Keep another process's downloads when opening a library (closes #105)

The download writer's temp files are now named .quak-<pid>-<random>.tmp.
removeLeftoverTempFiles moves from the backup into the download module and
deletes a .quak-*.tmp file only when the process ID in its name is no
longer running; the content cache calls it at open() instead of deleting
every temp file, so a download in progress in another process sharing the
cache survives. The README backup layout and TODO.md are updated.

Model: opus-5-5

src/backup.ts

@@ -45,7 +45,7 @@ import {
 import { copyFile, rename, rm } from "node:fs/promises";
 import { basename, dirname, extname, join, relative } from "node:path";
 
-import { fsyncPath } from "./download/index.js";
+import { fsyncPath, removeLeftoverTempFiles } from "./download/index.js";
 import { safeExtension, sanitizeFileName } from "./filename.js";
 import type { Collection, EnteFile } from "./model/types.js";
 
@@ -184,36 +184,6 @@ const copyAtomic = async (src: string, dest: string): Promise<void> => {
     }
 };
 
-// A process-ID check: signal 0 delivers nothing and only reports whether the
-// process exists. EPERM means it exists but belongs to another user.
-const isRunning = (pid: number): boolean => {
-    try {
-        process.kill(pid, 0);
-        return true;
-    } catch (err) {
-        return (err as NodeJS.ErrnoException).code === "EPERM";
-    }
-};
-
-// Delete the temp files `copyAtomic` leaves behind when a backup is killed
-// before its rename. Only files whose process is no longer running are
-// removed, so a backup running at the same time keeps its own. A reused
-// process ID can only keep a leftover a while longer, never remove a live one.
-const removeLeftoverTempFiles = (dir: string): void => {
-    let names: string[];
-    try {
-        names = readdirSync(dir);
-    } catch {
-        return;
-    }
-    for (const name of names) {
-        const match = /^\.quak-backup-.*-(\d+)-[0-9a-z]*\.tmp$/.exec(name);
-        if (match && !isRunning(Number(match[1]))) {
-            rmSync(join(dir, name), { force: true });
-        }
-    }
-};
-
 // Ensure `linkPath` is a symlink to `target`, rebuilding a missing, wrong, or
 // non-symlink entry. Throws on failure (a directory in the way, no permission)
 // so the caller records it and moves on rather than aborting the run.

src/download/index.ts

@@ -1,4 +1,5 @@
-import { randomUUID } from "node:crypto";
+import { randomBytes } from "node:crypto";
+import { readdirSync, rmSync } from "node:fs";
 import { open, rename, rm } from "node:fs/promises";
 import type { FileHandle } from "node:fs/promises";
 import { dirname, join } from "node:path";
@@ -180,6 +181,38 @@ export const fsyncPath = async (path: string): Promise<void> => {
     }
 };
 
+// A process-ID check: signal 0 delivers nothing and only reports whether the
+// process exists. EPERM means it exists but belongs to another user.
+const isRunning = (pid: number): boolean => {
+    try {
+        process.kill(pid, 0);
+        return true;
+    } catch (err) {
+        return (err as NodeJS.ErrnoException).code === "EPERM";
+    }
+};
+
+// Delete the temp files a killed process left in `dir`: the writer's
+// `.quak-<pid>-<random>.tmp` and the backup copy's
+// `.quak-backup-<name>-<pid>-<random>.tmp`. Only files whose process is no
+// longer running are removed, so another process writing into the same
+// directory keeps its own. A reused process ID can only keep a leftover a while
+// longer, never remove a live one.
+export const removeLeftoverTempFiles = (dir: string): void => {
+    let names: string[];
+    try {
+        names = readdirSync(dir);
+    } catch {
+        return;
+    }
+    for (const name of names) {
+        const match = /^\.quak-(?:.*-)?(\d+)-[0-9a-z]*\.tmp$/.exec(name);
+        if (match && !isRunning(Number(match[1]))) {
+            rmSync(join(dir, name), { force: true });
+        }
+    }
+};
+
 // Stage a write to `destination` atomically and durably, then rename it into
 // place. `fill` writes the contents into the open temp file handle — either the
 // whole buffer at once (`writeAtomic`) or chunk by chunk as they decrypt
@@ -205,8 +238,12 @@ const stageAtomic = async (
 ): Promise<void> => {
     const dir = dirname(destination);
     // The random suffix keeps concurrent downloads of the same destination
-    // from stepping on each other's temporary file.
-    const tmpPath = join(dir, `.quak-${randomUUID()}.tmp`);
+    // from stepping on each other's temporary file; the process ID lets
+    // `removeLeftoverTempFiles` tell a leftover from a write in progress.
+    const tmpPath = join(
+        dir,
+        `.quak-${process.pid}-${randomBytes(16).toString("hex")}.tmp`,
+    );
     try {
         const handle = await open(tmpPath, "w");
         try {

src/library/content.ts

@@ -39,6 +39,7 @@ import {
     downloadFile,
     downloadThumbnail,
     type ProgressCallback,
+    removeLeftoverTempFiles,
 } from "../download/index.js";
 import { safeExtension } from "../filename.js";
 import type { EnteFile } from "../model/types.js";
@@ -46,8 +47,6 @@ import type { Priority, RequestPools } from "./pools.js";
 
 const DIR_MODE = 0o700;
 const FILE_MODE = 0o600;
-const TEMP_PREFIX = ".quak-";
-const TEMP_SUFFIX = ".tmp";
 const GIB = 1024 * 1024 * 1024;
 // Owner ruling (#36): bound the originals cache at 100 GiB, but back off when
 // the volume has under 50 GiB free so the cache never crowds the disk.
@@ -654,6 +653,9 @@ export class ContentCache implements PhotoContent, ThumbnailsAPI {
     }
 
     private async scan(dir: string, into: Map<number, string>): Promise<void> {
+        // Another process sharing this cache may still be writing its temp
+        // files, so only those whose process has exited are removed.
+        removeLeftoverTempFiles(dir);
         let entries: string[];
         try {
             entries = await readdir(dir);
@@ -661,12 +663,6 @@ export class ContentCache implements PhotoContent, ThumbnailsAPI {
             return;
         }
         for (const name of entries) {
-            if (name.startsWith(TEMP_PREFIX) && name.endsWith(TEMP_SUFFIX)) {
-                await rm(join(dir, name), { force: true }).catch(
-                    () => undefined,
-                );
-                continue;
-            }
             const id = fileIDFromName(name);
             const path = join(dir, name);
             if (id !== undefined && existsSync(path)) into.set(id, path);