Panic: Send on a closed channel when running go routine in foor loop

Question

I'm attempting to make a concurrent version of grep. The program walks directories/subdirectories and returns back any matching strings to a provided pattern.

I am attempting to run the file searching concurrently, once I have all the files to search (see searchPaths function). Originally I was getting:

fatal error: all goroutines are asleep - deadlock

Until I added the close(out) at the end of searchPaths, to which it now returns:

Panic: Send on a closed channel when running go routine in foor loop

I am attempting to implement something similar to:

https://go.dev/blog/pipelines#fan-out-fan-in

Is it the case that I am closing the channel at the wrong point?

package main

import (
    "fmt"
    "io/fs"
    "io/ioutil"
    "log"
    "os"
    "path/filepath"
    "strings"
    "sync"
)

type SearchResult struct {
    line       string
    lineNumber int
}

type Display struct {
    filePath string
    SearchResult
}

var wg sync.WaitGroup

func (d Display) PrettyPrint() {
    fmt.Printf("Line Number: %v\nFilePath: %v\nLine: %v\n\n", d.lineNumber, d.filePath, d.line)
}

func searchLine(pattern string, line string, lineNumber int) (SearchResult, bool) {
    if strings.Contains(line, pattern) {
        return SearchResult{lineNumber: lineNumber + 1, line: line}, true
    }
    return SearchResult{}, false
}

func splitIntoLines(file string) []string {
    lines := strings.Split(file, "\n")
    return lines
}

func fileFromPath(path string) string {
    fileContent, err := ioutil.ReadFile(path)

    if err != nil {
        log.Fatal(err)
    }

    return string(fileContent)
}

func getRecursiveFilePaths(inputDir string) []string {
    var paths []string
    err := filepath.Walk(inputDir, func(path string, info fs.FileInfo, err error) error {
        if err != nil {
            fmt.Printf("prevent panic by handling failure accessing a path %q: %v\n", path, err)
            return err
        }
        if !info.IsDir() {
            paths = append(paths, path)
        }
        return nil
    })
    if err != nil {
        fmt.Printf("Error walking the path %q: %v\n", inputDir, err)
    }
    return paths
}

func searchPaths(paths []string, pattern string) <-chan Display {
    out := make(chan Display)

    for _, path := range paths {
        wg.Add(1)
        go func() {
            defer wg.Done()
            for _, display := range searchFile(path, pattern) {
                out <- display
            }
        }()
    }
    close(out)
    return out
}

func searchFile(path string, pattern string) []Display {
    var out []Display
    input := fileFromPath(path)
    lines := splitIntoLines(input)
    for index, line := range lines {
        if searchResult, ok := searchLine(pattern, line, index); ok {
            out = append(out, Display{path, searchResult})
        }
    }
    return out
}

func main() {
    pattern := os.Args[1]
    dirPath := os.Args[2]

    paths := getRecursiveFilePaths(dirPath)

    out := searchPaths(paths, pattern)
    wg.Wait()
    for d := range out {
        d.PrettyPrint()
    }

}

Answer 1

2 main issues with this code were

1. you need to close the channel only after wg.Wait() completes. you can do this in a seperate goroutine as shown below
2. as the path var in searchPaths func is reassigned multiple times as part of the for loop logic, it is not a good practice to use that var directly in the goroutines, a better approach will be to pass it as an argument.

package main

import (
    "fmt"
    "io/fs"
    "io/ioutil"
    "log"
    "os"
    "path/filepath"
    "strings"
    "sync"
)

type SearchResult struct {
    line       string
    lineNumber int
}

type Display struct {
    filePath string
    SearchResult
}

var wg sync.WaitGroup

func (d Display) PrettyPrint() {
    fmt.Printf("Line Number: %v\nFilePath: %v\nLine: %v\n\n", d.lineNumber, d.filePath, d.line)
}

func searchLine(pattern string, line string, lineNumber int) (SearchResult, bool) {
    if strings.Contains(line, pattern) {
        return SearchResult{lineNumber: lineNumber + 1, line: line}, true
    }
    return SearchResult{}, false
}

func splitIntoLines(file string) []string {
    lines := strings.Split(file, "\n")
    return lines
}

func fileFromPath(path string) string {
    fileContent, err := ioutil.ReadFile(path)

    if err != nil {
        log.Fatal(err)
    }

    return string(fileContent)
}

func getRecursiveFilePaths(inputDir string) []string {
    var paths []string
    err := filepath.Walk(inputDir, func(path string, info fs.FileInfo, err error) error {
        if err != nil {
            fmt.Printf("prevent panic by handling failure accessing a path %q: %v\n", path, err)
            return err
        }
        if !info.IsDir() {
            paths = append(paths, path)
        }
        return nil
    })
    if err != nil {
        fmt.Printf("Error walking the path %q: %v\n", inputDir, err)
    }
    return paths
}

func searchPaths(paths []string, pattern string) chan Display {
    out := make(chan Display)
    for _, path := range paths {
        wg.Add(1)
        go func(p string, w *sync.WaitGroup) { // as path var is changing value in the loop, it's better to provide it as a argument in goroutine
            defer w.Done()
            for _, display := range searchFile(p, pattern) {
                out <- display
            }
        }(path, &wg)
    }
    return out
}

func searchFile(path string, pattern string) []Display {
    var out []Display
    input := fileFromPath(path)
    lines := splitIntoLines(input)
    for index, line := range lines {
        if searchResult, ok := searchLine(pattern, line, index); ok {
            out = append(out, Display{path, searchResult})
        }
    }
    return out
}

func main() {
    pattern := os.Args[1]
    dirPath := os.Args[2]

    paths := getRecursiveFilePaths(dirPath)

    out := searchPaths(paths, pattern)

    go func(){
        wg.Wait() // waiting before closing the channel
        close(out)
    }()
    
    count := 0
    for d := range out {
        fmt.Println(count)
        d.PrettyPrint()
        count += 1
    }

}