Keep the forge credential out of what a check's container sees, and redact what a check publishes (issue 462)

The toolchain container mounts the workspace as HOME, so the credential kept
there, and a clone's recorded userinfo, were readable by any pull request; its
output is kept on the bus for days. The credential now lives in a private
directory outside the workspace only while cloning, clones record their URL
without userinfo, and every line said to the build's log and the verdict
passes a redactor copied from the journal tool (sharing it: issue 471).

internal/builder/builder.go

@@ -7,6 +7,7 @@ import (
 	"crypto/sha256"
 	"encoding/hex"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -159,13 +160,11 @@ func build(ctx context.Context, run Runner, publish Publisher,
 	}
 	// The credential is a file git reads, never an argument: a URL carrying a password in argv
 	// would be readable by anything that can list processes for as long as a clone runs.
-	credentials := ""
-	if forge.URL != "" {
-		credentials = filepath.Join(workspace, "git-credentials")
-		if err := os.WriteFile(credentials, []byte(forge.URL+"\n"), 0o600); err != nil {
-			return Result{}, err
-		}
+	credentials, forget, err := storeCredential(workspace, forge)
+	if err != nil {
+		return Result{}, err
 	}
+	defer forget()
 	tree := filepath.Join(workspace, "source")
 	if err := os.RemoveAll(tree); err != nil {
 		return Result{}, err
@@ -467,6 +466,55 @@ func contextURL(from catalogue.ArtifactContext, seats map[string]string) (string
 	return strings.TrimRight(base, "/") + "/" + strings.TrimSuffix(strings.Trim(from.Repository, "/"), ".git") + ".git", nil
 }
 
+// storeCredential writes the forge credential where git's credential store reads it, and the function that
+// removes it again; "" and nothing to remove when the builder holds none.
+//
+// **Never inside the workspace** (novox/hq issue 462): a merge check's toolchain container mounts the
+// workspace as its HOME, so a credential kept there — even one a build left behind — is readable by any pull
+// request's merge-check.sh, and what it prints is kept on the bus. So it lives in a directory of its own
+// outside the workspace, made private, and a credential an older builder left in the workspace is removed.
+func storeCredential(workspace string, forge GitCredential) (string, func(), error) {
+	if err := os.Remove(filepath.Join(workspace, "git-credentials")); err != nil && !errors.Is(err, os.ErrNotExist) {
+		return "", nil, fmt.Errorf("a credential left in the workspace cannot be removed: %w", err)
+	}
+	if forge.URL == "" {
+		return "", func() {}, nil
+	}
+	dir, err := os.MkdirTemp("", "mesh-forge-credential-")
+	if err != nil {
+		return "", nil, err
+	}
+	forget := func() { os.RemoveAll(dir) }
+	if withinDir(workspace, dir) {
+		forget()
+		return "", nil, fmt.Errorf("the temporary directory %s is inside the workspace %s, which a check's "+
+			"container mounts: the forge credential is not written where it could read it", dir, workspace)
+	}
+	path := filepath.Join(dir, "git-credentials")
+	if err := os.WriteFile(path, []byte(forge.URL+"\n"), 0o600); err != nil {
+		forget()
+		return "", nil, err
+	}
+	return path, forget, nil
+}
+
+// withinDir is whether path is dir or under it, both made absolute and resolved.
+func withinDir(dir, path string) bool {
+	d, err1 := filepath.Abs(dir)
+	p, err2 := filepath.Abs(path)
+	if err1 != nil || err2 != nil {
+		return true
+	}
+	if r, err := filepath.EvalSymlinks(d); err == nil {
+		d = r
+	}
+	if r, err := filepath.EvalSymlinks(p); err == nil {
+		p = r
+	}
+	rel, err := filepath.Rel(d, p)
+	return err != nil || rel == "." || filepath.IsLocal(rel)
+}
+
 // cloneWith is a git invocation that may offer a stored credential.
 //
 // The first `-c credential.helper=` clears every helper the environment might carry, so exactly