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fix: fsync state file before rename to survive crashes #23

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fix: fsync state file before rename to survive crashes #23

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12 changes: 9 additions & 3 deletions dist/server.js
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Original file line number Diff line number Diff line change
Expand Up @@ -3,7 +3,7 @@
import { z } from "zod";

// src/state.ts
import { chmod, mkdir, readFile, rename, writeFile } from "fs/promises";
import { chmod, mkdir, open, readFile, rename } from "fs/promises";
import { homedir } from "os";
import { dirname, join } from "path";
import { Data, Effect, Schema } from "effect";
Expand Down Expand Up @@ -109,8 +109,14 @@ function writeStateEffect(state) {
const file = statePath();
await mkdir(dirname(file), { recursive: true, mode: 448 });
const tmp = `${file}.${process.pid}.${Date.now()}.tmp`;
await writeFile(tmp, JSON.stringify(state, null, 2) + `
`, { mode: 384 });
const handle = await open(tmp, "w", 384);
try {
await handle.writeFile(JSON.stringify(state, null, 2) + `
`);
await handle.sync();
} finally {
await handle.close();
}
await rename(tmp, file);
await chmod(file, 384).catch(() => {
return;
Expand Down
13 changes: 11 additions & 2 deletions src/state.ts
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
import { readFileSync } from "node:fs"
import { chmod, mkdir, readFile, rename, writeFile } from "node:fs/promises"
import { chmod, mkdir, open, readFile, rename } from "node:fs/promises"
import { homedir } from "node:os"
import { dirname, join } from "node:path"
import { Data, Effect, Schema } from "effect"
Expand Down Expand Up @@ -237,7 +237,16 @@ function writeStateEffect(state: State) {
const file = statePath()
await mkdir(dirname(file), { recursive: true, mode: 0o700 })
const tmp = `${file}.${process.pid}.${Date.now()}.tmp`
await writeFile(tmp, JSON.stringify(state, null, 2) + "\n", { mode: 0o600 })
// Write to a temp file and fsync it before renaming into place. On crash
// (power loss / BSOD) an fsync'd temp file is durable, so the rename never
// exposes an empty or partially-flushed state file.
const handle = await open(tmp, "w", 0o600)
try {
await handle.writeFile(JSON.stringify(state, null, 2) + "\n")
await handle.sync()
} finally {
await handle.close()
}
await rename(tmp, file)
await chmod(file, 0o600).catch(() => undefined)
},
Expand Down