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Q&A Nodejs wrap async function in synchronous function

I am writing a spoof of the fs module; the actual storage mechanism is not the the filesystem but rather a database. Overall, however, the api will function exactly like fs. So far my code is compa...

1 answer  ·  posted 4y ago by Gershy‭  ·  last activity 4y ago by Moshi‭

#3: Post edited by user avatar Moshi‭ · 2020-09-03T23:42:34Z (almost 4 years ago)
I am writing a spoof of the `fs` module; the actual storage mechanism is not the the filesystem but rather a database. Overall, however, the api will function exactly like `fs`.

So far my code is comparable to the following:

```
let underlyingDb = require('module-for-accessing-db');

let mockFs = {
  
  promises: {
    
    readFile: async (fp, opts) => {
      let pk = convertFpToPk(fp);
      let res = await underlyingDb.query('SELECT * FROM "MockFs" WHERE "pk"=$1', [ pk ]);
      
      // Mock potential fs error:
      if (!res.rows.length) throw new Error(`ENOENT: no such file or directory...`);
      
      // TODO: check `opts` to see which encoding is desired
      return res.rows[0].fileContent;
    },
    writeFile: async (fp, data, opts) => {
      // Same idea as readFile, but with an INSERT ON CONFLICT statement
      // instead of SELECT...
    }
    
  },
  
  readFileSync: () => { /* ... */ }
  
};
```

As you can see it is straightforward for me to mock async file reading and writing.

I am now wondering, however, if it is possible to mock the `*Sync` methods in `fs`. Importantly, `module-for-accessing-db` *only* provides async access methods!

My initial instinct was to try the following (warning: gross!):

```
{
  readFileSync: (fp, data) => {
    let result = null;
    let err = null;
    
    (async () => {
      try {
        result = await mockFs.promises.readFile(fp, data);
      } catch(err0) {
        err = err0;
      }
    })();
    
    while (result === null && err === null) {} // Ew!!
    
    if (err) throw err;
    return result;
  }
}

```

Of course, this doesn't work. The `while` loop chokes out the entire event loop making it impossible for `result` or `err` to ever become set, even if the async database queries complete.

What are my options here? Do I need to switch the module I'm using to access the db? Do I need to write C code? Or is there a really clever technique to directly solve this problem, using only nodejs?
#2: Post edited by user avatar Gershy‭ · 2020-09-03T19:40:41Z (almost 4 years ago)
  • I am writing a spoof of the `fs` module; the actual storage mechanism is not the the filesystem but rather a database. Overall, however, the api will function exactly like `fs`.
  • So far my code is comparable to the following:
  • ```
  • let underlyingDb = require('module-for-accessing-db');
  • let mockFs = {
  • promises: {
  • readFile: async (fp, opts) => {
  • let pk = convertFpToPk(fp);
  • let res = await underlyingDb.query('SELECT * FROM "MockFs" WHERE "pk"=$1', [ pk ]);
  • // Mock potential fs error:
  • if (!res.rows) throw new Error(`ENOENT: no such file or directory...`);
  • // TODO: check `opts` to see which encoding is desired
  • return res.rows[0].fileContent;
  • },
  • writeFile: async (fp, data, opts) => {
  • // Same idea as readFile, but with an INSERT ON CONFLICT statement
  • // instead of SELECT...
  • }
  • },
  • readFileSync: () => { /* ... */ }
  • };
  • ```
  • As you can see it is straightforward for me to mock async file reading and writing.
  • I am now wondering, however, if it is possible to mock the `*Sync` methods in `fs`. Importantly, `module-for-accessing-db` *only* provides async access methods!
  • My initial instinct was to try the following (warning: gross!):
  • ```
  • {
  • readFileSync: (fp, data) => {
  • let result = null;
  • let err = null;
  • (async () => {
  • try {
  • result = await mockFs.promises.readFile(fp, data);
  • } catch(err0) {
  • err = err0;
  • }
  • })();
  • while (result === null && err === null) {} // Ew!!
  • if (err) throw err;
  • return result;
  • }
  • }
  • ```
  • Of course, this doesn't work. The `while` loop chokes out the entire event loop making it impossible for `result` or `err` to ever become set, even if the async database queries complete.
  • What are my options here? Do I need to switch the module I'm using to access the db? Do I need to write C code? Or is there a really clever technique to directly solve this problem, using only nodejs?
  • I am writing a spoof of the `fs` module; the actual storage mechanism is not the the filesystem but rather a database. Overall, however, the api will function exactly like `fs`.
  • So far my code is comparable to the following:
  • ```
  • let underlyingDb = require('module-for-accessing-db');
  • let mockFs = {
  • promises: {
  • readFile: async (fp, opts) => {
  • let pk = convertFpToPk(fp);
  • let res = await underlyingDb.query('SELECT * FROM "MockFs" WHERE "pk"=$1', [ pk ]);
  • // Mock potential fs error:
  • if (!res.rows.length) throw new Error(`ENOENT: no such file or directory...`);
  • // TODO: check `opts` to see which encoding is desired
  • return res.rows[0].fileContent;
  • },
  • writeFile: async (fp, data, opts) => {
  • // Same idea as readFile, but with an INSERT ON CONFLICT statement
  • // instead of SELECT...
  • }
  • },
  • readFileSync: () => { /* ... */ }
  • };
  • ```
  • As you can see it is straightforward for me to mock async file reading and writing.
  • I am now wondering, however, if it is possible to mock the `*Sync` methods in `fs`. Importantly, `module-for-accessing-db` *only* provides async access methods!
  • My initial instinct was to try the following (warning: gross!):
  • ```
  • {
  • readFileSync: (fp, data) => {
  • let result = null;
  • let err = null;
  • (async () => {
  • try {
  • result = await mockFs.promises.readFile(fp, data);
  • } catch(err0) {
  • err = err0;
  • }
  • })();
  • while (result === null && err === null) {} // Ew!!
  • if (err) throw err;
  • return result;
  • }
  • }
  • ```
  • Of course, this doesn't work. The `while` loop chokes out the entire event loop making it impossible for `result` or `err` to ever become set, even if the async database queries complete.
  • What are my options here? Do I need to switch the module I'm using to access the db? Do I need to write C code? Or is there a really clever technique to directly solve this problem, using only nodejs?
#1: Initial revision by user avatar Gershy‭ · 2020-09-03T19:19:39Z (almost 4 years ago)
Nodejs wrap async function in synchronous function
I am writing a spoof of the `fs` module; the actual storage mechanism is not the the filesystem but rather a database. Overall, however, the api will function exactly like `fs`.

So far my code is comparable to the following:

```
let underlyingDb = require('module-for-accessing-db');

let mockFs = {
  
  promises: {
    
    readFile: async (fp, opts) => {
      let pk = convertFpToPk(fp);
      let res = await underlyingDb.query('SELECT * FROM "MockFs" WHERE "pk"=$1', [ pk ]);
      
      // Mock potential fs error:
      if (!res.rows) throw new Error(`ENOENT: no such file or directory...`);
      
      // TODO: check `opts` to see which encoding is desired
      return res.rows[0].fileContent;
    },
    writeFile: async (fp, data, opts) => {
      // Same idea as readFile, but with an INSERT ON CONFLICT statement
      // instead of SELECT...
    }
    
  },
  
  readFileSync: () => { /* ... */ }
  
};
```

As you can see it is straightforward for me to mock async file reading and writing.

I am now wondering, however, if it is possible to mock the `*Sync` methods in `fs`. Importantly, `module-for-accessing-db` *only* provides async access methods!

My initial instinct was to try the following (warning: gross!):

```
{
  readFileSync: (fp, data) => {
    let result = null;
    let err = null;
    
    (async () => {
      try {
        result = await mockFs.promises.readFile(fp, data);
      } catch(err0) {
        err = err0;
      }
    })();
    
    while (result === null && err === null) {} // Ew!!
    
    if (err) throw err;
    return result;
  }
}

```

Of course, this doesn't work. The `while` loop chokes out the entire event loop making it impossible for `result` or `err` to ever become set, even if the async database queries complete.

What are my options here? Do I need to switch the module I'm using to access the db? Do I need to write C code? Or is there a really clever technique to directly solve this problem, using only nodejs?