MinGW: implement mmap

Add USE_WIN32_MMAP which triggers the use of windows' native
file memory mapping functionality in git_mmap()/git_munmap() functions.

As git functions currently use mmap with MAP_PRIVATE set only, this
implementation supports only that mode for now.

On Windows, offsets for memory mapped files need to match the allocation
granularity. Take this into account when calculating the packed git-
windowsize and file offsets. At the moment, the only function which makes
use of offsets in conjunction with mmap is use_pack() in sha1-file.c.

Git fast-import's code path tries to map a portion of the temporary
packfile that exceeds the current filesize, i.e. offset+length is
greater than the filesize. The NO_MMAP code worked with that since pread()
just reads the file content until EOF and returns gracefully, while
MapViewOfFile() aborts the mapping and returns 'Access Denied'.
Working around that by determining the filesize and adjusting the length
parameter.

Signed-off-by: Janos Laube <janos.dev@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 58 insertions, 0 deletions

diff --git a/compat/mingw.h b/compat/mingw.h
index 7e52f3607f..762eb143a7 100644
--- a/compat/mingw.h
+++ b/compat/mingw.h
@@ -159,6 +159,11 @@ int mingw_connect(int sockfd, struct sockaddr *sa, size_t sz);
 int mingw_rename(const char*, const char*);
 #define rename mingw_rename
 
+#ifdef USE_WIN32_MMAP
+int mingw_getpagesize(void);
+#define getpagesize mingw_getpagesize
+#endif
+
 /* Use mingw_lstat() instead of lstat()/stat() and
  * mingw_fstat() instead of fstat() on Windows.
  */
diff --git a/compat/win32mmap.c b/compat/win32mmap.c
new file mode 100644
index 0000000000..779d796cd5
--- /dev/null
+++ b/compat/win32mmap.c
@@ -0,0 +1,53 @@
+#include "../git-compat-util.h"
+
+/*
+ * Note that this doesn't return the actual pagesize, but
+ * the allocation granularity. If future Windows specific git code
+ * needs the real getpagesize function, we need to find another solution.
+ */
+int mingw_getpagesize(void)
+{
+	SYSTEM_INFO si;
+	GetSystemInfo(&si);
+	return si.dwAllocationGranularity;
+}
+
+void *git_mmap(void *start, size_t length, int prot, int flags, int fd, off_t offset)
+{
+	HANDLE hmap;
+	void *temp;
+	size_t len;
+	struct stat st;
+	uint64_t o = offset;
+	uint32_t l = o & 0xFFFFFFFF;
+	uint32_t h = (o >> 32) & 0xFFFFFFFF;
+
+	if (!fstat(fd, &st))
+		len = xsize_t(st.st_size);
+	else
+		die("mmap: could not determine filesize");
+
+	if ((length + offset) > len)
+		length = len - offset;
+
+	if (!(flags & MAP_PRIVATE))
+		die("Invalid usage of mmap when built with USE_WIN32_MMAP");
+
+	hmap = CreateFileMapping((HANDLE)_get_osfhandle(fd), 0, PAGE_WRITECOPY,
+		0, 0, 0);
+
+	if (!hmap)
+		return MAP_FAILED;
+
+	temp = MapViewOfFileEx(hmap, FILE_MAP_COPY, h, l, length, start);
+
+	if (!CloseHandle(hmap))
+		warning("unable to close file mapping handle\n");
+
+	return temp ? temp : MAP_FAILED;
+}
+
+int git_munmap(void *start, size_t length)
+{
+	return !UnmapViewOfFile(start);
+}