pad_utils: Rate limit identical `warnDeprecated` calls

src/static/js/pad_utils.js

@@ -93,6 +93,9 @@ const padutils = {
    * Prints a warning message followed by a stack trace (to make it easier to figure out what code
    * is using the deprecated function).
    *
+   * Identical deprecation warnings (as determined by the stack trace, if available) are rate
+   * limited to avoid log spam.
+   *
    * Most browsers include UI widget to examine the stack at the time of the warning, but this
    * includes the stack in the log message for a couple of reasons:
    *   - This makes it possible to see the stack if the code runs in Node.js.
@@ -106,6 +109,18 @@ const padutils = {
     const err = new Error();
     if (Error.captureStackTrace) Error.captureStackTrace(err, padutils.warnDeprecated);
     err.name = '';
+    // Rate limit identical deprecation warnings (as determined by the stack) to avoid log spam.
+    if (typeof err.stack === 'string') {
+      if (padutils.warnDeprecated._rl == null) {
+        padutils.warnDeprecated._rl =
+            {prevs: new Map(), now: () => Date.now(), period: 10 * 60 * 1000};
+      }
+      const rl = padutils.warnDeprecated._rl;
+      const now = rl.now();
+      const prev = rl.prevs.get(err.stack);
+      if (prev != null && now - prev < rl.period) return;
+      rl.prevs.set(err.stack, now);
+    }
     if (err.stack) args.push(err.stack);
     (padutils.warnDeprecated.logger || console).warn(...args);
   },

src/tests/backend/specs/pad_utils.js

@@ -14,6 +14,7 @@ describe(__filename, function () {
 
     afterEach(async function () {
       warnDeprecated.logger = backups.logger;
+      delete warnDeprecated._rl; // Reset internal rate limiter state.
     });
 
     it('includes the stack', async function () {
@@ -22,5 +23,21 @@ describe(__filename, function () {
       warnDeprecated();
       assert(got.includes(__filename));
     });
+
+    it('rate limited', async function () {
+      let got = 0;
+      warnDeprecated.logger = {warn: () => ++got};
+      warnDeprecated(); // Initialize internal rate limiter state.
+      const {period} = warnDeprecated._rl;
+      got = 0;
+      const testCases = [[0, 1], [0, 1], [period - 1, 1], [period, 2]];
+      for (const [now, want] of testCases) { // In a loop so that the stack trace is the same.
+        warnDeprecated._rl.now = () => now;
+        warnDeprecated();
+        assert.equal(got, want);
+      }
+      warnDeprecated(); // Should have a different stack trace.
+      assert.equal(got, testCases[testCases.length - 1][1] + 1);
+    });
   });
 });