don't update currency rates on transient errors (#4662)

## Explanation

We think in MetaMask/metamask-mobile#11035,
the intermittent 0 balances were caused by transient errors in requests
to crypto compare. Instead of setting the conversion rate to null in
this case, we'll omit the state update and keep the conversion rate from
the previous successful fetch.

## References

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* Fixes #12345
* Related to #67890
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### `@metamask/assets-controllers`

- **FIXED**: In `CurrencyRateController` if unexpected errors occur
during requests to crypto compare, the conversion rate in state will
remain unchanged instead of being set to null.

## Checklist

- [ ] I've updated the test suite for new or updated code as appropriate
- [ ] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [ ] I've highlighted breaking changes using the "BREAKING" category
above as appropriate

packages/assets-controllers/src/CurrencyRateController.test.ts

@@ -478,4 +478,35 @@ describe('CurrencyRateController', () => {
 
     controller.destroy();
   });
+
+  it('should not update state on unexpected / transient errors', async () => {
+    const cryptoCompareHost = 'https://min-api.cryptocompare.com';
+    nock(cryptoCompareHost)
+      .get('/data/price?fsym=ETH&tsyms=XYZ')
+      .reply(500) // HTTP 500 transient error
+      .persist();
+
+    const state = {
+      currentCurrency: 'xyz',
+      currencyRates: {
+        ETH: {
+          conversionDate: 123,
+          conversionRate: 123,
+          usdConversionRate: 123,
+        },
+      },
+    };
+    const messenger = getRestrictedMessenger();
+    const controller = new CurrencyRateController({ messenger, state });
+
+    // Error should still be thrown
+    await expect(controller.updateExchangeRate('ETH')).rejects.toThrow(
+      `Fetch failed with status '500' for request 'https://min-api.cryptocompare.com/data/price?fsym=ETH&tsyms=XYZ'`,
+    );
+
+    // But state should not be changed
+    expect(controller.state).toStrictEqual(state);
+
+    controller.destroy();
+  });
 });

packages/assets-controllers/src/CurrencyRateController.ts

@@ -170,6 +170,7 @@ export class CurrencyRateController extends StaticIntervalPollingController<
       ? FALL_BACK_VS_CURRENCY // ETH
       : nativeCurrency;
 
+    let shouldUpdateState = true;
     try {
       if (
         currentCurrency &&
@@ -196,23 +197,27 @@ export class CurrencyRateController extends StaticIntervalPollingController<
           error.message.includes('market does not exist for this coin pair')
         )
       ) {
+        // Don't update state on transient / unexpected errors
+        shouldUpdateState = false;
         throw error;
       }
     } finally {
       try {
-        this.update(() => {
-          return {
-            currencyRates: {
-              ...currencyRates,
-              [nativeCurrency]: {
-                conversionDate,
-                conversionRate,
-                usdConversionRate,
+        if (shouldUpdateState) {
+          this.update(() => {
+            return {
+              currencyRates: {
+                ...currencyRates,
+                [nativeCurrency]: {
+                  conversionDate,
+                  conversionRate,
+                  usdConversionRate,
+                },
               },
-            },
-            currentCurrency,
-          };
-        });
+              currentCurrency,
+            };
+          });
+        }
       } finally {
         releaseLock();
       }