Replace Gson library with Moshi

[cbeyls]

! ! Warning: Do not merge before testing every API call and database read involving JSON !

Gson is obsolete and has been superseded by Moshi. But more importantly, parsing Kotlin objects using Gson is dangerous because Gson uses Java serialization and is not Kotlin-aware. This has two main consequences:

• Fields of non-null types may end up null at runtime. Parsing will succeed, but the code may crash later with a NullPointerException when trying to access a field member;
• Default values of constructor parameters are always ignored. When absent, reference types will be null, booleans will be false and integers will be zero.

On the other hand, Kotlin-aware parsers like Moshi or Kotlin Serialization will validate at parsing time that all received fields comply with the Kotlin contract and avoid errors at runtime, making apps more stable and schema mismatches easier to detect (as long as logs are accessible):

• Receiving a null value for a non-null type will generate a parsing error;
• Optional types are declared explicitly by adding a default value. A missing value with no default value declaration will generate a parsing error.

Migrating the entity declarations from Gson to Moshi will make the code more robust but is not an easy task because of the semantic differences.

With Gson, both nullable and optional fields are represented with a null value. After converting to Moshi, some nullable entities can become non-null with a default value (if they are optional and not nullable), others can stay nullable with no default value (if they are mandatory and nullable), and others can become nullable with a default value of null (if they are optional or nullable or both). That third option is the safest bet when it's not clear if a field is optional or not, except for lists which can usually be declared as non-null with a default value of an empty list (I have yet to see a nullable array type in the Mastodon API).

Fields that are currently declared as non-null present another challenge. In theory, they should remain as-is and everything will work fine. In practice, because Gson is not aware of nullable types at all, it's possible that some non-null fields currently hold a null value in some cases but the app does not report any error because the field is not accessed by Kotlin code in that scenario. After migrating to Moshi however, parsing such a field will now fail early if a null value or no value is received.

These fields will have to be identified by heavily testing the app and looking for parsing errors (JsonDataException) and/or by going through the Mastodon documentation. A default value needs to be added for missing optional fields, and their type could optionally be changed to nullable, depending on the case.

Gson is also currently used to serialize and deserialize objects to and from the local database, which is also challenging because backwards compatibility needs to be preserved. Fortunately, by default Gson omits writing null fields, so a field of type List<T>? could be replaced with a field of type List<T> with a default value of emptyList() and reading back the old data should still work. However, nullable lists that are written directly (not as a field of another object) will still be serialized to JSON as "null" so the deserializing code must still be handling null properly.

Finally, changing the database schema is out of scope for this pull request, so database entities that also happen to be serialized with Gson will keep their original types even if they could be made non-null as an improvement.

In the end this is all for the best, because the app will be more reliable and errors will be easier to detect by showing up earlier with a clear error message. Not to mention the performance benefits of using Moshi compared to Gson.

• Replace Gson reflection with Moshi Kotlin codegen to generate all parsers at compile time.
• Replace custom Rfc3339DateJsonAdapter with the one provided by moshi-adapters.
• Replace custom JsonDeserializer classes for Enum types with EnumJsonAdapter.create(T).withUnknownFallback() from moshi-adapters to support fallback values.
• Replace GuardedBooleanAdapter with the more generic GuardedAdapter which works with any type. Any nullable field may now be annotated with @Guarded.
• Remove Proguard rules related to Json entities. Each Json entity needs to be annotated with @JsonClass with no exception, and adding this annotation will ensure that R8/Proguard will handle the entities properly.
• Replace some nullable Boolean fields with non-null Boolean fields with a default value where possible.
• Replace some nullable list fields with non-null list fields with a default value of emptyList() where possible.
• Update TimelineDao to perform all Json conversions internally using Converters so no Gson or Moshi instance has to be passed to its methods.
• Create a custom DraftAttachmentJsonAdapter to serialize and deserialize DraftAttachment which is a special entity that supports more than one json name per field. A custom adapter is necessary because there is not direct equivalent of @SerializedName(alternate = [...]) in Moshi. Remove alternate names for some DraftAttachment fields which were used as a workaround to deserialize local data in 2-years old builds of Tusky.
• Update tests to make them work with Moshi.
• Simplify a few equals() implementations.
• Change a few functions to vals
• Turn NetworkModule into an object (since it contains no abstract methods).

Please test the app thoroughly before merging. There may be some fields currently declared as mandatory that are actually optional.

[Goooler]

Suggested change

	emojis = moshi.adapter<List<Emoji>?>().fromJson(emojis) ?: emptyList()
	emojis = moshi.adapter<List<Emoji>?>().fromJson(emojis).orEmpty()

[Goooler]

Suggested change

	return emojiListJson?.let { moshi.adapter<List<Emoji>?>().fromJson(it) } ?: emptyList()
	return emojiListJson?.let { moshi.adapter<List<Emoji>?>().fromJson(it) } .orEmpty()

[Goooler]

Suggested change

	return accountListJson?.let { moshi.adapter<List<ConversationAccountEntity>?>().fromJson(it) } ?: emptyList()
	return accountListJson?.let { moshi.adapter<List<ConversationAccountEntity>?>().fromJson(it) }.orEmpty()

[Goooler]

Suggested change

	return attachmentListJson?.let { moshi.adapter<List<Attachment>?>().fromJson(it) } ?: emptyList()
	return attachmentListJson?.let { moshi.adapter<List<Attachment>?>().fromJson(it) }.orEmpty()

[Goooler]

Suggested change

	return mentionListJson?.let { moshi.adapter<List<Status.Mention>?>().fromJson(it) } ?: emptyList()
	return mentionListJson?.let { moshi.adapter<List<Status.Mention>?>().fromJson(it) }.orEmpty()

[Goooler]

Suggested change

	return draftAttachmentListJson?.let { moshi.adapter<List<DraftAttachment>?>().fromJson(it) } ?: emptyList()
	return draftAttachmentListJson?.let { moshi.adapter<List<DraftAttachment>?>().fromJson(it) }.orEmpty()

[Goooler]

Suggested change

	get() {
	return text == null && attachments.isEmpty()
	}
	get() = text == null && attachments.isEmpty()

[Goooler]

Suggested change

	get() {
	return when (this) {
	PUBLIC -> "public"
	UNLISTED -> "unlisted"
	PRIVATE -> "private"
	DIRECT -> "direct"
	UNKNOWN -> "unknown"
	}
	get() {
	return when (this) {
	PUBLIC -> "public"
	UNLISTED -> "unlisted"
	PRIVATE -> "private"
	DIRECT -> "direct"
	UNKNOWN -> "unknown"
	}

Suggested change

	get() {
	return when (this) {
	PUBLIC -> "public"
	UNLISTED -> "unlisted"
	PRIVATE -> "private"
	DIRECT -> "direct"
	UNKNOWN -> "unknown"
	}
	get() = when (this) {
	PUBLIC -> "public"
	UNLISTED -> "unlisted"
	PRIVATE -> "private"
	DIRECT -> "direct"
	UNKNOWN -> "unknown"
	}

[charlag]

If we are going to use codegen anyway, why not Kotlin Serialization?

[charlag]

this is genuine question btw, amazing work too!

[cbeyls]

If we are going to use codegen anyway, why not Kotlin Serialization?

I don't know kotlinx serialization well enough to be able to write a custom streaming deserializer or implement something equivalent to GuardedAdapter. But If someone wants to do it, be my guest 😄

I'm going to travel for 3 weeks without a laptop, so I won't be able to update this branch until March 27. Fell free to do more tests and reviews in the meantime.

[Goooler]

It could be something like

import kotlinx.serialization.KSerializer
import kotlinx.serialization.Serializer
import kotlinx.serialization.encoding.Decoder
import kotlinx.serialization.encoding.Encoder
import kotlinx.serialization.json.Json
import kotlinx.serialization.json.JsonElement
import kotlinx.serialization.json.JsonNull

@Serializer(forClass = T::class)
object GuardedSerializer : KSerializer<T?> {
    private val json = Json { ignoreUnknownKeys = true }

    override val descriptor = SerialDescriptor("WithCustomDefault", StructureKind.OBJECT)

    override fun serialize(encoder: Encoder, value: T?) {
        // Write the value to the encoder
        encoder.encodeSerializableValue(json.encodeToJsonElement(value))
    }

    override fun deserialize(decoder: Decoder): T? {
        return try {
            // Try to decode the value
            json.decodeFromJsonElement(decoder.decodeSerializableValue(JsonElement.serializer()))
        } catch (e: Exception) {
            // If there's an error, return null
            null
        }
    }
}

[connyduck]

I found one bug with my mastodon.social account

Failed to load timeline com.squareup.moshi.JsonDataException: Required value 'keywords' missing at $[0].filtered[0].filter

[connyduck]

Suggested change

	val attachments: List<DraftAttachment> = buildList {
	val attachments: List<DraftAttachment> = buildList(mediaUris.size) {

[connyduck]

I wonder if this is still necessary. It only helps users that have unedited drafts that are over two years old. 🤔

[cbeyls]

This is indeed legacy code that adds unnecessary complexity to the project. I'll remove it.

[cbeyls]

I found one bug with my mastodon.social account

Failed to load timeline com.squareup.moshi.JsonDataException: Required value 'keywords' missing at $[0].filtered[0].filter

I made the "keywords" field optional with a default value of empty list. This is one of the cases where a non-null Kotlin field ends up null at runtime with Gson. I already fixed one or two more in the initial code I submitted.

[cbeyls]

If we are going to use codegen anyway, why not Kotlin Serialization?

Note that contrary to Moshi, the Kotlin Serialization adapter for Retrofit currently does not support streaming: the entire network response is first loaded into a big String, then that String is parsed. Moshi works directly at the byte stream level using Okio, which makes it more efficient in theory.

[charlag]

this is a good point, thank you

[connyduck]

I found one bug with my mastodon.social account
Failed to load timeline com.squareup.moshi.JsonDataException: Required value 'keywords' missing at $[0].filtered[0].filter

I made the "keywords" field optional with a default value of empty list. This is one of the cases where a non-null Kotlin field ends up null at runtime with Gson. I already fixed one or two more in the initial code I submitted.

Now I'm getting com.squareup.moshi.JsonDataException: Non-null value 'statusMatches' (JSON name 'status_matches') was null at $[1].filtered[0].status_matches

[cbeyls]

Now I'm getting com.squareup.moshi.JsonDataException: Non-null value 'statusMatches' (JSON name 'status_matches') was null at $[1].filtered[0].status_matches

The two list fields in FilterResult (currently unused in the code) are actually nullable according to the API documentation. I changed their declaration to reflect that. I need to check if other nullable lists exist in the Mastodon API.

[connyduck]

Loading the timeline works now and I can't find any other problems!

If they are unused we should probably comment them out to save some deserializing time and memory.

[cbeyls]

I went trough the entire Mastodon API entities documentation to find remaining optional fields that were declared as mandatory, as well as nullable fields that were declared as non-null.
I'm now quite confident that the code is stable and ready to be merged after a last round of tests.

[Tak]

Huh, I'm still getting the Filter.keywords exception after merging 6be3f07 locally - if I fix that, everything appears to be working normally for me

[cbeyls]

Huh, I'm still getting the Filter.keywords exception after merging 6be3f07 locally - if I fix that, everything appears to be working normally for me

Oops, I just realized I reverted my own change in the recent commit. Turns out the documentation says this field is mandatory but it's in fact optional. I added a comment to avoid a future mistake.