Add a utility function for showing manuals in Go

A subsequent commit will use this to implement the help command.

Just like the existing POSIX shell implementation, the standard error
needs to be redirected to standard output. For details see commit
5e63e9ec9b.

https://github.com/containers/toolbox/pull/318

src/meson.build

@@ -5,6 +5,7 @@ sources = files(
   'toolbox.go',
   'cmd/root.go',
   'pkg/shell/shell.go',
+  'pkg/utils/utils.go',
   'pkg/version/version.go',
 )
 

src/pkg/utils/utils.go

@@ -0,0 +1,51 @@
+/*
+ * Copyright © 2019 – 2020 Red Hat Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package utils
+
+import (
+	"os"
+	"os/exec"
+	"syscall"
+)
+
+func ShowManual(manual string) error {
+	manBinary, err := exec.LookPath("man")
+	if err != nil {
+		if errors.Is(err, exec.ErrNotFound) {
+			return errors.New("man(1) not found")
+		}
+
+		return errors.New("failed to lookup man(1)")
+	}
+
+	manualArgs := []string{"man", manual}
+	env := os.Environ()
+
+	stderrFd := os.Stderr.Fd()
+	stderrFdInt := int(stderrFd)
+	stdoutFd := os.Stdout.Fd()
+	stdoutFdInt := int(stdoutFd)
+	if err := syscall.Dup2(stdoutFdInt, stderrFdInt); err != nil {
+		return errors.New("failed to redirect standard error to standard output")
+	}
+
+	if err := syscall.Exec(manBinary, manualArgs, env); err != nil {
+		return errors.New("failed to invoke man(1)")
+	}
+
+	return nil
+}