Merge pull request juju#18431 from SimonRichardson/charm-downloader-a…

…nd-store

juju#18431

Infrastructure work for downloading a charm asynchronously. This supersedes the internal/charm/downloader, which will be removed once the async downloading of a charm is done.

This correctly handles the downloading of a charm, checking the integrity of a file once downloaded against the source of the file, and the removal of files on errors. By doing less, we can achieve more.

----

This pull request introduces several new functionalities and improvements, including the addition of a `String` method to the `Origin` struct, new charm store functionalities, and enhancements to the charm downloader. The most important changes are summarized below:

### New functionalities and methods:

* Added a `String` method to the `Origin` struct in `core/charm/origin.go` to provide a formatted string representation of the `Origin` object.

### Charm store enhancements:

* Introduced the `CharmStore` struct and implemented methods for storing charms, including error handling for file not found and hash checking, in `domain/application/charm/store/store.go`.
* Added unit tests for the `CharmStore` functionalities in `domain/application/charm/store/store_test.go`.
* Generated mock implementations for `ObjectStore` and `ModelObjectStoreGetter` interfaces using `go:generate` in `domain/application/charm/store/store_mock_test.go`.

### Charm downloader improvements:

* Added the `CharmDownloader` struct and implemented methods for downloading charms, verifying their hashes, and handling temporary files in `internal/charm/charmdownloader/downloader.go`.
* Documented the purpose and functionality of the `charmdownloader` package in `internal/charm/charmdownloader/doc.go`.

These changes enhance the charm store and downloader functionalities, improve error handling, and add comprehensive unit tests and documentation.

----

## QA Steps

Regression testing should be employed, all of this is just infrastructure work, broken off the juju#18398 PR.

## Jira

https://warthogs.atlassian.net/browse/JUJU-7166

core/charm/origin.go

@@ -63,6 +63,11 @@ func (o Origin) Validate() error {
 	return nil
 }
 
+func (o Origin) String() string {
+	return fmt.Sprintf("(source: %q, id: %s, hash: %s, revision: %v, channel: %v, platform: %s)",
+		o.Source, o.ID, o.Hash, o.Revision, o.Channel, o.Platform)
+}
+
 // Platform describes the platform used to install the charm with.
 type Platform struct {
 	Architecture string

domain/application/charm/store/package_test.go

@@ -0,0 +1,16 @@
+// Copyright 2024 Canonical Ltd.
+// Licensed under the AGPLv3, see LICENCE file for details.
+
+package store
+
+import (
+	"testing"
+
+	gc "gopkg.in/check.v1"
+)
+
+//go:generate go run go.uber.org/mock/mockgen -typed -package store -destination store_mock_test.go github.com/juju/juju/core/objectstore ObjectStore,ModelObjectStoreGetter
+
+func TestPackage(t *testing.T) {
+	gc.TestingT(t)
+}

domain/application/charm/store/store.go

@@ -0,0 +1,68 @@
+// Copyright 2024 Canonical Ltd.
+// Licensed under the AGPLv3, see LICENCE file for details.
+
+package store
+
+import (
+	"context"
+	"encoding/base32"
+	"fmt"
+	"os"
+
+	"github.com/juju/juju/core/objectstore"
+	"github.com/juju/juju/internal/errors"
+	"github.com/juju/juju/internal/uuid"
+)
+
+const (
+	// ErrNotFound is returned when the file is not found.
+	ErrNotFound = errors.ConstError("file not found")
+)
+
+// CharmStore provides an API for storing charms.
+type CharmStore struct {
+	objectStoreGetter objectstore.ModelObjectStoreGetter
+	encoder           *base32.Encoding
+}
+
+// NewCharmStore returns a new charm store instance.
+func NewCharmStore(objectStoreGetter objectstore.ModelObjectStoreGetter) *CharmStore {
+	return &CharmStore{
+		objectStoreGetter: objectStoreGetter,
+		encoder:           base32.StdEncoding.WithPadding(base32.NoPadding),
+	}
+}
+
+// Store the charm at the specified path into the object store. It is expected
+// that the archive already exists at the specified path. If the file isn't
+// found, a [ErrNotFound] is returned.
+func (s *CharmStore) Store(ctx context.Context, name string, path string, size int64, hash string) (objectstore.UUID, error) {
+	objectStore, err := s.objectStoreGetter.GetObjectStore(ctx)
+	if err != nil {
+		return "", errors.Errorf("getting object store: %w", err)
+	}
+
+	file, err := os.Open(path)
+	if errors.Is(err, os.ErrNotExist) {
+		return "", errors.Errorf("%q: %w", path, ErrNotFound)
+	} else if err != nil {
+		return "", errors.Errorf("cannot open file %q: %w", path, err)
+	}
+
+	// Ensure that we close any open handles to the file.
+	defer file.Close()
+
+	// Generate a unique path for the file.
+	unique, err := uuid.NewUUID()
+	if err != nil {
+		return "", errors.Errorf("cannot generate unique path")
+	}
+
+	// The name won't be unique and that's ok. In that case, we'll slap a
+	// unique identifier at the end of the name. This can happen if you have
+	// a charm with the same name but different content.
+	uniqueName := fmt.Sprintf("%s-%s", name, s.encoder.EncodeToString(unique[:]))
+
+	// Store the file in the object store.
+	return objectStore.PutAndCheckHash(ctx, uniqueName, file, size, hash)
+}