tdb: fix tdb_check() on read-only TDBs to actually work.

But make sure we can still do tdb_check() inside a transaction (weird,
but we previously allowed it).

ccan/tdb/check.c

@@ -327,9 +327,17 @@ int tdb_check(struct tdb_context *tdb,
 	struct tdb_record rec;
 	bool found_recovery = false;
 	tdb_len_t dead;
-
-	if (tdb_lockall_read(tdb) == -1)
-		return -1;
+	bool locked;
+
+	/* Read-only databases use no locking at all: it's best-effort.
+	 * We may have a write lock already, so skip that case too. */
+	if (tdb->read_only || tdb->allrecord_lock.count != 0) {
+		locked = false;
+	} else {
+		if (tdb_lockall_read(tdb) == -1)
+			return -1;
+		locked = true;
+	}
 
 	/* Make sure we know true size of the underlying file. */
 	tdb->methods->tdb_oob(tdb, tdb->map_size + 1, 1);
@@ -444,12 +452,16 @@ int tdb_check(struct tdb_context *tdb,
 	}
 
 	free(hashes);
-	tdb_unlockall_read(tdb);
+	if (locked) {
+		tdb_unlockall_read(tdb);
+	}
 	return 0;
 
 free:
 	free(hashes);
 unlock:
-	tdb_unlockall_read(tdb);
+	if (locked) {
+		tdb_unlockall_read(tdb);
+	}
 	return -1;
 }

ccan/tdb/test/run-readonly-check.c

@@ -0,0 +1,53 @@
+/* We should be able to tdb_check a O_RDONLY tdb, and we were previously allowed
+ * to tdb_check() inside a transaction (though that's paranoia!). */
+#define _XOPEN_SOURCE 500
+#include <ccan/tdb/tdb.h>
+#include <ccan/tdb/io.c>
+#include <ccan/tdb/tdb.c>
+#include <ccan/tdb/lock.c>
+#include <ccan/tdb/freelist.c>
+#include <ccan/tdb/traverse.c>
+#include <ccan/tdb/transaction.c>
+#include <ccan/tdb/error.c>
+#include <ccan/tdb/open.c>
+#include <ccan/tdb/check.c>
+#include <ccan/tap/tap.h>
+#include <stdlib.h>
+#include <err.h>
+#include "logging.h"
+
+int main(int argc, char *argv[])
+{
+	struct tdb_context *tdb;
+	TDB_DATA key, data;
+
+	plan_tests(11);
+	tdb = tdb_open_ex("run-readonly-check.tdb", 1024,
+			  TDB_DEFAULT,
+			  O_CREAT|O_TRUNC|O_RDWR, 0600, &taplogctx, NULL);
+
+	ok1(tdb);
+	key.dsize = strlen("hi");
+	key.dptr = (void *)"hi";
+	data.dsize = strlen("world");
+	data.dptr = (void *)"world";
+
+	ok1(tdb_store(tdb, key, data, TDB_INSERT) == 0);
+	ok1(tdb_check(tdb, NULL, NULL) == 0);
+
+	/* We are also allowed to do a check inside a transaction. */
+	ok1(tdb_transaction_start(tdb) == 0);
+	ok1(tdb_check(tdb, NULL, NULL) == 0);
+	ok1(tdb_close(tdb) == 0);
+
+	tdb = tdb_open_ex("run-readonly-check.tdb", 1024,
+			  TDB_DEFAULT, O_RDONLY, 0, &taplogctx, NULL);
+
+	ok1(tdb);
+	ok1(tdb_store(tdb, key, data, TDB_MODIFY) == -1);
+	ok1(tdb_error(tdb) == TDB_ERR_RDONLY);
+	ok1(tdb_check(tdb, NULL, NULL) == 0);
+	ok1(tdb_close(tdb) == 0);
+
+	return exit_status();
+}