is it safe to use `runCatching` in a `suspend fun`...

# coroutines

[alexhelder]

is it safe to use runCatching in a suspend fun when you are sure the underlying code would not throw CancellationException (in my case blocking file I/O stuff, FileInputStream etc)

[Youssef Shoaib [MOD]]

The issue is that it'd hog the thread completely.

[CLOVIS]

@Youssef Shoaib [MOD] why would it hog the thread? Wouldn't that happen with runBlocking ?

[Youssef Shoaib [MOD]]

Oh my bad I misread the question 🤦🏼

[CLOVIS]

I thought so 😅 happens to the best of us

[Youssef Shoaib [MOD]]

Yeah using runCatching might be okay then? Although there's other "fatal" exceptions that shouldn't be caught like OOMs.
Can you give an example of how you're using it? Are you swallowing the exception and printing it or something? That's okay at the top level for instance, but I'd avoid it in something deeply nested because of OOMs and other fatal exceptions existing

[alexhelder]

I’m generally aware of the runCatching + CancellationException issue, but wonder if runCatching is “safe” to use inside a coroutine if it is not calling any suspend functions like so (assuming you don’t expect anything thowing Java’s CancellationException):

suspend fun importNkey(uri: Uri) {
    withContext(ioDispatcher) {
        readNkeyFromFile2(uri) // plain non-suspend method calls Java blocking I/O stuff
    }.onSuccess { nkey ->
        dataStore.edit { prefs -> // edit() is a suspend fun
            // save in prefs
        }
    }
}

// not suspend !!!
private fun readNkeyFromFile2(uri: Uri): Result<NKey> =
    runCatching {
        context.contentResolver.openInputStream(uri)?.use { stream -> // throws IOException, others
            stream.bufferedReader().use { reader ->
                reader.useLines { lines ->
                    lines.first { it.startsWith(USER_SEED_PREFIX) } // throws if missing
                }
            }
        } ?: throw IOException() // openInputStream could be null if ContentProvider crashes
    }.mapCatching { seed ->
        NKey.fromSeed(seed.toCharArray()) // can throw if malformed
    }

[CLOVIS]

Consider what happens here if any Error is thrown. For example, OutOfMemoryError . Your code swallows it and tries to continue doing stuff in a half-broken JVM, which may cause further problems down the stream

[CLOVIS]

Result exists to rethrow exceptions elsewhere, NOT for exception recovery

[CLOVIS]

If you want to do something like this, use Arrow's Try or Either which are explicitly written to enable these kinds of behaviors