# RFC 008: Don't Panic

## Changelog

- 2021-12-17: initial draft (@tychoish)

## Abstract

Today, the Tendermint core codebase has panics in a number of cases as
a response to exceptional situations. These panics complicate testing,
and might make tendermint components difficult to use as a library in
some circumstances. This document outlines a project of converting
panics to errors and describes the situations where its safe to
panic.

## Background

Panics in Go are a great mechanism for aborting the current execution
for truly exceptional situations (e.g. memory errors, data corruption,
processes initialization); however, because they resemble exceptions
in other languages, it can be easy to over use them in the
implementation of software architectures. This certainly happened in
the history of Tendermint, and as we embark on the project of
stabilizing the package, we find ourselves in the right moment to
reexamine our use of panics, and largely where panics happen in the
code base.

There are still some situations where panics are acceptable and
desirable, but it's important that Tendermint, as a project, comes to
consensus--perhaps in the text of this document--on the situations
where it is acceptable to panic.

### References

- [Defer Panic and Recover](https://go.dev/blog/defer-panic-and-recover)
- [Why Go gets exceptions right](https://dave.cheney.net/tag/panic)
- [Don't panic](https://dave.cheney.net/practical-go/presentations/gophercon-singapore-2019.html#_dont_panic)

## Discussion

### Acceptable Panics

#### Initialization

It is unambiguously safe (and desirable) to panic in `init()`
functions in response to any kind of error. These errors are caught by
tests, and occur early enough in process initialization that they
won't cause unexpected runtime crashes.

Other code that is called early in process initialization MAY panic,
in some situations if it's not possible to return an error or cause
the process to abort early, although these situations should be
vanishingly slim.

#### Data Corruption

If Tendermint code encounters an inconsistency that could be
attributed to data corruption or a logical impossibility it is safer
to panic and crash the process than continue to attempt to make
progress in these situations.

Examples including reading data out of the storage engine that
is invalid or corrupt, or encountering an ambiguous situation where
the process should halt. Generally these forms of corruption are
detected after interacting with a trusted but external data source,
and reflect situations where the author thinks its safer to terminate
the process immediately rather than allow execution to continue.

#### Unrecoverable Consensus Failure

In general, a panic should be used in the case of unrecoverable
consensus failures. If a process detects that the network is
behaving in an incoherent way and it does not have a clearly defined
and mechanism for recovering, the process should panic.

#### Static Validity

It is acceptable to panic for invariant violations, within a library
or package, in situations that should be statically impossible,
because there is no way to make these kinds of assertions at compile
time.

For example, type-asserting `interface{}` values returned by
`container/list` and `container/heap` (and similar), is acceptable,
because package authors should have exclusive control of the inputs to
these containers. Packages should not expose the ability to add
arbitrary values to these data structures.

#### Controlled Panics Within Libraries

In some algorithms with highly recursive structures or very nested
call patterns, using a panic, in combination with conditional recovery
handlers results in more manageable code. Ultimately this is a limited
application, and implementations that use panics internally should
only recover conditionally, filtering out panics rather than ignoring
or handling all panics.

#### Request Handling

Code that handles responses to incoming/external requests
(e.g. `http.Handler`) should avoid panics, but practice this isn't
totally possible, and it makes sense that request handlers have some
kind of default recovery mechanism that will prevent one request from
terminating a service.

### Unacceptable Panics

In **no** other situation is it acceptable for the code to panic:

- there should be **no** controlled panics that callers are required
  to handle across library/package boundaries.
- callers of library functions should not expect panics.
- ensuring that arbitrary go routines can't panic.
- ensuring that there are no arbitrary panics in core production code,
  especially code that can run at any time during the lifetime of a
  process.
- all test code and fixture should report normal test assertions with
  a mechanism like testify's `require` assertion rather than calling
  panic directly.

The goal of this increased "panic rigor" is to ensure that any escaped
panic is reflects a fixable bug in Tendermint.

### Removing Panics

The process for removing panics involve a few steps, and will be part
of an ongoing process of code modernization:

- converting existing explicit panics to errors in cases where it's
  possible to return an error, the errors can and should be handled, and returning
  an error would not lead to data corruption or cover up data
  corruption.

- increase rigor around operations that can cause runtime errors, like
  type assertions, nil pointer errors, array bounds access issues, and
  either avoid these situations or return errors where possible.

- remove generic panic handlers which could cover and hide known
  panics.
