web: harden the cache purge scheduler

web/src/instrumentation.ts

@@ -12,8 +12,8 @@ import {
 // How often the clock is checked. Timers alone can't be trusted to fire at
 // midnight: they stand still while a host or VM is paused
 const CHECK_INTERVAL_MS = 30_000;
-// Until the first purge has gone through; the server isn't listening yet
-// when register() runs
+// Until the first purge has gone through. The server is already listening
+// when register() runs, so this is only for a purge that fails
 const STARTUP_RETRY_MS = 1_000;
 
 function keepCacheCurrent() {
@@ -24,6 +24,18 @@ function keepCacheCurrent() {
   let failures = 0;
 
   async function check() {
+    try {
+      await purgeWhatIsDue();
+    } catch (error) {
+      // nothing in there is expected to throw, but if something does the
+      // loop must survive it: without it the cache never rolls over
+      console.error("Checking the page cache failed", error);
+    } finally {
+      schedule();
+    }
+  }
+
+  async function purgeWhatIsDue() {
     const day = osloDay();
     let error: string | null = null;
 
@@ -50,14 +62,14 @@ function keepCacheCurrent() {
 
     if (error) {
       failures += 1;
-      // at startup, connection errors are expected for the first moments
+      // report the first failure once running, and then now and again; at
+      // startup only if it goes on for a minute
       if (failures === 1 ? purgedDay !== undefined : failures % 60 === 0) {
         console.error(`Purging the page cache keeps failing: ${error}`);
       }
     } else {
       failures = 0;
     }
-    schedule();
   }
 
   function schedule() {