From e8acda5d9dc62d3aaaa061320932f0666b03a678 Mon Sep 17 00:00:00 2001
From: Philippe Faist <phfaist@gmail.com>
Date: Fri, 14 Feb 2025 01:12:55 +0100
Subject: [PATCH] attempt atomic file write for cache

---
 src/citationmanager/_manager.js |  8 ++++-
 src/util/atomicfilewrite.js     | 58 +++++++++++++++++++++++++++++++++
 src/zooflm/citationcompiler.js  |  8 ++++-
 3 files changed, 72 insertions(+), 2 deletions(-)
 create mode 100644 src/util/atomicfilewrite.js

diff --git a/src/citationmanager/_manager.js b/src/citationmanager/_manager.js
index a77febd..87ca8df 100644
--- a/src/citationmanager/_manager.js
+++ b/src/citationmanager/_manager.js
@@ -6,6 +6,7 @@ const debug = debug_module('zoodb.citationmanager');
 import sha256 from 'hash.js/lib/hash/sha/256.js';
 
 import { promisifyMethods } from '../util/prify.js';
+import { writeFileAtomic } from '../util/atomicfilewrite.js';
 
 import { Cache, one_day } from './_cache.js';
 
@@ -111,7 +112,12 @@ export class CitationDatabaseManager
         const fsp = this.cache_fsp;
         // debug(`Saving database to cache file ‘${this.cache_file}’`);
         // to this.cache_file
-        await fsp.writeFile(this.cache_file, this.cache.exportJson());
+        await writeFileAtomic({
+            fsp,
+            fileName: this.cache_file,
+            data: this.cache.exportJson(),
+            processPid: (process != null) ? process.pid : 'XX',
+        });
     }
 
     /**
diff --git a/src/util/atomicfilewrite.js b/src/util/atomicfilewrite.js
new file mode 100644
index 0000000..5e6e13c
--- /dev/null
+++ b/src/util/atomicfilewrite.js
@@ -0,0 +1,58 @@
+import path from 'path';
+
+/**
+ * Write `data` to the file with the given `fileName`, overwriting it silently if it
+ * exists.  The write first happens to a temporary hidden file, and finally after the
+ * write succeeds, the temporary file is renamed to the target file.  This procedure
+ * prevents the output file from being corrupted if the process is interrupted while
+ * data is being written to the file.
+ * 
+ * Arguments:
+ * 
+ * - `fsp` a promisified filesystem object compatible with Node.js' promisified `fs`
+ *   object.
+ * 
+ * - `fileName` the name of the final file to write to.
+ * 
+ * - `data` the data to write to the file.  Can be string, typed array, or whatever
+ *   fsp's `writeFile()` can handle.
+ * 
+ * - `processPid` - specify `process.pid` here (or whatever you choose to use as relevant
+ *   PID for platforms/browser that might not have process PIDs).
+ */
+export async function writeFileAtomic({ fsp, fileName, data, useTempDir, processPid })
+{
+    let atomicWriteTempFileName = null;
+    let counter = 0;
+    let dataWritten = false;
+
+    const fDir = useTempDir ?? path.dirname(fileName) ?? '';
+    const fBaseTemplate = `.atomicwrite.${path.basename(fileName)}.${processPid}`
+
+    while (!dataWritten && counter <= 99) {
+        atomicWriteTempFileName = path.join(
+            fDir,
+            `${fBaseTemplate}.${counter}.tmp`
+        );
+        // do not accidentally overwrite a file; use 'wx' as open flags so that open fails
+        // if the file exists.
+        try {
+            await fsp.writeFile(atomicWriteTempFileName, data, {
+                flag: 'wx',
+            });
+            dataWritten = true;
+        } catch (err) {
+            // if file already exists, try another one
+            if (err && err.code === 'EEXIST') {
+                ++ counter;
+            } else {
+                // it's another error, send it up the chain
+                throw err;
+            }
+        }
+    }
+    if (!dataWritten) {
+        throw new Error(`writeFileAtomic: Failed to find temporary file to write to!`);
+    }
+    await fsp.rename(atomicWriteTempFileName, fileName);
+}
diff --git a/src/zooflm/citationcompiler.js b/src/zooflm/citationcompiler.js
index 9a51690..3e2edf9 100644
--- a/src/zooflm/citationcompiler.js
+++ b/src/zooflm/citationcompiler.js
@@ -8,6 +8,7 @@ const {
 
 import { split_prefix_label } from '../util/index.js';
 import { promisifyMethods } from '../util/prify.js';
+import { writeFileAtomic } from '../util/atomicfilewrite.js';
 
 import { Cache, one_day } from '../citationmanager/_cache.js';
 
@@ -399,7 +400,12 @@ export class CitationCompiler
     {
         const fsp = this.cache_fsp;
         debug(`Saving compiled citations to cache file ‘${this.cache_file}’`);
-        await fsp.writeFile(this.cache_file, this.cache.exportJson());
+        await writeFileAtomic({
+            fsp,
+            fileName: this.cache_file,
+            data: this.cache.exportJson(),
+            processPid: (process != null) ? process.pid : 'XX',
+        });
     }