Time out stalled GitHub clones after ten minutes

Progress streams now stay open until a job ends, so a hung clone would
otherwise hold its job and stream forever.

github.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"context"
 	"crypto/rand"
 	"crypto/subtle"
 	"encoding/base64"
@@ -178,8 +179,8 @@ func (s *Server) githubAuthFor(sid string) githubAuth {
 
 // gitClone builds the clone command. The token travels in environment-provided git config,
 // never in the URL or arguments, so it cannot leak through process lists or error output.
-func gitClone(repoURL, dest, token string) *exec.Cmd {
+func gitClone(ctx context.Context, repoURL, dest, token string) *exec.Cmd {
-	cmd := exec.Command("git", "clone", "--depth=1", "--single-branch", "--quiet", "--", repoURL, dest)
+	cmd := exec.CommandContext(ctx, "git", "clone", "--depth=1", "--single-branch", "--quiet", "--", repoURL, dest)
 	// Ignore the host's git config and credential helpers: only the visitor's own token may
 	// ever authenticate a clone.
 	cmd.Env = append(os.Environ(), "GIT_TERMINAL_PROMPT=0", "GIT_CONFIG_NOSYSTEM=1", "GIT_CONFIG_GLOBAL=/dev/null",

github_test.go

@@ -135,7 +135,7 @@ func TestGitCloneKeepsTokenOutOfArgsAndIgnoresHostCredentials(t *testing.T) {
 	if _, err := exec.LookPath("git"); err != nil {
 		t.Skip("git not installed")
 	}
-	cmd := gitClone("https://github.com/o/r", t.TempDir(), "ghu_secret")
+	cmd := gitClone(t.Context(), "https://github.com/o/r", t.TempDir(), "ghu_secret")
 	if strings.Contains(strings.Join(cmd.Args, " "), "ghu_secret") {
 		t.Fatal("token appears in git arguments")
 	}

main.go

@@ -3,6 +3,7 @@ package main
 import (
 	"bufio"
 	"bytes"
+	"context"
 	"crypto/rand"
 	"embed"
 	"encoding/json"
@@ -49,6 +50,7 @@ const (
 	idleExpiry   = time.Hour // snapshots unused and jobs untouched for this long are dropped
 	maxSnapshots = 8         // ponytail: fixed caps sized for the 2GB container; make them configurable if memory changes
 	maxIndexing  = 2         // concurrent index runs; each already uses every CPU
+	cloneTimeout = 10 * time.Minute
 )
 
 var sseKeepAlive = 15 * time.Second // variable so tests can shorten it
@@ -186,9 +188,14 @@ func (s *Server) createJob(w http.ResponseWriter, r *http.Request) {
 		token := s.githubAuthFor(owner).token
 		go func() {
 			checkout := filepath.Join(root, "repo")
-			output, err := gitClone(url, checkout, token).CombinedOutput()
+			ctx, cancel := context.WithTimeout(context.Background(), cloneTimeout)
+			output, err := gitClone(ctx, url, checkout, token).CombinedOutput()
+			cancel()
 			if err != nil {
 				text := string(output)
+				if ctx.Err() != nil {
+					text = "timed out after " + cloneTimeout.String()
+				}
 				if token != "" {
 					text = strings.ReplaceAll(text, token, "***")
 				}