fastboot: use filename instead of fd to generate filesystem image

mke2fs tool takes a filename and has its own open function to handle
output file. Change fastboot in preparation to switch from make_ext4
to mke2fs.

Test: fastboot format:ext4 userdata
Bug: 35219933
Change-Id: I7a31cb215f443a4a7cb0bfc23ec28c121a6101e6
diff --git a/fastboot/fs.cpp b/fastboot/fs.cpp
index 5d9ccfe..99ca7dd 100644
--- a/fastboot/fs.cpp
+++ b/fastboot/fs.cpp
@@ -4,6 +4,7 @@
 #include "make_f2fs.h"
 
 #include <errno.h>
+#include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -11,12 +12,20 @@
 #include <sys/types.h>
 #include <unistd.h>
 
+#include <android-base/unique_fd.h>
 #include <ext4_utils/make_ext4fs.h>
 #include <sparse/sparse.h>
 
-static int generate_ext4_image(int fd, long long partSize, const std::string& initial_dir,
+using android::base::unique_fd;
+
+static int generate_ext4_image(const char* fileName, long long partSize, const std::string& initial_dir,
                                        unsigned eraseBlkSize, unsigned logicalBlkSize)
 {
+    unique_fd fd(open(fileName, O_CREAT | O_RDWR | O_TRUNC, S_IRUSR | S_IWUSR));
+    if (fd == -1) {
+        fprintf(stderr, "Unable to open output file for EXT4 filesystem: %s\n", strerror(errno));
+        return -1;
+    }
     if (initial_dir.empty()) {
         make_ext4fs_sparse_fd_align(fd, partSize, NULL, NULL, eraseBlkSize, logicalBlkSize);
     } else {
@@ -27,11 +36,16 @@
 }
 
 #ifdef USE_F2FS
-static int generate_f2fs_image(int fd, long long partSize, const std::string& initial_dir,
+static int generate_f2fs_image(const char* fileName, long long partSize, const std::string& initial_dir,
                                unsigned /* unused */, unsigned /* unused */)
 {
     if (!initial_dir.empty()) {
-        fprintf(stderr, "Unable to set initial directory on F2FS filesystem\n");
+        fprintf(stderr, "Unable to set initial directory on F2FS filesystem: %s\n", strerror(errno));
+        return -1;
+    }
+    unique_fd fd(open(fileName, O_CREAT | O_RDWR | O_TRUNC, S_IRUSR | S_IWUSR));
+    if (fd == -1) {
+        fprintf(stderr, "Unable to open output file for F2FS filesystem: %s\n", strerror(errno));
         return -1;
     }
     return make_f2fs_sparse_fd(fd, partSize, NULL, NULL);
@@ -42,7 +56,7 @@
     const char* fs_type;  //must match what fastboot reports for partition type
 
     //returns 0 or error value
-    int (*generate)(int fd, long long partSize, const std::string& initial_dir,
+    int (*generate)(const char* fileName, long long partSize, const std::string& initial_dir,
                     unsigned eraseBlkSize, unsigned logicalBlkSize);
 
 } generators[] = {
@@ -61,8 +75,8 @@
     return nullptr;
 }
 
-int fs_generator_generate(const struct fs_generator* gen, int tmpFileNo, long long partSize,
+int fs_generator_generate(const struct fs_generator* gen, const char* fileName, long long partSize,
     const std::string& initial_dir, unsigned eraseBlkSize, unsigned logicalBlkSize)
 {
-    return gen->generate(tmpFileNo, partSize, initial_dir, eraseBlkSize, logicalBlkSize);
+    return gen->generate(fileName, partSize, initial_dir, eraseBlkSize, logicalBlkSize);
 }