Load the manifest check downloads from memory, not a temp file (closes #223)

check given a URL wrote the manifest it downloaded to a file in the temp
directory before loading it, so it failed wherever that directory is
missing or cannot be written, such as the scratch image script/cibuild
builds. It now keeps the manifest in memory, as fetch, list and export
do. NewChecker reads a manifest handed to it as a reader through the new
CheckerOptions.Manifest field, in place of ManifestPath, because it
otherwise reads the manifest from the filesystem holding the files it
checks.

Model: opus-5-5

internal/cli/check.go

@@ -2,6 +2,7 @@
 package cli
 
 import (
+	"bytes"
 	"context"
 	"encoding/hex"
 	"errors"
@@ -94,42 +95,29 @@ func findManifest(fs afero.Fs, dir string) (string, error) {
 	return path, nil
 }
 
-// fetchManifestToTemp downloads a manifest URL to a temporary file and
-// returns the temp file path. The caller is responsible for removing it.
-func (mfa *CLIApp) fetchManifestToTemp(
-	ctx context.Context, url string,
-) (string, error) {
-	rc, fetchErr := mfa.openManifestReader(ctx, url)
-	if fetchErr != nil {
-		return "", fetchErr
+// downloadManifest downloads the manifest at url into memory. It refuses a
+// manifest larger than mfa.maxManifestSize.
+func (mfa *CLIApp) downloadManifest(ctx context.Context, url string) ([]byte, error) {
+	rc, err := mfa.openManifestReader(ctx, url)
+	if err != nil {
+		return nil, err
 	}
 
-	tmpFile, tmpErr := afero.TempFile(mfa.Fs, "", "mfer-manifest-*.mf")
-	if tmpErr != nil {
-		_ = rc.Close()
+	defer func() { _ = rc.Close() }()
 
-		return "", tmpErr
-	}
-
-	tmpPath := tmpFile.Name()
-
-	// Copying stops one byte past mfa.maxManifestSize, which is enough to
+	// Reading stops one byte past mfa.maxManifestSize, which is enough to
 	// tell that the manifest is too large.
-	written, cpErr := io.Copy(tmpFile, io.LimitReader(rc, mfa.maxManifestSize+1))
-	_ = rc.Close()
-	_ = tmpFile.Close()
-
-	if cpErr == nil && written > mfa.maxManifestSize {
-		cpErr = fmt.Errorf("%w of %d bytes", errManifestTooLarge, mfa.maxManifestSize)
+	manifestData, err := io.ReadAll(io.LimitReader(rc, mfa.maxManifestSize+1))
+	if err != nil {
+		return nil, fmt.Errorf("download manifest: %w", err)
 	}
 
-	if cpErr != nil {
-		_ = mfa.Fs.Remove(tmpPath)
-
-		return "", fmt.Errorf("download manifest: %w", cpErr)
+	if int64(len(manifestData)) > mfa.maxManifestSize {
+		return nil, fmt.Errorf("download manifest: %w of %d bytes",
+			errManifestTooLarge, mfa.maxManifestSize)
 	}
 
-	return tmpPath, nil
+	return manifestData, nil
 }
 
 // verifyRequiredSigner enforces the --require-signature fingerprint
@@ -299,32 +287,28 @@ func (mfa *CLIApp) checkManifestOperation(
 		return err
 	}
 
-	// Done before a URL is swapped for the temp file it is downloaded to,
-	// whose directory is not the base.
 	basePath := resolveBasePath(cmd, manifestPath)
 
-	// URL manifests need to be downloaded to a temp file for the checker
+	opts := &mfer.CheckerOptions{BasePath: basePath, Fs: mfa.Fs}
+
+	// A manifest given by URL is downloaded into memory and read from
+	// there.
 	if isHTTPURL(manifestPath) {
-		tmpPath, tmpErr := mfa.fetchManifestToTemp(ctx, manifestPath)
-		if tmpErr != nil {
-			return tmpErr
+		manifestData, downloadErr := mfa.downloadManifest(ctx, manifestPath)
+		if downloadErr != nil {
+			return downloadErr
 		}
 
-		defer func() { _ = mfa.Fs.Remove(tmpPath) }()
-
-		manifestPath = tmpPath
+		opts.Manifest = bytes.NewReader(manifestData)
+	} else {
+		opts.ManifestPath = manifestPath
 	}
 
 	showProgress := cmd.Bool("progress")
 
 	log.Infof("checking manifest %s with base %s", manifestPath, basePath)
 
-	// Create checker
-	chk, err := mfer.NewChecker(&mfer.CheckerOptions{
-		ManifestPath: manifestPath,
-		BasePath:     basePath,
-		Fs:           mfa.Fs,
-	})
+	chk, err := mfer.NewChecker(opts)
 	if err != nil {
 		return fmt.Errorf("load manifest: %w", err)
 	}