add documentation for error cases

rust/protocol/src/error.rs

@@ -1,8 +1,12 @@
 //
-// Copyright 2020-2021 Signal Messenger, LLC.
+// Copyright 2020-2022 Signal Messenger, LLC.
 // SPDX-License-Identifier: AGPL-3.0-only
 //
 
+//! Errors that may occur during various stages of the Signal Protocol.
+
+#![warn(missing_docs)]
+
 use crate::curve::KeyType;
 
 use displaydoc::Display;
@@ -11,82 +15,208 @@ use uuid::Uuid;
 
 use std::panic::UnwindSafe;
 
+#[cfg(doc)]
+pub use crate::{
+    address::ProtocolAddress,
+    curve::{PrivateKey, PublicKey},
+    group_cipher::{group_decrypt, group_encrypt},
+    protocol::{SenderKeyMessage, SignalMessage, CIPHERTEXT_MESSAGE_CURRENT_VERSION},
+    sealed_sender::{sealed_sender_decrypt, sealed_sender_multi_recipient_encrypt},
+    sender_keys::SenderKeyRecord,
+    session_cipher::{message_decrypt, message_encrypt},
+    state::{PreKeyId, PreKeyRecord, SignedPreKeyId, SignedPreKeyRecord},
+    storage::{IdentityKeyStore, PreKeyStore, SenderKeyStore, SessionStore, SignedPreKeyStore},
+};
+
+/// Return type for all fallible operations in this crate.
 pub type Result<T> = std::result::Result<T, SignalProtocolError>;
 
+/// Error states recognized by the Signal Protocol.
 #[derive(Debug, Display, Error)]
+#[ignore_extra_doc_attributes]
 pub enum SignalProtocolError {
     /// invalid argument: {0}
+    ///
+    /// Raised if an invalid argument is provided to any Signal API methods.
+    ///
+    /// Prefer to use lifetimes, static-sized slices, and dedicated wrapper structs in API
+    /// signatures to minimize the need to raise this error to FFI boundaries.
     InvalidArgument(String),
     /// invalid state for call to {0} to succeed: {1}
+    ///
+    /// Raised if some optional value was missing before performing some operation which needed it.
+    ///
+    /// Prefer to avoid returning [std::result::Result] and [Option] from struct methods in cases
+    /// where they're not necessary for trait polymorphism, as well as using dedicated wrapper
+    /// structs in API signatures, to minimize the need to raise this error.
     InvalidState(&'static str, String),
 
     /// protobuf encoding was invalid
+    ///
+    /// Raised if a field in a protobuf is invalid in some way.
+    ///
+    /// Prefer to raise [Self::InvalidState] except in methods which directly decode protobufs.
     InvalidProtobufEncoding,
 
     /// ciphertext serialized bytes were too short <{0}>
+    ///
+    /// Raised if some ciphertext was deserialized from a too-small slice.
+    ///
+    /// Prefer to make API method signatures and wrapper structs consume and produce static-sized
+    /// byte slices to minimize the need to raise this error.
     CiphertextMessageTooShort(usize),
     /// ciphertext version was too old <{0}>
+    ///
+    /// Raised if the ciphertext version decoded from a protobuf is older than this client.
+    ///
+    /// The current client's ciphertext version is at [CIPHERTEXT_MESSAGE_CURRENT_VERSION].
     LegacyCiphertextVersion(u8),
     /// ciphertext version was unrecognized <{0}>
+    ///
+    /// Raised if the ciphertext version decoded from a protobuf is newer than this client.
+    ///
+    /// The current client's ciphertext version is at [CIPHERTEXT_MESSAGE_CURRENT_VERSION].
     UnrecognizedCiphertextVersion(u8),
     /// unrecognized message version <{0}>
+    ///
+    /// Raised if the ciphertext version decoded from a protobuf fails to match the cached version
+    /// for the message chain that message originates from.
+    ///
+    /// *TODO: This case should wrap the same numeric type as [Self::LegacyCiphertextVersion] and
+    /// [Self::UnrecognizedCiphertextVersion]. This dissonance is addressed in
+    /// <https://github.com/signalapp/libsignal-client/pull/289>.*
     UnrecognizedMessageVersion(u32),
 
     /// fingerprint version number mismatch them {0} us {1}
+    ///
+    /// Raised if a fingerprint version decoded from a protobuf has an unexpected value.
     FingerprintVersionMismatch(u32, u32),
     /// fingerprint parsing error
+    ///
+    /// Raised if a field in a fingerprint protobuf is invalid in some way.
+    ///
+    /// Similar to [Self::InvalidProtobufEncoding].
     FingerprintParsingError,
 
     /// no key type identifier
+    ///
+    /// Raised if a [PublicKey] is deserialized from an empty slice.
+    ///
+    /// Prefer to use static-sized slices in API method signatures and struct fields to minimize the
+    /// need to raise this error.
     NoKeyTypeIdentifier,
     /// bad key type <{0:#04x}>
+    ///
+    /// Raised if a [KeyType] decoded from a [u8] has an unrecognized value.
     BadKeyType(u8),
     /// bad key length <{1}> for key with type <{0}>
+    ///
+    /// Raised if a [PublicKey] or [PrivateKey] is deserialized from a slice of incorrect length.
+    ///
+    /// Prefer to use static-sized slices in API method signatures and struct fields to minimize the
+    /// need to raise this error.
     BadKeyLength(KeyType, usize),
 
     /// invalid signature detected
+    ///
+    /// Raised if signature validation fails for a [SignedPreKeyRecord] or a [SenderKeyMessage].
     SignatureValidationFailed,
 
     /// untrusted identity for address {0}
+    ///
+    /// Raised if an identity verification check fails in [message_encrypt] or [message_decrypt].
     UntrustedIdentity(crate::ProtocolAddress),
 
     /// invalid prekey identifier
+    ///
+    /// Raised if a [PreKeyId] could not be resolved to a [PreKeyRecord] by a [PreKeyStore].
     InvalidPreKeyId,
     /// invalid signed prekey identifier
+    ///
+    /// Raised if a [SignedPreKeyId] could not be resolved to a [SignedPreKeyRecord] by
+    /// a [SignedPreKeyStore].
     InvalidSignedPreKeyId,
 
     /// invalid MAC key length <{0}>
+    ///
+    /// Raised if a MAC key is deserialized from an incorrectly-sized slice.
+    ///
+    /// Prefer to use static-sized slices in API method signatures and struct fields to minimize the
+    /// need to raise this error.
     InvalidMacKeyLength(usize),
 
     /// missing sender key state for distribution ID {distribution_id}
-    NoSenderKeyState { distribution_id: Uuid },
+    ///
+    /// Raised if a [SenderKeyStore] is unable to locate a [SenderKeyRecord] for a given
+    /// *([ProtocolAddress], [Uuid])* pair in [group_encrypt] or [group_decrypt].
+    NoSenderKeyState {
+        /// Unique identifier for the sender key session.
+        distribution_id: Uuid,
+    },
 
     /// session with {0} not found
+    ///
+    /// Raised if an [IdentityKeyStore] does not contain an entry for a [ProtocolAddress], or
+    /// alternately if a [SessionStore] does not contain a session for a given [ProtocolAddress].
     SessionNotFound(crate::ProtocolAddress),
     /// invalid session: {0}
+    ///
+    /// Raised if a [SessionStore] does not contain a remote identity key to validate.
+    ///
+    /// Similar to [Self::InvalidState].
     InvalidSessionStructure(&'static str),
     /// invalid sender key session with distribution ID {distribution_id}
-    InvalidSenderKeySession { distribution_id: Uuid },
+    ///
+    /// Raised if a [SenderKeyStore] could not load the session for the given ID.
+    ///
+    /// Similar to [Self::NoSenderKeyState].
+    InvalidSenderKeySession {
+        /// Unique identifier for the sender key session.
+        distribution_id: Uuid,
+    },
     /// session for {0} has invalid registration ID {1:X}
+    ///
+    /// Raised if a sealed sender message has a registration id that doesn't map to any
+    /// known session.
     InvalidRegistrationId(crate::ProtocolAddress, u32),
 
     /// message with old counter {0} / {1}
+    ///
+    /// Raised if the same message is decrypted twice so it can be discarded.
     DuplicatedMessage(u32, u32),
     /// invalid {0:?} message: {1}
+    ///
+    /// Raised if a [SignalMessage] could not be decrypted or some field had an unexpected value.
+    ///
+    /// *TODO: what differentiates this from [Self::CiphertextMessageTooShort]?*
     InvalidMessage(crate::CiphertextMessageType, &'static str),
 
     /// error while invoking an ffi callback: {0}
+    ///
+    /// Raised to propagate an error from an FFI callback.
     FfiBindingError(String),
     /// error in method call '{0}': {1}
+    ///
+    /// Raised to propagate an error through to an FFI exception along with a boxed handle.
     ApplicationCallbackError(
         &'static str,
         #[source] Box<dyn std::error::Error + Send + Sync + UnwindSafe + 'static>,
     ),
 
     /// invalid sealed sender message: {0}
+    ///
+    /// Raised if an [crate::sealed_sender::UnidentifiedSenderMessage] could not be
+    /// deserialized successfully.
+    ///
+    /// *TODO: this sounds a lot like [Self::InvalidProtobufEncoding] or [Self::UntrustedIdentity]?*
     InvalidSealedSenderMessage(String),
     /// unknown sealed sender message version {0}
+    ///
+    /// Raised if an version decoded from a [crate::sealed_sender::UnidentifiedSenderMessage]
+    /// was unrecognized.
     UnknownSealedSenderVersion(u8),
     /// self send of a sealed sender message
+    ///
+    /// Raised if [sealed_sender_decrypt] finds that the message came from this exact client.
     SealedSenderSelfSend,
 }