dd: fix handling of 'conv=notrunc seek=N'

Commit e6680912a (dd: create a sparse file when seek=N is used)
broke the use of 'conv=notrunc seek=N' to modify existing files.

Rename seek_sparse() to make_sparse() and:

- add an argument to specify the start of the sparse region;

- call make_sparse() before ftruncate();

- call make_sparse() only if:
  * we can determine the size of the file;
  * the file is not open in append mode;
  * the file is being extended.

This should fix GitHub issue #186.

3 files changed, 26 insertions, 7 deletions

diff --git a/coreutils/dd.c b/coreutils/dd.c
index fc8b1dbb2..042355e24 100644
--- a/coreutils/dd.c
+++ b/coreutils/dd.c
@@ -466,6 +466,27 @@ int dd_main(int argc UNUSED_PARAM, char **argv)
 
                 xmove_fd(xopen(outfile, oflag), ofd);
 
+#if ENABLE_PLATFORM_MINGW32
+                {
+                        off_t len = (off_t)seek * ((G.flags & FLAG_SEEK_BYTES) ? 1 : obs);
+                        struct stat st;
+                        int ret = fstat(ofd, &st);
+
+                        if (ret == 0 && !(G.flags & FLAG_APPEND) && len > st.st_size)
+                                make_sparse(ofd, st.st_size, len);
+
+                        if (seek && !(G.flags & FLAG_NOTRUNC)) {
+                                if (ftruncate(ofd, len) < 0) {
+                                        if (ret < 0
+                                         || S_ISREG(st.st_mode)
+                                         || S_ISDIR(st.st_mode)
+                                        ) {
+                                                goto die_outfile;
+                                        }
+                                }
+                        }
+                }
+#else
                 if (seek && !(G.flags & FLAG_NOTRUNC)) {
                         size_t blocksz = (G.flags & FLAG_SEEK_BYTES) ? 1 : obs;
                         if (ftruncate(ofd, seek * blocksz) < 0) {
@@ -479,6 +500,7 @@ int dd_main(int argc UNUSED_PARAM, char **argv)
                                 }
                         }
                 }
+#endif
         } else {
                 outfile = bb_msg_standard_output;
         }
@@ -502,9 +524,6 @@ int dd_main(int argc UNUSED_PARAM, char **argv)
         }
         if (seek) {
                 size_t blocksz = (G.flags & FLAG_SEEK_BYTES) ? 1 : obs;
-#if ENABLE_PLATFORM_MINGW32
-                seek_sparse(ofd, seek * blocksz);
-#endif
                 if (lseek(ofd, seek * blocksz, SEEK_CUR) < 0)
                         goto die_outfile;
         }
diff --git a/include/mingw.h b/include/mingw.h
index 0fead43c9..39d716521 100644
--- a/include/mingw.h
+++ b/include/mingw.h
@@ -537,5 +537,5 @@ int chdir_system_drive(void);
 char *xabsolute_path(char *path);
 char *get_drive_cwd(const char *path, char *buffer, int size);
 void fix_path_case(char *path);
-void seek_sparse(int fd, size_t size);
+void make_sparse(int fd, off_t start, off_t end);
 int skip_ansi_emulation(int reset);
diff --git a/win32/mingw.c b/win32/mingw.c
index 7f8fecdc3..5c4c39b9d 100644
--- a/win32/mingw.c
+++ b/win32/mingw.c
@@ -1751,7 +1751,7 @@ void fix_path_case(char *path)
         }
 }
 
-void seek_sparse(int fd, size_t size)
+void make_sparse(int fd, off_t start, off_t end)
 {
         DWORD dwTemp;
         HANDLE fh;
@@ -1762,8 +1762,8 @@ void seek_sparse(int fd, size_t size)
 
         DeviceIoControl(fh, FSCTL_SET_SPARSE, NULL, 0, NULL, 0, &dwTemp, NULL);
 
-        fzdi.FileOffset.QuadPart = 0;
-        fzdi.BeyondFinalZero.QuadPart = size;
+        fzdi.FileOffset.QuadPart = start;
+        fzdi.BeyondFinalZero.QuadPart = end;
         DeviceIoControl(fh, FSCTL_SET_ZERO_DATA, &fzdi, sizeof(fzdi),
                                          NULL, 0, &dwTemp, NULL);
 }