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// URLs implements a breadth-first search webcrawler based on the
// example given in section 8.6 of The Go Programming Language.
package main
import (
"flag"
"fmt"
"log"
"net/url"
"runtime/pprof"
"strings"
"time"
)
func main() {
// Setting shorfile helps especially for when we log errors
// without returning them.
log.SetFlags(log.LstdFlags | log.Lshortfile)
maxConcurrency := flag.Int("c", 0, "Maximum number of concurrent queue pushes")
urlArg := flag.String("url", "", "Entry-point URL")
maxURLs := flag.Int("max", 0, "Maximum number of URLs to collect (omitted or 0 means no limit)")
maxDepth := flag.Int("depth", 0, "Maximum URL depth (omitted or 0 means no limit)")
shortcode := flag.String("shortcode", "", "URL shortcode")
shortcodeFilename := flag.String("scfile", "urls.csv", "Shortcode CSV file")
flag.Parse()
// Vet the given CLI arguments for things like negative or
// missing values.
if *maxConcurrency == 0 {
log.Fatal("Missing -c argument")
}
if *maxConcurrency < 1 {
log.Fatalf("Invalid -c argument: %d", *maxConcurrency)
}
startRawURL, err := chooseFrom(*urlArg, *shortcode, *shortcodeFilename)
if err != nil {
log.Fatal(err)
}
if *maxURLs < 0 {
log.Fatalf("Invalid -max argument: %d", *maxURLs)
}
if *maxDepth < 0 {
log.Fatalf("Invalid -depth argument: %d", *maxDepth)
}
startURL, err := convertToURL(startRawURL)
if err != nil {
log.Fatal(err)
}
// Our web crawlers use concurrency: check if any goroutines
// have leaked.
getLeakProfile(func() {
classic(startURL, *maxConcurrency, *maxURLs, *maxDepth)
})
}
// chooseFrom determines whether to use a -url or -shortcode
// argument. If both are present or absent, an error is returned. If
// exactly one is present, return that one.
func chooseFrom(urlArg, shortcode, shortcodeFilename string) (string, error) {
urlAbsent := (urlArg == "")
shortcodeAbsent := (shortcode == "")
if urlAbsent == shortcodeAbsent {
messageSlug := "present"
if urlAbsent {
messageSlug = "missing; need exactly one"
}
return "", fmt.Errorf("-url and -shortcode flags both %s", messageSlug)
}
if urlAbsent {
return getURLFromShortcode(shortcodeFilename, shortcode)
}
return urlArg, nil
}
// convertToURL parses the given rawURL into a [url.URL]. If the
// rawURL is missing a scheme, "https://" is prepended before parsing.
//
// Return the parsed URL, along with any error.
func convertToURL(rawURL string) (url.URL, error) {
if !strings.HasPrefix(rawURL, "http://") && !strings.HasPrefix(rawURL, "https://") {
rawURL = "https://" + rawURL
fmt.Printf("start url: %s\n", rawURL)
}
u, err := url.Parse(rawURL)
if err != nil {
return url.URL{}, fmt.Errorf("can't parse %s: %w", rawURL, err)
}
return *u, nil
}
// getLeakProfile runs a leaky program snippet, extracts the goroutine leak profile,
// and writes it to stdout.
func getLeakProfile(leakySnippet func()) {
prof := pprof.Lookup("goroutineleak")
defer func() {
time.Sleep(2 * time.Second)
var content strings.Builder
prof.WriteTo(&content, 2)
// Ignore non leaked goroutines
leaks := strings.SplitSeq(content.String(), "\n\n")
for leak := range leaks {
if strings.Contains(leak, "(leaked)") {
fmt.Println(leak + "\n")
}
}
}()
leakySnippet()
}
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