMap.set(key, resource);
-  resources.explicitScriptPreloads.add(resource);
+  resources.bootstrapScripts.add(resource);
   pushLinkImpl(resource.chunks, props);
 } // This function is only safe to call at Request start time since it assumes
 // that each module has not already been preloaded. If we find a need to preload
@@ -7525,6 +7530,7 @@ function preloadBootstrapModule(resources, src, nonce, integrity, crossOrigin) {
   var props = {
     rel: "modulepreload",
     href: src,
+    fetchPriority: "low",
     nonce: nonce,
     integrity: integrity,
     crossOrigin: crossOrigin
@@ -7536,7 +7542,7 @@ function preloadBootstrapModule(resources, src, nonce, integrity, crossOrigin) {
     props: props
   };
   resources.preloadsMap.set(key, resource);
-  resources.explicitScriptPreloads.add(resource);
+  resources.bootstrapScripts.add(resource);
   pushLinkImpl(resource.chunks, props);
   return;
 }

compiled/facebook-www/ReactDOMServerStreaming-prod.modern.js

@@ -3409,6 +3409,10 @@ function flushCompletedQueues(request, destination) {
         resources.fontPreloads.forEach(flushResourceInPreamble, destination);
         resources.fontPreloads.clear();
         resources.precedences.forEach(flushAllStylesInPreamble, destination);
+        resources.bootstrapScripts.forEach(
+          flushResourceInPreamble,
+          destination
+        );
         resources.scripts.forEach(flushResourceInPreamble, destination);
         resources.scripts.clear();
         resources.explicitStylesheetPreloads.forEach(
@@ -3819,6 +3823,7 @@ exports.renderToStream = function (children, options) {
       fontPreloads: new Set(),
       precedences: new Map(),
       stylePrecedences: new Map(),
+      bootstrapScripts: new Set(),
       scripts: new Set(),
       explicitStylesheetPreloads: new Set(),
       explicitScriptPreloads: new Set(),
@@ -3883,13 +3888,14 @@ exports.renderToStream = function (children, options) {
           rel: "preload",
           href: externalRuntimeConfig,
           as: "script",
+          fetchPriority: "low",
           nonce: void 0,
           integrity: integrity,
           crossOrigin: scriptConfig
         },
         resource = { type: "preload", chunks: [], state: 0, props: props };
       resources.preloadsMap.set("[script]" + externalRuntimeConfig, resource);
-      resources.explicitScriptPreloads.add(resource);
+      resources.bootstrapScripts.add(resource);
       pushLinkImpl(resource.chunks, props);
       JSCompiler_inline_result.push(
         '<script src="',
@@ -3927,6 +3933,7 @@ exports.renderToStream = function (children, options) {
         (scriptConfig = {
           rel: "modulepreload",
           href: bootstrapScriptContent,
+          fetchPriority: "low",
           nonce: void 0,
           integrity: externalRuntimeConfig,
           crossOrigin: integrity
@@ -3938,7 +3945,7 @@ exports.renderToStream = function (children, options) {
           props: scriptConfig
         }),
         resources.preloadsMap.set("[script]" + bootstrapScriptContent, props),
-        resources.explicitScriptPreloads.add(props),
+        resources.bootstrapScripts.add(props),
         pushLinkImpl(props.chunks, scriptConfig),
         JSCompiler_inline_result.push(
           '<script type="module" src="',