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file-ops-fallback.js
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file-ops-fallback.js
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import { LOCK_TYPES, isSafeToWrite, getPageSize } from '../sqlite-util';
import idbReady from 'safari-14-idb-fix';
function positionToKey(pos, blockSize) {
// We are forced to round because of floating point error. `pos`
// should always be divisible by `blockSize`
return Math.round(pos / blockSize);
}
async function openDb(name) {
await idbReady();
return new Promise((resolve, reject) => {
let req = globalThis.indexedDB.open(name, 2);
req.onsuccess = event => {
let db = event.target.result;
db.onversionchange = () => {
console.log('closing because version changed');
db.close();
};
db.onclose = () => {};
resolve(db);
};
req.onupgradeneeded = event => {
let db = event.target.result;
if (!db.objectStoreNames.contains('data')) {
db.createObjectStore('data');
}
};
req.onblocked = e => console.log('blocked', e);
req.onerror = req.onabort = e => reject(e.target.error);
});
}
// Using a separate class makes it easier to follow the code, and
// importantly it removes any reliance on internal state in
// `FileOpsFallback`. That would be problematic since these method
// happen async; the args to `write` must be closed over so they don't
// change
class Persistance {
constructor(dbName, onFallbackFailure) {
this.dbName = dbName;
this._openDb = null;
this.hasAlertedFailure = false;
this.onFallbackFailure = onFallbackFailure;
}
async getDb() {
if (this._openDb) {
return this._openDb;
}
this._openDb = await openDb(this.dbName);
return this._openDb;
}
closeDb() {
if (this._openDb) {
this._openDb.close();
this._openDb = null;
}
}
// Both `readAll` and `write` rely on IndexedDB transactional
// semantics to work, otherwise we'd have to coordinate them. If
// there are pending writes, the `readonly` transaction in `readAll`
// will block until they are all flushed out. If `write` is called
// multiple times, `readwrite` transactions can only run one at a
// time so it will naturally apply the writes sequentially (and
// atomically)
async readAll() {
let db = await this.getDb(this.dbName);
let blocks = new Map();
let trans = db.transaction(['data'], 'readonly');
let store = trans.objectStore('data');
return new Promise((resolve, reject) => {
// Open a cursor and iterate through the entire file
let req = store.openCursor(IDBKeyRange.lowerBound(-1));
req.onerror = reject;
req.onsuccess = e => {
let cursor = e.target.result;
if (cursor) {
blocks.set(cursor.key, cursor.value);
cursor.continue();
} else {
resolve(blocks);
}
};
});
}
async write(writes, cachedFirstBlock, hasLocked) {
let db = await this.getDb(this.dbName);
// We need grab a readwrite lock on the db, and then read to check
// to make sure we can write to it
let trans = db.transaction(['data'], 'readwrite');
let store = trans.objectStore('data');
await new Promise((resolve, reject) => {
let req = store.get(0);
req.onsuccess = e => {
if (hasLocked) {
if (!isSafeToWrite(req.result, cachedFirstBlock)) {
if (this.onFallbackFailure && !this.hasAlertedFailure) {
this.hasAlertedFailure = true;
this.onFallbackFailure();
}
reject(new Error('Fallback mode unable to write file changes'));
return;
}
}
// Flush all the writes
for (let write of writes) {
store.put(write.value, write.key);
}
trans.onsuccess = () => resolve();
trans.onerror = () => reject();
};
req.onerror = reject;
});
}
}
export class FileOpsFallback {
constructor(filename, onFallbackFailure) {
this.filename = filename;
this.dbName = this.filename.replace(/\//g, '-');
this.cachedFirstBlock = null;
this.writeQueue = null;
this.blocks = new Map();
this.lockType = 0;
this.transferBlockOwnership = false;
this.persistance = new Persistance(this.dbName, onFallbackFailure);
}
async readIfFallback() {
this.transferBlockOwnership = true;
this.blocks = await this.persistance.readAll();
return this.readMeta();
}
lock(lockType) {
// Locks always succeed here. Essentially we're only working
// locally (we can't see any writes from anybody else) and we just
// want to track the lock so we know when it downgrades from write
// to read
this.cachedFirstBlock = this.blocks.get(0);
this.lockType = lockType;
return true;
}
unlock(lockType) {
if (this.lockType > LOCK_TYPES.SHARED && lockType === LOCK_TYPES.SHARED) {
// Within a write lock, we delay all writes until the end of the
// lock. We probably don't have to do this since we already
// delay writes until an `fsync`, however this is an extra
// measure to make sure we are writing everything atomically
this.flush();
}
this.lockType = lockType;
return true;
}
delete() {
let req = globalThis.indexedDB.deleteDatabase(this.dbName);
req.onerror = () => {
console.warn(`Deleting ${this.filename} database failed`);
};
req.onsuccess = () => {};
}
open() {
this.writeQueue = [];
this.lockType = 0;
}
close() {
this.flush();
if (this.transferBlockOwnership) {
this.transferBlockOwnership = false;
} else {
this.blocks = new Map();
}
this.persistance.closeDb();
}
readMeta() {
let metaBlock = this.blocks.get(-1);
if (metaBlock) {
let block = this.blocks.get(0);
return {
size: metaBlock.size,
blockSize: getPageSize(new Uint8Array(block))
};
}
return null;
}
writeMeta(meta) {
this.blocks.set(-1, meta);
this.queueWrite(-1, meta);
}
readBlocks(positions, blockSize) {
let res = [];
for (let pos of positions) {
res.push({
pos,
data: this.blocks.get(positionToKey(pos, blockSize))
});
}
return res;
}
writeBlocks(writes, blockSize) {
for (let write of writes) {
let key = positionToKey(write.pos, blockSize);
this.blocks.set(key, write.data);
this.queueWrite(key, write.data);
}
// No write lock; flush them out immediately
if (this.lockType <= LOCK_TYPES.SHARED) {
this.flush();
}
}
queueWrite(key, value) {
this.writeQueue.push({ key, value });
}
flush() {
if (this.writeQueue.length > 0) {
this.persistance.write(
this.writeQueue,
this.cachedFirstBlock,
this.lockType > LOCK_TYPES.SHARED
);
this.writeQueue = [];
}
this.cachedFirstBlock = null;
}
}