libgit2: ensure context timeout cancels transfer

With the information from the refactor still fresh in mind, I continue
to find new paths now I mentally tamed the git2go beast.

`libgit2` seems to assume that a transport will eventually tell by
itself that it has timed out. This also means that at present any
timeout configuration does not seem have an effect. It will continue
to transfer until the remote (or _something_ else) tells it is no
longer transfering.

This commit introduces a simple check (without tests) which was used
to confirm the theory in combination with the tests in
`pkg/git/strategy` (by setting it to a very low timeout and observing
it fail).

A future iteration should probably take the data given to the callback
into account to ensure it doesn't error out if the given data[1]
reports it has successfully received all objects. Another candidate
for this check may be `CompletionCallback`, but one should study the
C code (and likely some Go code as well) before this.

In addition, to ensure the same timeout is taken into account for push
operations, `PushTransferProgressCallback` may require a likewise
helper.

[1]: https://github.com/libgit2/git2go/blob/main/remote.go#L50-L58

Signed-off-by: Hidde Beydals <[email protected]>

Author: hiddeco

pkg/git/libgit2/checkout.go

@@ -63,7 +63,7 @@ func (c *CheckoutBranch) Checkout(ctx context.Context, path, url string, opts *g
 	repo, err := git2go.Clone(url, path, &git2go.CloneOptions{
 		FetchOptions: &git2go.FetchOptions{
 			DownloadTags:    git2go.DownloadTagsNone,
-			RemoteCallbacks: RemoteCallbacks(opts),
+			RemoteCallbacks: RemoteCallbacks(ctx, opts),
 		},
 		CheckoutBranch: c.Branch,
 	})
@@ -92,7 +92,7 @@ func (c *CheckoutTag) Checkout(ctx context.Context, path, url string, opts *git.
 	repo, err := git2go.Clone(url, path, &git2go.CloneOptions{
 		FetchOptions: &git2go.FetchOptions{
 			DownloadTags:    git2go.DownloadTagsAll,
-			RemoteCallbacks: RemoteCallbacks(opts),
+			RemoteCallbacks: RemoteCallbacks(ctx, opts),
 		},
 	})
 	if err != nil {
@@ -115,7 +115,7 @@ func (c *CheckoutCommit) Checkout(ctx context.Context, path, url string, opts *g
 	repo, err := git2go.Clone(url, path, &git2go.CloneOptions{
 		FetchOptions: &git2go.FetchOptions{
 			DownloadTags:    git2go.DownloadTagsNone,
-			RemoteCallbacks: RemoteCallbacks(opts),
+			RemoteCallbacks: RemoteCallbacks(ctx, opts),
 		},
 	})
 	if err != nil {
@@ -146,7 +146,7 @@ func (c *CheckoutSemVer) Checkout(ctx context.Context, path, url string, opts *g
 	repo, err := git2go.Clone(url, path, &git2go.CloneOptions{
 		FetchOptions: &git2go.FetchOptions{
 			DownloadTags:    git2go.DownloadTagsAll,
-			RemoteCallbacks: RemoteCallbacks(opts),
+			RemoteCallbacks: RemoteCallbacks(ctx, opts),
 		},
 	})
 	if err != nil {

pkg/git/libgit2/transport.go

@@ -19,6 +19,7 @@ package libgit2
 import (
 	"bufio"
 	"bytes"
+	"context"
 	"crypto/md5"
 	"crypto/sha1"
 	"crypto/sha256"
@@ -43,16 +44,31 @@ var (
 
 // RemoteCallbacks constructs RemoteCallbacks with credentialsCallback and
 // certificateCallback, and the given options if the given opts is not nil.
-func RemoteCallbacks(opts *git.AuthOptions) git2go.RemoteCallbacks {
+func RemoteCallbacks(ctx context.Context, opts *git.AuthOptions) git2go.RemoteCallbacks {
 	if opts != nil {
 		return git2go.RemoteCallbacks{
+			TransferProgressCallback: transferProgressCallback(ctx),
 			CredentialsCallback:      credentialsCallback(opts),
 			CertificateCheckCallback: certificateCallback(opts),
 		}
 	}
 	return git2go.RemoteCallbacks{}
 }
 
+// transferProgressCallback constructs TransferProgressCallbacks which signals
+// libgit2 it should stop the transfer when the given context is closed (due to
+// e.g. a timeout).
+func transferProgressCallback(ctx context.Context) git2go.TransferProgressCallback {
+	return func(_ git2go.TransferProgress) git2go.ErrorCode {
+		select {
+		case <-ctx.Done():
+			return git2go.ErrorCodeUser
+		default:
+			return git2go.ErrorCodeOK
+		}
+	}
+}
+
 // credentialsCallback constructs CredentialsCallbacks with the given options
 // for git.Transport, and returns the result.
 func credentialsCallback(opts *git.AuthOptions) git2go.CredentialsCallback {