mailcap/mailcap.go

// Package mailcap is a port of the python3 mailcap library to golang and
// provides developers with the ability to query mimetypes and keys against 
// the various mailcap files on a system to acquire an appropriate command 
// to run in order to view, edit, etc a given file of a given mimetype.
package mailcap

import (
	"os"
	"os/exec"
	"strings"
	"strconv"
	"fmt"
	"bufio"
	"sort"
)

// Entry is a map of strings keyed with strings that represents a single
// mailcap entry. It represents one prgram for one mimetype. An entry can
// contain options for various keys (test, view, edit, etc).
type Entry map[string]string

// Mailcap is the main struct to interact with. It contains a Cap, as
// caps, that represents the full mailcap db for the system. It has a 
// number of receivers to facilitate retrieving a command from a mimetype.
type Mailcap struct {
	DB  map[string][]Entry
}

// Creates and initializes the mailcap struct/db
// and returns a pointer to Mailcap struct
func NewMailcap() *Mailcap {
	mc := Mailcap{make(map[string][]Entry)}
	mc.getCaps()
	return &mc
}

// Returns []Entry for a given mimetype and an error or nil.
func (m *Mailcap) GetAllMime(mime string) ([]Entry, error) {
	if v, ok := m.DB[mime]; ok {
		return v, nil
	} 
	return nil, fmt.Errorf("Cannot find %s\n", mime)
}

// Retrieves an Entry for a given mimetype and key.
// Key is required to make sure the returned Entry has
// the capability that is desired. Common keys: "view", "edit".
// Also takes a bool, needsTerm, that can be set to true if the
// command must be executable in a terminal only environment.
//
// Returns and Entry and an error
func (m *Mailcap) FindMatch(mime, key string, needsTerm bool) (Entry, error) {
	entries := m.lookup(mime, key)
	for _, v := range entries {
		exitCode := 0;
		if _, ok := v["needsterminal"]; needsTerm && !ok {
			continue
		}
		if t, ok := v["test"]; ok {
			cmdArgs := strings.Split(t, " ")
			if len(cmdArgs) < 2 {
				continue
			}

			com := exec.Command(cmdArgs[0], cmdArgs[1:]...)
			if err := com.Run(); err != nil {
				if exitError, ok := err.(*exec.ExitError); ok {
					exitCode = exitError.ExitCode()
				}
			}
		}
		if exitCode != 0 {
			continue
		}
		v["action"] = key
		return v, nil
	}

	return nil, fmt.Errorf("Unable to find key %q in entries for %s", key, mime)
}

// Called on an Entry type that has an action set (either by calling
// SetAction on the Entry or by it being returned by FindMatch. 
// Returns a pointer to type exec.Command and an error (or nil).
// The caller needs to remember to set Std in/out/err on the returned
// Cmd as is needed by their use case.
func (e Entry) Command(path string) (*exec.Cmd, error) {
	key, ok := e["action"]
	if !ok {
		return nil, fmt.Errorf("No action has been set for this entry, use SetAction to set the action")
	} 
	command, ok := e[key]
	if !ok {
		return nil, fmt.Errorf("Entry does not have the key %q", key)
	}

	matchFields := strings.Fields(command)
	for index, field := range matchFields {
		if field == "%s" || field == "'%s'" {
			matchFields[index] = path
		}
	}

	c := exec.Command(matchFields[0], matchFields[1:]...)
	return c, nil
}

// Sets the action (key) for the entry calling it. This action
// is used when calling execute. The action is generally set by the
// Mailcap receiver FindMatch and SetAction is provided in order
// to easily manipulate an Entry found in the db directly. This
// is not recommended, but is provided as a lower level way of 
// using the lib.
func (e Entry) SetAction(action string) error {
	if _, ok := e[action]; !ok {
		return fmt.Errorf("This entry does not have the action %q available", action)
	}
	e["action"] = action
	return nil
}

// Retrieve the command string for a particular action. The
// string will not have a path inserted into it and will have
// %s to represent the place that the path will go.
// Returns a string and error (or nil).
func (e Entry) CommandString(action string) (string, error) {
	if v, ok := e[action]; ok {
		return v, nil
	}
	return "", fmt.Errorf("This entry does not have the action %q available", action)
}

// Get the available actions/attributes for an entry as
// a string slice
func (e Entry) Actions() []string {
	o := make([]string, len(e))
	i := 0
	for k, _ := range e {
		o[i] = k
		i++
	}
	return o
}

// Look up all of the Entry types available for 
// a given mime and key. Returns an Entry slice (Fields)
func (m *Mailcap) lookup(mime, key string) []Entry {
	f := make([]Entry, 0, 5)
	if val, ok := m.DB[mime]; ok {
		f = append(f, val...)
	}
	splitMime := strings.SplitN(mime,"/",2)
	catchAllMime := splitMime[0] + "/*"
	if val, ok := m.DB[catchAllMime]; ok && mime != catchAllMime {
		f = append(f, val...)
	}
	output := make([]Entry, 0, len(f))
	for _, v := range f {
		if _, ok := v[key]; ok {
			output = append(output, v)
		}
	}

	sort.SliceStable(
		output, 
		func(i, j int) bool { 
			return output[i]["lineno"] < output[j]["lineno"] 
		})

	return output
}

// Top level private initialization method
// Creates the mailcap db and loads it into the
// Caps paramater of the Mailcap struct in question
func (m *Mailcap) getCaps() {
	lnNum := 0
	moreCaps := make(map[string][]Entry)
	for _, mailcapFile := range getMailcapFileList() {
		file, err := os.Open(mailcapFile)
		if err != nil {
			continue
		}
		moreCaps, lnNum = readMailcapFile(file, lnNum)
		for k, v := range moreCaps {
			if _, ok := m.DB[k]; ok {
				for _, item := range v {
					m.DB[k] = append(m.DB[k], item)
				}
			} else {
				m.DB[k] = v
			}
		}
		file.Close()
	} 
}


// Retrieve a slice of strings with all mailcap files
// found on the system.
func getMailcapFileList() (mCapSlice []string) {
	var home string = "."
	if val, ok := os.LookupEnv("MAILCAPS"); ok {
		mCapSlice = strings.Split(val, string(os.PathListSeparator))
	} else {
		if val, ok := os.LookupEnv("HOME"); ok {
			home = val
		} 
		mCapSlice = []string{
			home + "/.mailcap",
			"/etc/mailcap",
			"/usr/etc/mailcap",
			"/usr/local/etc/mailcap",
		}
	}
	return
}

// Reads an individual mailcap file and returns a Cap
// and a line number
func readMailcapFile(f *os.File,ln int) (map[string][]Entry, int) {
	caps := make(map[string][]Entry)
	reader := bufio.NewReader(f)
	for {
		l, e := reader.ReadString('\n')
		if e != nil {
			break
		}
		if strings.TrimSpace(l) == "" || l[0] == '#' {
			continue
		}

		// Handle continuations for long lines
		nxtLn := l
		for ;nxtLn[len(nxtLn)-3:] == "\\\n"; {
			var er error
			nxtLn, er = reader.ReadString('\n')
			if er != nil || strings.TrimSpace(nxtLn) == "" {
				nxtLn = "\n"
			}
			l = l[:len(l)-2] + nxtLn
		}

		// Parse the line
		key, fields, err := parseLine(l)
		if err != nil {
			continue
		}
		if ln >= 0 {
			fields["lineno"] = strconv.Itoa(ln)
			ln += 1
		}
		types := strings.Split(key, "/")
		for i, t := range types {
			types[i] = strings.TrimSpace(t)
		}
		key = strings.Join(types, "/")
		key = strings.ToLower(key)
		if _, ok := caps[key]; ok {
			caps[key] = append(caps[key], fields)
		} else {
			caps[key] = make([]Entry,0,10)
			caps[key] = append(caps[key], fields)
		}
	}

	return caps, ln
}

// Parses an individual mailcap line, returns the key
// as a string, all of the fields, and an error or nil.
func parseLine(ln string) (string, Entry, error) {
	outputFields := make(Entry)
	i := 0
	n := len(ln)
	fields := make([]string, 0, 10)
	var field string
	for ;i < n; {
		field, i = parseField(ln, i, n)
		fields = append(fields, field)
		i += 1
	}
	if len(fields) < 2 {
		return "", nil,fmt.Errorf("Not enough fields present in line")
	}
	key, view := fields[0], fields[1]
	outputFields["view"] = view
	rest := make([]string,0,0)
	if len(fields) > 2 {
		rest = fields[2:]
	}
	for _, f := range rest {
		var fkey, fvalue string
		i = strings.Index(f, "=")
		if i < 0 {
			fkey = f
			fvalue = ""
		} else {
			fkey = strings.TrimSpace(f[:i])
			fvalue = strings.TrimSpace(f[i+1:])
		}
		if _, ok := outputFields[fkey]; !ok {
			// If the key doesnt exist in the map, add it
			outputFields[fkey] = fvalue
		}
	}
	return key, outputFields, nil
}

// Gets one key-value pair from mailcap entry
func parseField(ln string, i, n int) (string, int) {
	start := i
	for ;i < n; {
		c := ln[i]
		if c == ';' {
			break
		} else if c == '\\' {
			i += 2
		} else {
			i += 1
		}
	}
	return strings.TrimSpace(ln[start:i]), i
}