feat(service-worker): Add fetch retry mechanism and environment variable support

Author: clairernovotny

doc/features-environment-variables.md

@@ -26,6 +26,7 @@ The bootstrapper provides a set of environment variables that reflect the config
 - `UNO_BOOTSTRAP_MONO_RUNTIME_MODE`, which specifies the runtime mode configuration (see above for valid values)
 - `UNO_BOOTSTRAP_LINKER_ENABLED`, which is set to `True` if the linker was enabled, otherwise `False`
 - `UNO_BOOTSTRAP_DEBUGGER_ENABLED`, which is set to `True` if the debugging support was enabled, otherwise `False`
+- `UNO_BOOTSTRAP_FETCH_RETRIES`, which specifies how many ties `fetch` should retry on a failed request. Defaults to `1`
 - `UNO_BOOTSTRAP_MONO_RUNTIME_CONFIGURATION`, which provides the mono runtime configuration, which can be can either be `release` or `debug`.
 - `UNO_BOOTSTRAP_MONO_RUNTIME_FEATURES`, which provides a list of comma separated feature enabled in the runtime (e.g. `threads`)
 - `UNO_BOOTSTRAP_MONO_PROFILED_AOT`, which specifies if the package was built using a PG-AOT profile.

src/Uno.Wasm.Bootstrap/Embedded/service-worker.js

@@ -4,6 +4,9 @@ if (unoConfig.environmentVariables["UNO_BOOTSTRAP_DEBUGGER_ENABLED"] !== "True")
     console.debug("[ServiceWorker] Initializing");
     let uno_enable_tracing = unoConfig.uno_enable_tracing;
 
+    // Get the number of fetch retries from environment variables or default to 1
+    const fetchRetries = parseInt(unoConfig.environmentVariables["UNO_BOOTSTRAP_FETCH_RETRIES"] || "1");
+
     self.addEventListener('install', function (e) {
         console.debug('[ServiceWorker] Installing offline worker');
         e.waitUntil(
@@ -114,10 +117,36 @@ if (unoConfig.environmentVariables["UNO_BOOTSTRAP_DEBUGGER_ENABLED"] !== "True")
                         return cachedResponse;
                     }
 
+                    // Add retry mechanism - attempt to fetch again if retries are configured
+                    if (fetchRetries > 0) {
+                        console.debug(`[ServiceWorker] Resource not in cache, attempting ${fetchRetries} network retries for: ${requestClone.url}`);
+
+                        // Try multiple fetch attempts with exponential backoff
+                        for (let retryCount = 0; retryCount < fetchRetries; retryCount++) {
+                            try {
+                                // Exponential backoff between retries (500ms, 1s, 2s, etc.)
+                                const retryDelay = Math.pow(2, retryCount) * 500;
+                                await new Promise(resolve => setTimeout(resolve, retryDelay));
+
+                                if (uno_enable_tracing) {
+                                    console.debug(`[ServiceWorker] Retry attempt ${retryCount + 1}/${fetchRetries} for: ${requestClone.url}`);
+                                }
+
+                                // Need a fresh request clone for each retry
+                                return await fetch(event.request.clone());
+                            } catch (retryErr) {
+                                if (uno_enable_tracing) {
+                                    console.debug(`[ServiceWorker] Retry ${retryCount + 1} failed: ${retryErr.message}`);
+                                }
+                                // Continue to next retry attempt
+                            }
+                        }
+                    }
+
                     // Graceful error handling with a proper HTTP response
                     // Rather than letting the fetch fail with a generic error,
                     // we return a controlled 503 Service Unavailable response
-                    console.error(`[ServiceWorker] Resource not available in cache or network: ${requestClone.url}`);
+                    console.error(`[ServiceWorker] Resource not available in cache or network after ${fetchRetries} retries: ${requestClone.url}`);
                     return new Response('Network error occurred, and resource was not found in cache.', {
                         status: 503,
                         statusText: 'Service Unavailable',

src/Uno.Wasm.Bootstrap/ShellTask.cs

@@ -125,6 +125,8 @@ public partial class ShellTask_v0 : Task
 
 		public bool GenerateAOTProfile { get; set; }
 
+		public string FetchRetries { get; set; } = "1";
+
 		public bool EnableThreads { get; set; }
 
 		public ITaskItem[]? ReferencePath { get; set; }
@@ -590,6 +592,7 @@ private void GenerateConfig()
 				AddEnvironmentVariable("UNO_BOOTSTRAP_MONO_PROFILED_AOT", isProfiledAOT.ToString());
 				AddEnvironmentVariable("UNO_BOOTSTRAP_LINKER_ENABLED", (PublishTrimmed && RunILLink).ToString());
 				AddEnvironmentVariable("UNO_BOOTSTRAP_DEBUGGER_ENABLED", (!Optimize).ToString());
+				AddEnvironmentVariable("UNO_BOOTSTRAP_FETCH_RETRIES", FetchRetries);
 				AddEnvironmentVariable("UNO_BOOTSTRAP_MONO_RUNTIME_CONFIGURATION", "Release");
 				AddEnvironmentVariable("UNO_BOOTSTRAP_MONO_RUNTIME_FEATURES", BuildRuntimeFeatures());
 				AddEnvironmentVariable("UNO_BOOTSTRAP_APP_BASE", PackageAssetsFolder);