soumya92/barista

// Copyright 2018 Google 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 shell

import (
    "bufio"
    "os/exec"
    "syscall"

    "barista.run/bar"
    "barista.run/base/value"
    "barista.run/outputs"
)

// TailModule represents a bar.Module that displays the last line of output from
// a shell command in the bar.
type TailModule struct {
    cmd  string
    args []string
    outf value.Value // of func(string) bar.Output
}

// Tail constructs a module that displays the last line of output from a long
// running command.
func Tail(cmd string, args ...string) *TailModule {
    t := &TailModule{cmd: cmd, args: args}
    t.outf.Set(func(text string) bar.Output {
        return outputs.Text(text)
    })
    return t
}

// Stream starts the module.
func (m *TailModule) Stream(s bar.Sink) {
    cmd := exec.Command(m.cmd, m.args...)
    // Prevent SIGUSR for bar pause/resume from propagating to the
    // child process. Some commands don't play nice with signals.
    cmd.SysProcAttr = &syscall.SysProcAttr{
        Setpgid: true,
        Pgid:    0,
    }
    stdout, err := cmd.StdoutPipe()
    if s.Error(err) {
        return
    }
    if s.Error(cmd.Start()) {
        return
    }
    var out *string
    outf := m.outf.Get().(func(string) bar.Output)
    errChan := make(chan error)
    outChan := make(chan string)
    go func() {
        scanner := bufio.NewScanner(stdout)
        for scanner.Scan() {
            outChan <- scanner.Text()
        }
        errChan <- cmd.Wait()
    }()
    for {
        select {
        case e := <-errChan:
            s.Error(e)
            return
        case <-m.outf.Next():
            outf = m.outf.Get().(func(string) bar.Output)
        case txt := <-outChan:
            out = &txt
        }
        if out != nil {
            s.Output(outf(*out))
        }
    }
}

// Output sets the output format for each line of output.
func (m *TailModule) Output(format func(string) bar.Output) *TailModule {
    m.outf.Set(format)
    return m
}