Add a working Windows tmpfile(3) to fastboot.

Windows' tmpfile(3) implementation requires administrator rights because
it creates temporary files in the root directory. Write an alternative
that uses the user's temporary directory instead.

Bug: http://b/21558406
Change-Id: Ic9aece5c69429797a332a97681a76b76ac3551bf

fastboot/fastboot.cpp

@@ -416,6 +416,35 @@ static void* unzip_file(ZipArchiveHandle zip, const char* entry_name, unsigned*
     return data;
 }
 
+#if defined(_WIN32)
+
+// TODO: move this to somewhere it can be shared.
+
+#include <windows.h>
+
+// Windows' tmpfile(3) requires administrator rights because
+// it creates temporary files in the root directory.
+static FILE* win32_tmpfile() {
+    char temp_path[PATH_MAX];
+    DWORD nchars = GetTempPath(sizeof(temp_path), temp_path);
+    if (nchars == 0 || nchars >= sizeof(temp_path)) {
+        fprintf(stderr, "GetTempPath failed, error %ld\n", GetLastError());
+        return nullptr;
+    }
+
+    char filename[PATH_MAX];
+    if (GetTempFileName(temp_path, "fastboot", 0, filename) == 0) {
+        fprintf(stderr, "GetTempFileName failed, error %ld\n", GetLastError());
+        return nullptr;
+    }
+
+    return fopen(filename, "w+bTD");
+}
+
+#define tmpfile win32_tmpfile
+
+#endif
+
 static int unzip_to_file(ZipArchiveHandle zip, char* entry_name) {
     FILE* fp = tmpfile();
     if (fp == NULL) {