config: implement config snapshotting

In order to have consistent views of the config files for remotes,
submodules et al. and a configuration that represents what is currently
stored on-disk, we need a way to provide a view of the configuration
that does not change.

The goal here is to provide the snapshotting part by creating a
read-only copy of the state of the configuration at a particular point
in time, which does not change when a repository's main config changes.

include/git2/config.h

@@ -226,6 +226,19 @@ GIT_EXTERN(int) git_config_open_level(
  */
 GIT_EXTERN(int) git_config_open_global(git_config **out, git_config *config);
 
+/**
+ * Create a snapshot of the configuration
+ *
+ * Create a snapshot of the current state of a configuration, which
+ * allows you to look into a consistent view of the configuration for
+ * looking up complex values (e.g. a remote, submodule).
+ *
+ * @param out pointer in which to store the snapshot config object
+ * @param config configuration to snapshot
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_config_snapshot(git_config **out, git_config *config);
+
 
 /**
  * Reload changed config files

include/git2/sys/config.h

@@ -64,6 +64,8 @@ struct git_config_backend {
 	int (*del_multivar)(struct git_config_backend *, const char *key, const char *regexp);
 	int (*iterator)(git_config_iterator **, struct git_config_backend *);
 	int (*refresh)(struct git_config_backend *);
+	/** Produce a read-only version of this backend */
+	int (*snapshot)(struct git_config_backend **, struct git_config_backend *);
 	void (*free)(struct git_config_backend *);
 };
 #define GIT_CONFIG_BACKEND_VERSION 1

src/config.c

@@ -137,6 +137,38 @@ int git_config_open_ondisk(git_config **out, const char *path)
 	return error;
 }
 
+int git_config_snapshot(git_config **out, git_config *in)
+{
+	int error;
+	size_t i;
+	file_internal *internal;
+	git_config *config;
+
+	*out = NULL;
+
+	if (git_config_new(&config) < 0)
+		return -1;
+
+	git_vector_foreach(&in->files, i, internal) {
+		git_config_backend *b;
+
+		if ((error = internal->file->snapshot(&b, internal->file)) < 0)
+			goto on_error;
+
+		if ((error = git_config_add_backend(config, b, internal->level, 0)) < 0) {
+			b->free(b);
+			goto on_error;
+		}
+	}
+
+	*out = config;
+	return error;
+
+on_error:
+	git_config_free(config);
+	return error;
+}
+
 static int find_internal_file_by_level(
 	file_internal **internal_out,
 	const git_config *cfg,

tests/config/snapshot.c

+#include "clar_libgit2.h"
+
+void test_config_snapshot__create_snapshot(void)
+{
+	int32_t tmp;
+	git_config *cfg, *snapshot;
+	const char *filename = "config-ext-change";
+
+	cl_git_mkfile(filename, "[old]\nvalue = 5\n");
+
+	cl_git_pass(git_config_open_ondisk(&cfg, filename));
+
+	cl_git_pass(git_config_get_int32(&tmp, cfg, "old.value"));
+	cl_assert_equal_i(5, tmp);
+
+	cl_git_pass(git_config_snapshot(&snapshot, cfg));
+
+	/* Change the value on the file itself (simulate external process) */
+	cl_git_mkfile(filename, "[old]\nvalue = 56\n");
+
+	cl_git_pass(git_config_get_int32(&tmp, cfg, "old.value"));
+	cl_assert_equal_i(5, tmp);
+
+	cl_git_pass(git_config_refresh(cfg));
+
+	cl_git_pass(git_config_get_int32(&tmp, cfg, "old.value"));
+	cl_assert_equal_i(56, tmp);
+
+	cl_git_pass(git_config_get_int32(&tmp, snapshot, "old.value"));
+	cl_assert_equal_i(5, tmp);
+	
+	git_config_free(snapshot);
+	git_config_free(cfg);
+}
+
+static int count_me(const git_config_entry *entry, void *payload)
+{
+	int *n = (int *) payload;
+
+	GIT_UNUSED(entry);
+
+	(*n)++;
+
+	return 0;
+}
+
+void test_config_snapshot__multivar(void)
+{
+	int count = 0;
+	git_config *cfg, *snapshot;
+	const char *filename = "config-file";
+
+	cl_git_mkfile(filename, "[old]\nvalue = 5\nvalue = 6\n");
+
+	cl_git_pass(git_config_open_ondisk(&cfg, filename));
+	cl_git_pass(git_config_get_multivar_foreach(cfg, "old.value", NULL, count_me, &count));
+
+	cl_assert_equal_i(2, count);
+
+	cl_git_pass(git_config_snapshot(&snapshot, cfg));
+	git_config_free(cfg);
+
+	count = 0;
+	cl_git_pass(git_config_get_multivar_foreach(snapshot, "old.value", NULL, count_me, &count));
+
+	cl_assert_equal_i(2, count);
+
+	git_config_free(snapshot);
+}