cli-plugins/manager: ignore broken symlinks

Before this patch, a broken symlink would print a warning;

    docker info > /dev/null
    WARNING: Plugin "/Users/thajeztah/.docker/cli-plugins/docker-feedback" is not valid: failed to fetch metadata: fork/exec /Users/thajeztah/.docker/cli-plugins/docker-feedback: no such file or directory

After this patch, such symlinks are ignored:

    docker info > /dev/null

We should consider what the best approach is for these, as this
patch will make them completely invisible, but we may still be
iterating over them for discovery.

We should als consider passing a "seen" map to de-duplicate entries.
Entries can be either a direct symlink or in a symlinked path (for
which we can filepath.EvalSymlinks). We need to benchmark the overhead
of resolving the symlink vs possibly calling the plugin (to get their
metadata) further down the line.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli-plugins/manager/manager.go

@@ -2,6 +2,7 @@ package manager
 
 import (
 	"context"
+	"errors"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -81,9 +82,14 @@ func addPluginCandidatesFromDir(res map[string][]string, d string) {
 		return
 	}
 	for _, dentry := range dentries {
-		switch dentry.Type() & os.ModeType {
-		case 0, os.ModeSymlink:
-			// Regular file or symlink, keep going
+		switch mode := dentry.Type() & os.ModeType; mode {
+		case os.ModeSymlink:
+			if _, err := os.Stat(filepath.Join(d, dentry.Name())); errors.Is(err, os.ErrNotExist) {
+				// Ignore broken symlink
+				continue
+			}
+		case 0:
+			// Regular file, keep going
 		default:
 			// Something else, ignore.
 			continue

cli-plugins/manager/manager_test.go

@@ -37,7 +37,7 @@ func TestListPluginCandidates(t *testing.T) {
 			"plugins3-target", // Will be referenced as a symlink from below
 			fs.WithFile("docker-plugin1", ""),
 			fs.WithDir("ignored3"),
-			fs.WithSymlink("docker-brokensymlink", "broken"),           // A broken symlink is still a candidate (but would fail tests later)
+			fs.WithSymlink("docker-brokensymlink", "broken"),           // A broken symlink is ignored
 			fs.WithFile("non-plugin-symlinked", ""),                    // This shouldn't appear, but ...
 			fs.WithSymlink("docker-symlinked", "non-plugin-symlinked"), // ... this link to it should.
 		),
@@ -71,9 +71,6 @@ func TestListPluginCandidates(t *testing.T) {
 		"hardlink2": {
 			dir.Join("plugins2", "docker-hardlink2"),
 		},
-		"brokensymlink": {
-			dir.Join("plugins3", "docker-brokensymlink"),
-		},
 		"symlinked": {
 			dir.Join("plugins3", "docker-symlinked"),
 		},