Re: [PATCH] git-count-objects: Fix a disk-space under-estimate on Cygwin

[Junio C Hamano <gitster@pobox.com>]

When estimating the on-disk footprint of a file, we either used st_blocks
that is counted in 512-byte blocks (traditional unix behaviour), or on
platforms that do not have such st_blocks field in struct stat, simply the
file size itself.  Building with NO_ST_BLOCKS_IN_STRUCT_STAT will choose
the latter implementaion.

POSIX.1 says in its <sys/stat.h> description on this issue:

    The unit for the st_blocks member of the stat structure is not
    defined within POSIX.1-2008. In some implementations it is 512
    bytes. It may differ on a file system basis. There is no
    correlation between values of the st_blocks and st_blksize,
    and the f_bsize (from <sys/statvfs.h>) structure members.

Even though the above explicitly states st_blksize does not have any
correlation, at least on one system (Cygwin on NTFS), the st_blocks field
seems to count in blocks of st_blksize bytes.  A new Makefile variable
ST_BLOCKS_COUNTS_IN_BLKSIZE chooses to use this for the on-disk footprint.

---

Not signed-off yet, but this might be a workable alternative.  Testing on
wider platforms (especially not on "Linux with ext?") might find this
alternative useful.  I dunno.

 Makefile          |    8 +++++++-
 git-compat-util.h |    4 +++-
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index 79195c8..5f2c5de 100644
--- a/Makefile
+++ b/Makefile
@@ -148,6 +148,9 @@ all::
 # Define USE_STDEV below if you want git to care about the underlying device
 # change being considered an inode change from the update-index perspective.
 #
+# Define ST_BLOCKS_COUNTS_IN_ST_BLKSIZE if your platform has st_blocks
+# field that counts the on-disk footprint in st_blksize-byte blocks.
+#
 # Define NO_ST_BLOCKS_IN_STRUCT_STAT if your platform does not have st_blocks
 # field that counts the on-disk footprint in 512-byte blocks.
 #
@@ -776,7 +779,7 @@ ifeq ($(uname_O),Cygwin)
 	NO_FAST_WORKING_DIRECTORY = UnfortunatelyYes
 	NO_TRUSTABLE_FILEMODE = UnfortunatelyYes
 	OLD_ICONV = UnfortunatelyYes
-	NO_ST_BLOCKS_IN_STRUCT_STAT = YesPlease
+	ST_BLOCKS_COUNTS_IN_ST_BLKSIZE = YesPlease
 	# There are conflicting reports about this.
 	# On some boxes NO_MMAP is needed, and not so elsewhere.
 	# Try commenting this out if you suspect MMAP is more efficient
@@ -1126,6 +1129,9 @@ endif
 ifdef NO_D_INO_IN_DIRENT
 	BASIC_CFLAGS += -DNO_D_INO_IN_DIRENT
 endif
+ifdef ST_BLOCKS_COUNTS_IN_ST_BLKSIZE
+	BASIC_CFLAGS += -DST_BLOCKS_COUNTS_IN_ST_BLKSIZE
+endif
 ifdef NO_ST_BLOCKS_IN_STRUCT_STAT
 	BASIC_CFLAGS += -DNO_ST_BLOCKS_IN_STRUCT_STAT
 endif
diff --git a/git-compat-util.h b/git-compat-util.h
index ef60803..bba9c9e 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -240,7 +240,9 @@ extern int git_munmap(void *start, size_t length);
 
 #endif /* NO_MMAP */
 
-#ifdef NO_ST_BLOCKS_IN_STRUCT_STAT
+#ifdef ST_BLOCKS_COUNTS_IN_ST_BLKSIZE
+#define on_disk_bytes(st) ((st).st_blocks * (st).st_blksize)
+#elif defined(NO_ST_BLOCKS_IN_STRUCT_STAT) && NO_ST_BLOCKS_IN_STRUCT_STAT
 #define on_disk_bytes(st) ((st).st_size)
 #else
 #define on_disk_bytes(st) ((st).st_blocks * 512)