Tracking Issue: Postcard Q2 '25 Triage

[jamesmunns]

I've been relatively busy lately, and the issues and PRs have backed up. I'm going to attempt to triage the state of the Postcard Cinematic Universe, and get things to a maintainable place.

My priorities are:

• Merge any relevant open PRs
• Cut non-breaking releases when possible to unblock people waiting on features
• Where necessary, backport any changes (maybe postcard-schema changes)
• Where necessary, prepare for breaking changes (e.g. postcard 2.0, postcard-schema changes)
• Discuss future plans for more researchy ventures, like ergot

I can't promise how long it will take to get everything in this triage list sorted, but hopefully triaging will make it easier to track progress. If you are a company that uses postcard, I am currently available starting July via my company. Having this work sponsored would help me prioritize this.

This triage includes:

• postcard
  • 1.x release branch
  • 2.x work
• postcard-schema
  • 0.2.x release branch
  • 0.3.x work
• postcard-dyn
• postcard-derive
• postcard-rpc
  • prpcrtt
• cobs

Feel free to follow this issue. Please do not ping me asking when merges/releases will occur. If you are someone interested in helping with this process, and have been an active contributor before, feel free to ping me to be added as a maintainer. I probably will not accept if you aren't someone I've interacted with in the context of postcard or in the various embedded chats.

CC @robamu @max-heller

[jamesmunns]

postcard (1.x)

Issues

issues related to the current stable release of the postcard crate. Currently unsorted/prio'd.

• "Error" related issues - these might end up being moved to postcard 2.0 changes, if it requires breaking changes to the errors used by postcard.
  • Errors are not properly propagated #38
  • Display Custom Error Messages #80
  • Implement MaxSize for postcard::Error #224
  • More detailled reporting of errors from flavors #232 - (also flavors)
• "Flavor" related issues
  • Add MaybeUninit flavor #59
  • Consider splitting try_take_n on de_flavors::Flavor #93
  • Thoughts on implementing bytes::BufMut and friends for reducing/eliminating memory moves? #131 - might be postcard-rpc oriented too.
  • Support serde_spanned::Spanned<T> deserialization #248 - maybe only vaguely flavor related?
• io/embedded-io related issues
  • How should the size of from_io's "sliding buffer" be chosen? #140
  • Support dynamic buffer allocation for from_io #162
• heapless related issues
  • Decide how to deal with heapless-v0.8 #121 - we might want to bridge postcard 1.0 and 2.0 and have better namespaced/featured items
  • Un-revert heapless change #165
• fixint related issues
  • Feature request: configure fixint at the container level #95
  • fixint for Option #216
• Document trust/safety for Postcard Deserializer #48 - docs improvement
• How to read partially corrupted stream of messages with COBS encoding? #71
  • Related: take_from_bytes_cobs error should return the remainder of the buffer #203
  • Might be moved to postcard 2.0 if it requires breaking changes to implement.
• Enum encoding / decoding inconsistencies between platforms #110 - needs more info/repro
• Postcard error: Found a varint that didn't terminate. Is the usize too big for this platform? #113 - might be a weird custom serde_with error?
• Deserializer::deserialize_struct is incorrect #114 - could be a bug? Might help with other serde attrs?
• Document how to use CRC combined with COBS #117
• Remove changelog #207
• When can serialized_size fail and is it still experimental? #225

PRs

• "Error" related PRs
  • Display Custom Error Messages With alloc Feature #81 - related to Display Custom Error Messages #80
  • Do not discard error messages when using std #157 - related to Errors are not properly propagated #38 and Display Custom Error Messages #80
    • Breaking change, defer to postcard 2.0
• "Fixint" related PRs
  • Attempts at more terse fixint opt-ins #96 - related to Feature request: configure fixint at the container level #95
• "eio"/"io" related PRs
  • Support std::io::Read flavor with minimal temporary buffer #212 - related to How should the size of from_io's "sliding buffer" be chosen? #140
• Re-export used crates #116
• tinyvec #126 - out of date, we will probably move most flavor impls into a separate crate as part of 2.0
• Add a performant way to (de-)serialize fixed-size byte arrays #127
• Add support for skipped fields #174 - likely not reasonable to merge.
• allow to_extend to work with reference to write #210 - alternative to in to_extend take collection by mutable reference #208 which is a breaking change, not sure which is more reasonable.
• Allow using postcard without serde derive #219
• Update cobs to 0.3.0 #226 - I need to check if this is a breaking change
• Update postcard’s dep. on postcard-derive to 0.2 #228 - I need to check if this is a breaking change
• Version bump defmt to 1.x.x #234 - probably not a breaking change.
• Fix missing license files in published crates #227

[jamesmunns]

postcard-schema (0.2.x)

Issues

Issues related to the current stable release of the postcard-schema crate. Currently unsorted.

• Consider using type_name() when it stabilizes #68 - still waiting on rustc stabilization, will likely get moved to postcard-schema v0.3/"future".
• Textual representation of the wire format of a type (recursively) #143
• schema: How to support niches like "byte array"? #147

PRs

• MaxSize as a const fn #179 - out of date, might be better handled otherwise.
• Add postcard-schema implementations for types from fixed #221 - might want forwards porting to 0.3.x too
• Implement Schema for serde_big_array::Array type. #231 - might want forwards porting to 0.3.x too
• Mark Key::from_bytes as safe #236
• Implement Schema and MaxSize for more core types #240

[jamesmunns]

postcard-dyn

Issues

None, currently.

PRs

None, currently

[jamesmunns]

postcard-derive

Issues

Issues related to the current stable release of the postcard-derive crate.

• Consider optional "snapshot" feature for postcard-schema #204

PRs

• TODO

[jamesmunns]

postcard-rpc

Issues

Issues related to the current stable release of the postcard-rpc crate.

• "ergot would fix this" issues
  • host_server and target_client libraries postcard-rpc#37
  • [server] How to support multiple interfaces at once? postcard-rpc#67
  • [server] Should we support "internal" interfaces? postcard-rpc#68
  • [server] Consider breaking message handling into a three-stage pipeline postcard-rpc#69 - ergot sort of handles this, mostly because you need to split up interfaces and server handlers
  • [protocol] Allow for "reverse endpoints"? postcard-rpc#74
• "transport" issues
  • Support for other transports (SPI, BLE, TCP/IP, RTT, ...) postcard-rpc#26
  • Upstream the TCP host-client impl from poststation-sdk postcard-rpc#86
  • Question for use in an application... postcard-rpc#90
  • Look at how we handle "wait for connection" in Wire traits postcard-rpc#101 - might require breaking change
  • Host does not parse data over 64 bytes long postcard-rpc#108
• "macros are sad" issues
  • endpoints! and topics! macros don't accept generic type parameters postcard-rpc#56
  • [server] How to handle "reusable" endpoint/topic definitions? postcard-rpc#71
  • postcard-rpc: pull in postcard-schema directly postcard-rpc#105
  • define_dispatch incompatible with deny(missing_docs) postcard-rpc#106
  • Support generics in Context postcard-rpc#119
• "Errors" issues
  • Revisit HostClient Errors postcard-rpc#89 - might require breaking change
  • Review Errors and make them #[non_exhaustive] postcard-rpc#100
  • • postcard_rpc::host_client::IoClosed should implement core::error::Error postcard-rpc#79
• Use diagnostic for target server trait postcard-rpc#13
• Move to main postcard repo? postcard-rpc#22
• Why does WireTx require send_log_{str,fmt}? postcard-rpc#61
• wire receive error on host postcard-rpc#62
• [client] Take more advantage of pipelined NUSB sending postcard-rpc#70
• Example comms-01 doesn't call ping_handler on embedded device postcard-rpc#75
• dead overview link on crates.io postcard-rpc#95
• docs: "how to think in postcard-rpc" postcard-rpc#102
• Global logger for defmt postcard-rpc#103
• Expose metadata about the device postcard-rpc#114
• Misleading compiler error when using &strs instead of Strings with hostclient postcard-rpc#117
• add impl Debug for HostClient postcard-rpc#118

PRs

• "Transport" PRs
  • [server] Add support for embedded-io-async v0.6 postcard-rpc#83
  • Add ESP32C6 embedded-io-async example postcard-rpc#96 (example for the above)
• "Macros are sad" PRs
  • define_dispatch: add docs for app type postcard-rpc#109 - related to define_dispatch incompatible with deny(missing_docs) postcard-rpc#106
  • add postcard & postcard-rpc re-exports for macros postcard-rpc#110 - related to postcard-rpc: pull in postcard-schema directly postcard-rpc#105
• replace embassy_usb_driver import with re-export from embassy_usb postcard-rpc#49
• feat: ariel-os example initial postcard-rpc#97
• add necessary #[derive(Debug)] for HostClient postcard-rpc#111
• add impl Error for WireError postcard-rpc#112
• Update embassy-sync to 0.7 postcard-rpc#113
• Fix incorrect trailing comma in book postcard-rpc#115
• add udev rules example to docs postcard-rpc#116
• Update for postcard-schema-ng traits and types. postcard-rpc#120

prpcrtt

Work required to get RTT support as a transport added as a feature for postcard-rpc.

[jamesmunns]

cobs

Issues related to the current stable release of the cobs crate.

• TODO

[jamesmunns]

postcard-schema (0.3.x)

Issues

Issues related to the currently breaking changes to postcard-schema, and towards a new release.

• Desirable changes for postcard-schema #205
• [postcard-schema] Key hash calculation could suffer collision if path and schema alias #217

PRs

• postcard-dyn: reserialize from arbitrary Deserializer into arbitrary Serializer #194

[jamesmunns]

postcard (2.x)

Issues

Issues related to a new major library version of postcard. No breaking changes to the 1.0 wire format will be made.

• Consider removing O type parameter of serialize_with_flavor #103
• Decide how to deal with heapless-v0.8 #121
• Rename feature use-std to just std #124
• Can the to_allocvec function ever return an Err? #132
• Deserializing corrupt bytes to Vec panics on low-memory devices #135 - maybe be smarter when it comes to allocation and refuse to allocate (dramatically) more space than is in the received message? We may not always know the remaining message, if we are doing some kind of streaming.
• RUSTSEC-2023-0089: Unmaintained warning, because of atomic-polyfill -> heapless dependency #223

PRs

• Depend directly on serde_derive #130
• in to_extend take collection by mutable reference #208

[jamesmunns]

Outdated/Wontfix

Outdated/Wontfix issues/PRs closed as part of this triaging effort.

• Serialize/deserialize with footer? #16
• From Read / To Write #27
• Add public method to return 'input' from Deserializer #28
• How to implement forward compatibility #75
• Can serialize serde_json::Value, but can't deserialize it #77
• Failing to deserialize  #79
• Builds targeting AVR fail with an error #82
• Question: how to explore a data dump? #84
• Use Cases for a "self describing postcard" #92
• Serializing NaiveDatetime on AVR crashes #99
• Specify the discriminant for enum variants #109
• RFC: Thoughts on Postcard 2.0? #128
• Feature request: partial backwards compatibility #159
• Consider cutting new release #229
• postcard-schema bump to 0.2.2? #238
• COBS with footer PoC #17
• examples have a ping method that is not called postcard-rpc#98
• Upgrade heapless to 0.8. #244

Not Now

These are issues/PRs that definitely won't be handled as part of this triage

• Revisit char encoding #101 - needs a breaking wire format change
• Tracking issue: Unsupported serde attributes #125 - issue with serde itself
• Add initial implementation of Postcard-C #220