podman cp: treat /dev/stdout correctly

/dev/stdout should not be treated as "-" to remain compatible with
Docker and to have a more consistent and idiomatic interface.

Fixes: #9362
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

cmd/podman/containers/cp.go

@@ -121,7 +121,9 @@ func copyFromContainer(container string, containerPath string, hostPath string)
 		return err
 	}
 
+	isStdout := false
 	if hostPath == "-" {
+		isStdout = true
 		hostPath = os.Stdout.Name()
 	}
 
@@ -152,10 +154,16 @@ func copyFromContainer(container string, containerPath string, hostPath string)
 		hostBaseName = filepath.Base(hostInfo.LinkTarget)
 	}
 
+	if !isStdout {
+		if err := validateFileInfo(hostInfo); err != nil {
+			return errors.Wrap(err, "invalid destination")
+		}
+	}
+
 	reader, writer := io.Pipe()
 	hostCopy := func() error {
 		defer reader.Close()
-		if hostInfo.LinkTarget == os.Stdout.Name() {
+		if isStdout {
 			_, err := io.Copy(os.Stdout, reader)
 			return err
 		}
@@ -363,3 +371,12 @@ func containerParentDir(container string, containerPath string) (string, error)
 	workDir = filepath.Join(workDir, containerPath)
 	return filepath.Dir(workDir), nil
 }
+
+// validateFileInfo returns an error if the specified FileInfo doesn't point to
+// a directory or a regular file.
+func validateFileInfo(info *copy.FileInfo) error {
+	if info.Mode.IsDir() || info.Mode.IsRegular() {
+		return nil
+	}
+	return errors.Errorf("%q must be a directory or a regular file", info.LinkTarget)
+}

test/system/065-cp.bats

@@ -508,6 +508,10 @@ load helpers
     run_podman exec cpcontainer sh -c "echo '$rand_content' > /tmp/file.txt"
     run_podman exec cpcontainer touch /tmp/empty.txt
 
+    # Make sure that only "-" gets special treatment. "/dev/stdout"
+    run_podman 125 cp cpcontainer:/tmp/file.txt /dev/stdout
+    is "$output" 'Error: invalid destination: "/dev/stdout" must be a directory or a regular file'
+
     # Copying from stdout will always compress.  So let's copy the previously
     # created file from the container via stdout, untar the archive and make
     # sure the file exists with the expected content.