common: change default network from testnet to mainnet for new installs.

Changelog-changed: Default network (new installs) is now bitcoin, not testnet.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/configdir.c

@@ -10,6 +10,9 @@
 #include <common/utils.h>
 #include <common/version.h>
 #include <errno.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
 
 bool deprecated_apis = true;
 
@@ -241,6 +244,34 @@ void parse_config_files(const char *config_filename,
 	parse_implied_config_file(config_basedir, chainparams->network_name, early);
 }
 
+/* Could be a yet-to-be-upgraded dir (definitely testnet), or could be
+ * it's been upgraded to testnet. */
+static bool smells_like_old_testnet(const char *config_basedir)
+{
+	struct stat st;
+	/* Doubles as convenient top-level ctx for this function */
+	const char *base = default_base_configdir(NULL);
+
+	if (!config_basedir)
+		config_basedir = base;
+
+	/* If it doesn't exist, it's not testnet. */
+	if (stat(config_basedir, &st) != 0) {
+		tal_free(base);
+		return false;
+	}
+
+	/* Does it have a bitcoin/ subdir and no testnet/ subdir? */
+	if (stat(path_join(base, config_basedir, "bitcoin"), &st) == 0
+	    && stat(path_join(base, config_basedir, "testnet"), &st) != 0) {
+		tal_free(base);
+		return false;
+	}
+
+	tal_free(base);
+	return true;
+}
+
 void initial_config_opts(const tal_t *ctx,
 			 int argc, char *argv[],
 			 char **config_filename,
@@ -318,10 +349,16 @@ void initial_config_opts(const tal_t *ctx,
 		parse_implied_config_file(*config_basedir, NULL, true);
 	opt_early_parse_incomplete(argc, argv, opt_log_stderr_exit);
 
-	/* We use a global (in common/utils.h) for the chainparams.
-	 * We default to testnet for now. */
-	if (!chainparams)
-		chainparams = chainparams_for_network("testnet");
+	/* We use a global (in common/utils.h) for the chainparams. */
+	if (!chainparams) {
+		/* Use bitcoin default on new installations. */
+		if (deprecated_apis && smells_like_old_testnet(*config_basedir)) {
+			warnx("WARNING: default network changing in 2020:"
+			      " please set network=testnet in config!");
+			chainparams = chainparams_for_network("testnet");
+		} else
+			chainparams = chainparams_for_network("bitcoin");
+	}
 
 	if (!*config_basedir)
 		*config_basedir = default_base_configdir(ctx);