mount: Correctly return context.Canceled for interrupted syscalls

bazil/fuse expects us to return context.Canceled to signal that a
syscall was successfully interrupted. Returning a wrapped version of
that error however causes the fuse library to signal an EIO (input/output
error). Thus unwrap context.Canceled errors before returning them.

changelog/unreleased/issue-3567

@@ -0,0 +1,11 @@
+Bugfix: Improve handling of interrupted syscalls in `mount` command
+
+Accessing restic's fuse mount could result in "input / output" errors when
+using programs in which syscalls can be interrupted. This is for example the
+case for go programs.
+
+We have corrected the error handling for interrupted syscalls.
+
+https://github.com/restic/restic/issues/3567
+https://github.com/restic/restic/issues/3694
+https://github.com/restic/restic/pull/3875

internal/fuse/dir.go

@@ -5,6 +5,7 @@ package fuse
 
 import (
 	"context"
+	"errors"
 	"os"
 	"path/filepath"
 	"sync"
@@ -44,6 +45,16 @@ func newDir(ctx context.Context, root *Root, inode, parentInode uint64, node *re
 	}, nil
 }
 
+// returing a wrapped context.Canceled error will instead result in returing
+// an input / output error to the user. Thus unwrap the error to match the
+// expectations of bazil/fuse
+func unwrapCtxCanceled(err error) error {
+	if errors.Is(err, context.Canceled) {
+		return context.Canceled
+	}
+	return err
+}
+
 // replaceSpecialNodes replaces nodes with name "." and "/" by their contents.
 // Otherwise, the node is returned.
 func replaceSpecialNodes(ctx context.Context, repo restic.Repository, node *restic.Node) ([]*restic.Node, error) {
@@ -57,7 +68,7 @@ func replaceSpecialNodes(ctx context.Context, repo restic.Repository, node *rest
 
 	tree, err := restic.LoadTree(ctx, repo, *node.Subtree)
 	if err != nil {
-		return nil, err
+		return nil, unwrapCtxCanceled(err)
 	}
 
 	return tree.Nodes, nil
@@ -91,7 +102,7 @@ func (d *dir) open(ctx context.Context) error {
 	tree, err := restic.LoadTree(ctx, d.root.repo, *d.node.Subtree)
 	if err != nil {
 		debug.Log("  error loading tree %v: %v", d.node.Subtree, err)
-		return err
+		return unwrapCtxCanceled(err)
 	}
 	items := make(map[string]*restic.Node)
 	for _, n := range tree.Nodes {

internal/fuse/file.go

@@ -105,7 +105,7 @@ func (f *openFile) getBlobAt(ctx context.Context, i int) (blob []byte, err error
 	blob, err = f.root.repo.LoadBlob(ctx, restic.DataBlob, f.node.Content[i], nil)
 	if err != nil {
 		debug.Log("LoadBlob(%v, %v) failed: %v", f.node.Name, f.node.Content[i], err)
-		return nil, err
+		return nil, unwrapCtxCanceled(err)
 	}
 
 	f.root.blobCache.Add(f.node.Content[i], blob)

internal/fuse/snapshots_dir.go

@@ -58,7 +58,7 @@ func (d *SnapshotsDir) ReadDirAll(ctx context.Context) ([]fuse.Dirent, error) {
 	// update snapshots
 	meta, err := d.dirStruct.UpdatePrefix(ctx, d.prefix)
 	if err != nil {
-		return nil, err
+		return nil, unwrapCtxCanceled(err)
 	} else if meta == nil {
 		return nil, fuse.ENOENT
 	}
@@ -97,7 +97,7 @@ func (d *SnapshotsDir) Lookup(ctx context.Context, name string) (fs.Node, error)
 
 	meta, err := d.dirStruct.UpdatePrefix(ctx, d.prefix)
 	if err != nil {
-		return nil, err
+		return nil, unwrapCtxCanceled(err)
 	} else if meta == nil {
 		return nil, fuse.ENOENT
 	}