feat: ADR 040: New DB interface (#9573)

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## New DB interface to replace `tm-db`

This introduces a new interface to wrap the backend KV store DB while supporting versioning and ACID transactions.
Part of [ADR-040](https://github.com/cosmos/cosmos-sdk/blob/eb7d939f86c6cd7b4218492364cdda3f649f06b5/docs/architecture/adr-040-storage-and-smt-state-commitments.md) changes.

This has been revised to consist of only the interface types. All implementations and utilities will be in follow-up PRs.

Partially resolves: vulcanize#2

---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [x] added `!` to the type prefix if API or client breaking change
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [x] provided a link to the relevant issue or specification
- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules) - N/A
- [x] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [x] added a changelog entry to `CHANGELOG.md`
- [x] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [x] updated the relevant documentation or specification
- [x] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

CHANGELOG.md

@@ -128,6 +128,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 * [\#9540](https://github.com/cosmos/cosmos-sdk/pull/9540) Add output flag for query txs command.
 * (errors) [\#8845](https://github.com/cosmos/cosmos-sdk/pull/8845) Add `Error.Wrap` handy method
 * [\#8518](https://github.com/cosmos/cosmos-sdk/pull/8518) Help users of multisig wallets debug signature issues.
+* [\#9573](https://github.com/cosmos/cosmos-sdk/pull/9573) ADR 040 implementation: New DB interface
 
 
 ### Client Breaking Changes

db/go.mod

@@ -0,0 +1,7 @@
+go 1.15
+
+module github.com/cosmos/cosmos-sdk/db
+
+require (
+	github.com/stretchr/testify v1.7.0
+)

db/go.sum

@@ -0,0 +1,11 @@
+github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8=
+github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
+github.com/stretchr/testify v1.7.0 h1:nwc3DEeHmmLAfoZucVR881uASk0Mfjw8xYJ99tb5CcY=
+github.com/stretchr/testify v1.7.0/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c h1:dUUwHk2QECo/6vqA44rthZ8ie2QXMNeKRTHCNY2nXvo=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

db/types.go

@@ -0,0 +1,195 @@
+package db
+
+import "errors"
+
+var (
+	// ErrBatchClosed is returned when a closed or written batch is used.
+	ErrBatchClosed = errors.New("batch has been written or closed")
+
+	// ErrKeyEmpty is returned when attempting to use an empty or nil key.
+	ErrKeyEmpty = errors.New("key cannot be empty")
+
+	// ErrValueNil is returned when attempting to set a nil value.
+	ErrValueNil = errors.New("value cannot be nil")
+
+	// ErrVersionDoesNotExist is returned when a DB version does not exist.
+	ErrVersionDoesNotExist = errors.New("version does not exist")
+
+	// ErrOpenTransactions is returned when open transactions exist which must
+	// be discarded/committed before an operation can complete.
+	ErrOpenTransactions = errors.New("open transactions exist")
+
+	// ErrReadOnly is returned when a write operation is attempted on a read-only transaction.
+	ErrReadOnly = errors.New("cannot modify read-only transaction")
+
+	// ErrInvalidVersion is returned when an operation attempts to use an invalid version ID.
+	ErrInvalidVersion = errors.New("invalid version")
+)
+
+// DBConnection represents a connection to a versioned database.
+// Records are accessed via transaction objects, and must be safe for concurrent creation
+// and read and write access.
+// Past versions are only accessible read-only.
+type DBConnection interface {
+	// Opens a read-only transaction at the current working version.
+	Reader() DBReader
+
+	// Opens a read-only transaction at a specified version.
+	// Returns ErrVersionDoesNotExist for invalid versions.
+	ReaderAt(uint64) (DBReader, error)
+
+	// Opens a read-write transaction at the current version.
+	ReadWriter() DBReadWriter
+
+	// Opens a write-only transaction at the current version.
+	Writer() DBWriter
+
+	// Returns all saved versions
+	Versions() (VersionSet, error)
+
+	// Saves the current contents of the database and returns the next version ID, which will be
+	// `Versions().Last()+1`.
+	// Returns an error if any open DBWriter transactions exist.
+	// TODO: rename to something more descriptive?
+	SaveNextVersion() (uint64, error)
+
+	// Attempts to save database at a specific version ID, which must be greater than or equal to
+	// what would be returned by `SaveNextVersion`.
+	// Returns an error if any open DBWriter transactions exist.
+	SaveVersion(uint64) error
+
+	// Deletes a saved version. Returns ErrVersionDoesNotExist for invalid versions.
+	DeleteVersion(uint64) error
+
+	// Close closes the database connection.
+	Close() error
+}
+
+// DBReader is a read-only transaction interface. It is safe for concurrent access.
+// Callers must call Discard when done with the transaction.
+//
+// Keys cannot be nil or empty, while values cannot be nil. Keys and values should be considered
+// read-only, both when returned and when given, and must be copied before they are modified.
+type DBReader interface {
+	// Get fetches the value of the given key, or nil if it does not exist.
+	// CONTRACT: key, value readonly []byte
+	Get([]byte) ([]byte, error)
+
+	// Has checks if a key exists.
+	// CONTRACT: key, value readonly []byte
+	Has(key []byte) (bool, error)
+
+	// Iterator returns an iterator over a domain of keys, in ascending order. The caller must call
+	// Close when done. End is exclusive, and start must be less than end. A nil start iterates
+	// from the first key, and a nil end iterates to the last key (inclusive). Empty keys are not
+	// valid.
+	// CONTRACT: No writes may happen within a domain while an iterator exists over it.
+	// CONTRACT: start, end readonly []byte
+	Iterator(start, end []byte) (Iterator, error)
+
+	// ReverseIterator returns an iterator over a domain of keys, in descending order. The caller
+	// must call Close when done. End is exclusive, and start must be less than end. A nil end
+	// iterates from the last key (inclusive), and a nil start iterates to the first key (inclusive).
+	// Empty keys are not valid.
+	// CONTRACT: No writes may happen within a domain while an iterator exists over it.
+	// CONTRACT: start, end readonly []byte
+	// TODO: replace with an extra argument to Iterator()?
+	ReverseIterator(start, end []byte) (Iterator, error)
+
+	// Discards the transaction, invalidating any future operations on it.
+	Discard()
+}
+
+// DBWriter is a write-only transaction interface.
+// It is safe for concurrent writes, following an optimistic (OCC) strategy, detecting any write
+// conflicts and returning an error on commit, rather than locking the DB.
+// This can be used to wrap a write-optimized batch object if provided by the backend implementation.
+type DBWriter interface {
+	// Set sets the value for the given key, replacing it if it already exists.
+	// CONTRACT: key, value readonly []byte
+	Set([]byte, []byte) error
+
+	// Delete deletes the key, or does nothing if the key does not exist.
+	// CONTRACT: key readonly []byte
+	Delete([]byte) error
+
+	// Flushes pending writes and discards the transaction.
+	Commit() error
+
+	// Discards the transaction, invalidating any future operations on it.
+	Discard()
+}
+
+// DBReadWriter is a transaction interface that allows both reading and writing.
+type DBReadWriter interface {
+	DBReader
+	DBWriter
+}
+
+// Iterator represents an iterator over a domain of keys. Callers must call Close when done.
+// No writes can happen to a domain while there exists an iterator over it, some backends may take
+// out database locks to ensure this will not happen.
+//
+// Callers must make sure the iterator is valid before calling any methods on it, otherwise
+// these methods will panic. This is in part caused by most backend databases using this convention.
+//
+// As with DBReader, keys and values should be considered read-only, and must be copied before they are
+// modified.
+//
+// Typical usage:
+//
+// var itr Iterator = ...
+// defer itr.Close()
+//
+// for ; itr.Valid(); itr.Next() {
+//   k, v := itr.Key(); itr.Value()
+//   ...
+// }
+// if err := itr.Error(); err != nil {
+//   ...
+// }
+type Iterator interface {
+	// Domain returns the start (inclusive) and end (exclusive) limits of the iterator.
+	// CONTRACT: start, end readonly []byte
+	Domain() (start []byte, end []byte)
+
+	// Next moves the iterator to the next key in the database, as defined by order of iteration;
+	// returns whether the iterator is valid. Once invalid, it remains invalid forever.
+	Next() bool
+
+	// Key returns the key at the current position. Panics if the iterator is invalid.
+	// CONTRACT: key readonly []byte
+	Key() (key []byte)
+
+	// Value returns the value at the current position. Panics if the iterator is invalid.
+	// CONTRACT: value readonly []byte
+	Value() (value []byte)
+
+	// Error returns the last error encountered by the iterator, if any.
+	Error() error
+
+	// Close closes the iterator, relasing any allocated resources.
+	Close() error
+}
+
+// VersionSet specifies a set of existing versions
+type VersionSet interface {
+	// Last returns the most recent saved version, or 0 if none.
+	Last() uint64
+	// Count returns the number of saved versions.
+	Count() int
+	// Iterator returns an iterator over all saved versions.
+	Iterator() VersionIterator
+	// Equal returns true iff this set is identical to another.
+	Equal(VersionSet) bool
+	// Exists returns true if a saved version exists.
+	Exists(uint64) bool
+}
+
+type VersionIterator interface {
+	// Next advances the iterator to the next element.
+	// Returns whether the iterator is valid; once invalid, it remains invalid forever.
+	Next() bool
+	// Value returns the version ID at the current position.
+	Value() uint64
+}