feat(charm): add charm archive retrieval domain method

Add a method to the charm service to retrieve a charm archive from the
object store. Return alongside the hash of the charm.

domain/application/charm/store/store.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"encoding/base32"
 	"fmt"
+	"io"
 	"os"
 
 	"github.com/juju/juju/core/objectstore"
@@ -19,7 +20,7 @@ const (
 	ErrNotFound = errors.ConstError("file not found")
 )
 
-// CharmStore provides an API for storing charms.
+// CharmStore provides an API for storing and retrieving charm blobs.
 type CharmStore struct {
 	objectStoreGetter objectstore.ModelObjectStoreGetter
 	encoder           *base32.Encoding
@@ -66,3 +67,19 @@ func (s *CharmStore) Store(ctx context.Context, name string, path string, size i
 	// Store the file in the object store.
 	return objectStore.PutAndCheckHash(ctx, uniqueName, file, size, hash)
 }
+
+// Get retrieves a ReadCloser for the charm archive at the give path from
+// the underlying storage.
+// NOTE: It is up to the caller to verify the integrity of the data from the charm
+// hash stored in DQLite.
+func (s *CharmStore) Get(ctx context.Context, archivePath string) (io.ReadCloser, error) {
+	store, err := s.objectStoreGetter.GetObjectStore(ctx)
+	if err != nil {
+		return nil, errors.Errorf("getting object store: %w", err)
+	}
+	reader, _, err := store.Get(ctx, archivePath)
+	if err != nil {
+		return nil, errors.Errorf("getting charm: %w", err)
+	}
+	return reader, nil
+}

domain/application/charm/store/store_test.go

@@ -10,6 +10,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"strings"
 
 	"github.com/juju/testing"
 	jc "github.com/juju/testing/checkers"
@@ -111,6 +112,30 @@ func (s *storeSuite) TestStoreFailed(c *gc.C) {
 	c.Assert(err, gc.ErrorMatches, ".*boom")
 }
 
+func (s *storeSuite) TestGet(c *gc.C) {
+	defer s.setupMocks(c).Finish()
+
+	archive := io.NopCloser(strings.NewReader("archive-content"))
+	s.objectStore.EXPECT().Get(gomock.Any(), "foo").Return(archive, 0, nil)
+
+	storage := NewCharmStore(s.objectStoreGetter)
+	reader, err := storage.Get(context.Background(), "foo")
+	c.Assert(err, jc.ErrorIsNil)
+	content, err := io.ReadAll(reader)
+	c.Assert(err, jc.ErrorIsNil)
+	c.Check(string(content), gc.Equals, "archive-content")
+}
+
+func (s *storeSuite) TestGetFailed(c *gc.C) {
+	defer s.setupMocks(c).Finish()
+
+	s.objectStore.EXPECT().Get(gomock.Any(), "foo").Return(nil, 0, errors.Errorf("boom"))
+
+	storage := NewCharmStore(s.objectStoreGetter)
+	_, err := storage.Get(context.Background(), "foo")
+	c.Assert(err, gc.ErrorMatches, ".*boom")
+}
+
 func (s *storeSuite) setupMocks(c *gc.C) *gomock.Controller {
 	ctrl := gomock.NewController(c)
 

domain/application/service/application_test.go

@@ -1236,6 +1236,7 @@ func (s *applicationWatcherServiceSuite) setupMocks(c *gc.C) *gomock.Controller
 		s.watcherFactory,
 		nil,
 		nil,
+		nil,
 		s.clock,
 		loggertesting.WrapCheckLog(c),
 	)

domain/application/service/charm.go

@@ -6,6 +6,7 @@ package service
 import (
 	"context"
 	"fmt"
+	"io"
 	"regexp"
 
 	"github.com/juju/errors"
@@ -17,6 +18,7 @@ import (
 	"github.com/juju/juju/domain/application/charm"
 	applicationerrors "github.com/juju/juju/domain/application/errors"
 	internalcharm "github.com/juju/juju/internal/charm"
+	internalerrors "github.com/juju/juju/internal/errors"
 )
 
 var (
@@ -99,7 +101,12 @@ type CharmState interface {
 	// GetCharmArchivePath returns the archive storage path for the charm using
 	// the charm ID. If the charm does not exist, a
 	// [applicationerrors.CharmNotFound] error is returned.
-	GetCharmArchivePath(ctx context.Context, charmID corecharm.ID) (string, error)
+	GetCharmArchivePath(context.Context, corecharm.ID) (string, error)
+
+	// GetCharmArchiveMetadata returns the archive storage path and hash for the
+	// charm using the charm ID.
+	// If the charm does not exist, a [errors.CharmNotFound] error is returned.
+	GetCharmArchiveMetadata(context.Context, corecharm.ID) (archivePath string, hash string, err error)
 
 	// IsCharmAvailable returns whether the charm is available for use. If the
 	// charm does not exist, a [applicationerrors.CharmNotFound] error is
@@ -137,6 +144,14 @@ type CharmState interface {
 	ListCharmsWithOriginByNames(ctx context.Context, names []string) ([]charm.CharmWithOrigin, error)
 }
 
+// CharmStore defines the interface for storing and retrieving charms archive blobs
+// from the underlying storage.
+type CharmStore interface {
+	// GetCharm retrieves a ReadCloser for the charm archive at the give path from
+	// the underlying storage.
+	Get(ctx context.Context, archivePath string) (io.ReadCloser, error)
+}
+
 // GetCharmID returns a charm ID by name. It returns an error if the charm
 // can not be found by the name.
 // This can also be used as a cheap way to see if a charm exists without
@@ -411,16 +426,40 @@ func (s *Service) GetCharmLXDProfile(ctx context.Context, id corecharm.ID) (inte
 // returned.
 func (s *Service) GetCharmArchivePath(ctx context.Context, id corecharm.ID) (string, error) {
 	if err := id.Validate(); err != nil {
-		return "", fmt.Errorf("charm id: %w", err)
+		return "", internalerrors.Errorf("charm id: %w", err)
 	}
 
 	path, err := s.st.GetCharmArchivePath(ctx, id)
 	if err != nil {
-		return "", errors.Trace(err)
+		return "", internalerrors.Errorf("getting charm archive path: %w", err)
 	}
 	return path, nil
 }
 
+// GetCharmArchive returns a ReadCloser stream for the charm archive for a given
+// charm id, along with the hash of the charm archive. Clients can use the hash
+// to verify the integrity of the charm archive.
+//
+// If the charm does not exist, a [applicationerrors.CharmNotFound] error is
+// returned.
+func (s *Service) GetCharmArchive(ctx context.Context, id corecharm.ID) (io.ReadCloser, string, error) {
+	if err := id.Validate(); err != nil {
+		return nil, "", internalerrors.Errorf("charm id: %w", err)
+	}
+
+	archivePath, hash, err := s.st.GetCharmArchiveMetadata(ctx, id)
+	if err != nil {
+		return nil, "", internalerrors.Errorf("getting charm archive metadata: %w", err)
+	}
+
+	reader, err := s.charmStore.Get(ctx, archivePath)
+	if err != nil {
+		return nil, "", internalerrors.Errorf("getting charm archive: %w", err)
+	}
+
+	return reader, hash, nil
+}
+
 // IsCharmAvailable returns whether the charm is available for use. This
 // indicates if the charm has been uploaded to the controller.
 // This will return true if the charm is available, and false otherwise.

domain/application/service/charm_test.go

@@ -5,6 +5,8 @@ package service
 
 import (
 	"context"
+	"io"
+	"strings"
 
 	"github.com/juju/errors"
 	jc "github.com/juju/testing/checkers"
@@ -398,6 +400,24 @@ func (s *charmServiceSuite) TestGetCharmArchivePathInvalidUUID(c *gc.C) {
 	c.Assert(err, jc.ErrorIs, errors.NotValid)
 }
 
+func (s *charmServiceSuite) TestGetCharmArchive(c *gc.C) {
+	defer s.setupMocks(c).Finish()
+
+	id := charmtesting.GenCharmID(c)
+	archive := io.NopCloser(strings.NewReader("archive-content"))
+
+	s.state.EXPECT().GetCharmArchiveMetadata(gomock.Any(), id).Return("archive-path", "hash", nil)
+	s.charmStore.EXPECT().Get(gomock.Any(), "archive-path").Return(archive, nil)
+
+	reader, hash, err := s.service.GetCharmArchive(context.Background(), id)
+	c.Assert(err, jc.ErrorIsNil)
+	c.Check(hash, gc.Equals, "hash")
+
+	content, err := io.ReadAll(reader)
+	c.Assert(err, jc.ErrorIsNil)
+	c.Check(string(content), gc.Equals, "archive-content")
+}
+
 func (s *charmServiceSuite) TestSetCharmAvailable(c *gc.C) {
 	defer s.setupMocks(c).Finish()
 

domain/application/service/package_test.go

@@ -26,7 +26,7 @@ import (
 	dummystorage "github.com/juju/juju/internal/storage/provider/dummy"
 )
 
-//go:generate go run go.uber.org/mock/mockgen -typed -package service -destination package_mock_test.go github.com/juju/juju/domain/application/service State,DeleteSecretState,ResourceStoreGetter,WatcherFactory,AgentVersionGetter,Provider
+//go:generate go run go.uber.org/mock/mockgen -typed -package service -destination package_mock_test.go github.com/juju/juju/domain/application/service State,DeleteSecretState,ResourceStoreGetter,WatcherFactory,AgentVersionGetter,Provider,CharmStore
 //go:generate go run go.uber.org/mock/mockgen -typed -package service -destination charm_mock_test.go github.com/juju/juju/internal/charm Charm
 
 func TestPackage(t *testing.T) {
@@ -41,6 +41,7 @@ type baseSuite struct {
 	state              *MockState
 	charm              *MockCharm
 	secret             *MockDeleteSecretState
+	charmStore         *MockCharmStore
 	agentVersionGetter *MockAgentVersionGetter
 	provider           *MockProvider
 
@@ -67,6 +68,7 @@ func (s *baseSuite) setupMocksWithProvider(c *gc.C, fn func(ctx context.Context)
 	s.state = NewMockState(ctrl)
 	s.charm = NewMockCharm(ctrl)
 	s.secret = NewMockDeleteSecretState(ctrl)
+	s.charmStore = NewMockCharmStore(ctrl)
 
 	s.storageRegistryGetter = corestorage.ConstModelStorageRegistry(func() storage.ProviderRegistry {
 		return storage.ChainedProviderRegistry{
@@ -83,6 +85,7 @@ func (s *baseSuite) setupMocksWithProvider(c *gc.C, fn func(ctx context.Context)
 		s.modelID,
 		s.agentVersionGetter,
 		fn,
+		s.charmStore,
 		s.clock,
 		loggertesting.WrapCheckLog(c),
 	)
@@ -103,6 +106,7 @@ func (s *baseSuite) setupMocksWithAtomic(c *gc.C, fn func(domain.AtomicContext)
 	s.state = NewMockState(ctrl)
 	s.charm = NewMockCharm(ctrl)
 	s.secret = NewMockDeleteSecretState(ctrl)
+	s.charmStore = NewMockCharmStore(ctrl)
 
 	s.storageRegistryGetter = corestorage.ConstModelStorageRegistry(func() storage.ProviderRegistry {
 		return storage.ChainedProviderRegistry{
@@ -121,6 +125,7 @@ func (s *baseSuite) setupMocksWithAtomic(c *gc.C, fn func(domain.AtomicContext)
 		func(ctx context.Context) (Provider, error) {
 			return s.provider, nil
 		},
+		s.charmStore,
 		s.clock,
 		loggertesting.WrapCheckLog(c),
 	)