From e5f97ace8b13c8bec9d9f8d4575bbb9a7f4ed5dc Mon Sep 17 00:00:00 2001
From: Mononaut <github@monospace.live>
Date: Mon, 1 May 2023 14:30:30 -0600
Subject: [PATCH] delay writing disk cache until block handler completes

---
 backend/src/api/blocks.ts     |  4 ++++
 backend/src/api/disk-cache.ts | 33 +++++++++++++++++++++++++++++++++
 backend/src/index.ts          |  2 ++
 3 files changed, 39 insertions(+)

diff --git a/backend/src/api/blocks.ts b/backend/src/api/blocks.ts
index 2837d40a0..c50c38107 100644
--- a/backend/src/api/blocks.ts
+++ b/backend/src/api/blocks.ts
@@ -533,6 +533,8 @@ class Blocks {
     // warn if this run stalls the main loop for more than 2 minutes
     const timer = this.startTimer();
 
+    diskCache.lock();
+
     let fastForwarded = false;
     const blockHeightTip = await bitcoinApi.$getBlockHeightTip();
     this.updateTimerProgress(timer, 'got block height tip');
@@ -697,6 +699,8 @@ class Blocks {
       this.updateTimerProgress(timer, `async callbacks completed for ${this.currentBlockHeight}`);
     }
 
+    diskCache.unlock();
+
     this.clearTimer(timer);
   }
 
diff --git a/backend/src/api/disk-cache.ts b/backend/src/api/disk-cache.ts
index 7a38e7da0..220d22b8e 100644
--- a/backend/src/api/disk-cache.ts
+++ b/backend/src/api/disk-cache.ts
@@ -22,6 +22,11 @@ class DiskCache {
   private static CHUNK_FILES = 25;
   private isWritingCache = false;
 
+  private semaphore: { resume: (() => void)[], locks: number } = {
+    resume: [],
+    locks: 0,
+  };
+
   constructor() {
     if (!cluster.isPrimary) {
       return;
@@ -77,6 +82,7 @@ class DiskCache {
           fs.renameSync(DiskCache.TMP_FILE_NAMES.replace('{number}', i.toString()), DiskCache.FILE_NAMES.replace('{number}', i.toString()));
         }
       } else {
+        await this.$yield();
         await fsPromises.writeFile(DiskCache.TMP_FILE_NAME, JSON.stringify({
           network: config.MEMPOOL.NETWORK,
           cacheSchemaVersion: this.cacheSchemaVersion,
@@ -86,6 +92,7 @@ class DiskCache {
           mempoolArray: mempoolArray.splice(0, chunkSize),
         }), { flag: 'w' });
         for (let i = 1; i < DiskCache.CHUNK_FILES; i++) {
+          await this.$yield();
           await fsPromises.writeFile(DiskCache.TMP_FILE_NAMES.replace('{number}', i.toString()), JSON.stringify({
             mempool: {},
             mempoolArray: mempoolArray.splice(0, chunkSize),
@@ -240,6 +247,32 @@ class DiskCache {
       logger.warn('Failed to parse rbf cache. Skipping. Reason: ' + (e instanceof Error ? e.message : e));
     }
   }
+
+  private $yield(): Promise<void> {
+    if (this.semaphore.locks) {
+      logger.debug('Pause writing mempool and blocks data to disk cache (async)');
+      return new Promise((resolve) => {
+        this.semaphore.resume.push(resolve);
+      });
+    } else {
+      return Promise.resolve();
+    }
+  }
+
+  public lock(): void {
+    this.semaphore.locks++;
+  }
+
+  public unlock(): void {
+    this.semaphore.locks = Math.max(0, this.semaphore.locks - 1);
+    if (!this.semaphore.locks && this.semaphore.resume.length) {
+      const nextResume = this.semaphore.resume.shift();
+      if (nextResume) {
+        logger.debug('Resume writing mempool and blocks data to disk cache (async)');
+        nextResume();
+      }
+    }
+  }
 }
 
 export default new DiskCache();
diff --git a/backend/src/index.ts b/backend/src/index.ts
index 384f96c38..3887aac2c 100644
--- a/backend/src/index.ts
+++ b/backend/src/index.ts
@@ -205,6 +205,8 @@ class Server {
         logger.debug(`AxiosError: ${e?.message}`);
       }
       setTimeout(this.runMainUpdateLoop.bind(this), 1000 * this.currentBackendRetryInterval);
+    } finally {
+      diskCache.unlock();
     }
   }