mkcomposefs: Add --print-digest-only

In some cases, it's very useful to just see what the composefs
digest of an input tree is, without actually writing an image.  One
could do this externally by allocating a tempfile, but it's nicer for
us to do internally as we can entirely avoid writing at all.

Signed-off-by: Colin Walters <walters@verbum.org>

tests/integration.sh

@@ -9,13 +9,16 @@ nonhardlink_ls() {
     ls "$@" | sed -e 's,^\([^ ]*\)  *\([0-9][0-9]*\)\(.*\)$,\1\3,'
 }
 
-cfsroot=/composefs
+cfsroot=${cfsroot:-/composefs}
 rm ${cfsroot}/tmp -rf
 mkdir -p ${cfsroot}/{objects,roots,tmp}
 
 cd ${cfsroot}/tmp
 testsrc=/usr/bin
-mkcomposefs --digest-store=${cfsroot}/objects ${testsrc} ${cfsroot}/roots/test.cfs
+mkcomposefs --print-digest --digest-store=${cfsroot}/objects ${testsrc} ${cfsroot}/roots/test.cfs | tee digest.txt
+prev_digest=$(cat digest.txt)
+new_digest=$(mkcomposefs --by-digest --print-digest-only ${testsrc})
+test "$prev_digest" = "$new_digest"
 
 mkdir -p mnt
 mount.composefs -o basedir=${cfsroot}/objects ${cfsroot}/roots/test.cfs mnt

tools/mkcomposefs.c

@@ -369,7 +369,7 @@ static int fill_payload(struct lcfs_node_s *node, const char *path, size_t len,
 static void usage(const char *argv0)
 {
 	fprintf(stderr,
-		"usage: %s [--use-epoch] [--skip-xattrs] [--absolute] [--by-digest] [--digest-store=path] [--print-digest] [--skip-devices] [--compute-digest] SOURCEDIR IMAGE\n",
+		"usage: %s [--use-epoch] [--skip-xattrs] [--absolute] [--by-digest] [--digest-store=path] [--print-digest] [--print-digest-only] [--skip-devices] [--compute-digest] SOURCEDIR IMAGE\n",
 		argv0);
 }
 
@@ -382,6 +382,7 @@ static void usage(const char *argv0)
 #define OPT_DIGEST_STORE 108
 #define OPT_PRINT_DIGEST 109
 #define OPT_FORMAT 110
+#define OPT_PRINT_DIGEST_ONLY 111
 
 static ssize_t write_cb(void *_file, void *buf, size_t count)
 {
@@ -441,6 +442,12 @@ int main(int argc, char **argv)
 			flag: NULL,
 			val: OPT_PRINT_DIGEST
 		},
+		{
+			name: "print-digest-only",
+			has_arg: no_argument,
+			flag: NULL,
+			val: OPT_PRINT_DIGEST_ONLY
+		},
 		{
 			name: "format",
 			has_arg: required_argument,
@@ -455,6 +462,7 @@ int main(int argc, char **argv)
 	bool absolute_path = false;
 	bool by_digest = false;
 	bool print_digest = false;
+	bool print_digest_only = false;
 	const char *format = "erofs";
 	struct lcfs_node_s *root;
 	const char *out = NULL;
@@ -496,6 +504,9 @@ int main(int argc, char **argv)
 		case OPT_PRINT_DIGEST:
 			print_digest = true;
 			break;
+		case OPT_PRINT_DIGEST_ONLY:
+			print_digest = print_digest_only = true;
+			break;
 		case ':':
 			fprintf(stderr, "option needs a value\n");
 			exit(EXIT_FAILURE);
@@ -513,15 +524,30 @@ int main(int argc, char **argv)
 		usage(bin);
 		exit(1);
 	}
+	dir_path = argv[0];
 
-	if (argc != 2) {
-		fprintf(stderr, "No destination path specified\n");
+	if (argc > 2) {
+		fprintf(stderr, "Too many arguments\n");
+		usage(bin);
+		exit(1);
+	}
+	if (argc == 1) {
+		if (!print_digest_only) {
+			fprintf(stderr, "No destination path specified\n");
+			usage(bin);
+			exit(1);
+		}
+	} else if (!print_digest_only) {
+		assert(argc == 2);
+		out = argv[1];
+	} else {
+		fprintf(stderr,
+			"Cannot specify destination path with --print-digest-only\n");
 		usage(bin);
 		exit(1);
 	}
 
-	dir_path = argv[0];
-	out = argv[1];
+	assert(out || print_digest_only);
 
 	if (digest_store_path != NULL)
 		by_digest = true; /* implied */
@@ -533,7 +559,9 @@ int main(int argc, char **argv)
 		error(EXIT_FAILURE, 0,
 		      "Can't specify both --absolute and --by-digest");
 
-	if (strcmp(out, "-") == 0) {
+	if (print_digest_only) {
+		out_file = NULL;
+	} else if (strcmp(out, "-") == 0) {
 		if (isatty(1))
 			error(EXIT_FAILURE, 0, "stdout is a tty.  Refusing to use it");
 		out_file = stdout;
@@ -590,8 +618,10 @@ int main(int argc, char **argv)
 		error(EXIT_FAILURE, errno, "Unknown format %s", format);
 	}
 
-	options.file = out_file;
-	options.file_write_cb = write_cb;
+	if (out_file) {
+		options.file = out_file;
+		options.file_write_cb = write_cb;
+	}
 	if (print_digest)
 		options.digest_out = digest;