[test] standardize on database file name and move I/O functions to util

src/mbgl/util/io.cpp

@@ -2,6 +2,7 @@
 
 #include <cstdio>
 #include <cerrno>
+#include <cstring>
 #include <iostream>
 #include <sstream>
 #include <fstream>
@@ -9,6 +10,10 @@
 namespace mbgl {
 namespace util {
 
+IOException::IOException(int err, const std::string& msg)
+    : std::runtime_error(msg + ": " + std::strerror(errno)), code(err) {
+}
+
 void write_file(const std::string &filename, const std::string &data) {
     FILE *fd = fopen(filename.c_str(), "wb");
     if (fd) {
@@ -42,10 +47,22 @@ optional<std::string> readFile(const std::string &filename) {
 
 void deleteFile(const std::string& filename) {
     const int ret = std::remove(filename.c_str());
-    if (ret != 0) {
-        throw IOException(errno, "failed to unlink file");
+    if (ret != 0 && errno != ENOENT) {
+        throw IOException(errno, "Could not delete file " + filename);
     }
 }
 
+void copyFile(const std::string& destination, const std::string& source) {
+    std::ifstream src(source, std::ios::binary);
+    if (!src.good()) {
+        throw IOException(errno, "Cannot read file " + destination);
+    }
+    std::ofstream dst(destination, std::ios::binary);
+    if (!dst.good()) {
+        throw IOException(errno, "Cannot write file " + destination);
+    }
+    dst << src.rdbuf();
+}
+
 } // namespace util
 } // namespace mbgl

src/mbgl/util/io.hpp

@@ -9,8 +9,7 @@ namespace mbgl {
 namespace util {
 
 struct IOException : std::runtime_error {
-    IOException(int err, const char* msg) : std::runtime_error(msg), code(err) {
-    }
+    IOException(int err, const std::string& msg);
     const int code = 0;
 };
 
@@ -19,6 +18,7 @@ std::string read_file(const std::string &filename);
 
 optional<std::string> readFile(const std::string &filename);
 void deleteFile(const std::string& filename);
+void copyFile(const std::string& destination, const std::string& source);
 
 } // namespace util
 } // namespace mbgl