Add a simple mmap wrapper for cross-platform mmap usage

Win32 has a variant of mmap that is harder to use than POSIX, but
to run natively and efficiently on Win32 we need some form of it.

gitfo_map_ro() provides a basic mmap function for use in locations
where we need read-only random data access to large ranges of a file,
such as a pack-*.idx.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

src/fileops.c

 #include "common.h"
 #include "fileops.h"
+#include <sys/mman.h>
 
 int gitfo_open(const char *path, int flags)
 {
@@ -106,6 +107,20 @@ void gitfo_free_buf(gitfo_buf *obj)
 	obj->data = NULL;
 }
 
+int gitfo_map_ro(gitfo_map *out, git_file fd, off_t begin, size_t len)
+{
+	out->data = mmap(NULL, len, PROT_READ, MAP_SHARED, fd, begin);
+	if (out->data == (void*)-1)
+		return git_os_error();
+	out->len = len;
+	return GIT_SUCCESS;
+}
+
+void gitfo_free_map(gitfo_map *out)
+{
+	munmap(out->data, out->len);
+}
+
 /* cached diskio */
 struct gitfo_cache {
 	git_file fd;

src/fileops.h

@@ -30,6 +30,10 @@ typedef struct {  /* file io buffer  */
 	size_t len;  /* data length  */
 } gitfo_buf;
 
+typedef struct {  /* memory mapped buffer */
+	void *data;   /* data bytes */
+	size_t len;   /* data length */
+} gitfo_map;
 
 extern int gitfo_exists(const char *path);
 extern int gitfo_open(const char *path, int flags);
@@ -44,6 +48,33 @@ extern int gitfo_read_file(gitfo_buf *obj, const char *path);
 extern void gitfo_free_buf(gitfo_buf *obj);
 
 /**
+ * Read-only map all or part of a file into memory.
+ * When possible this function should favor a virtual memory
+ * style mapping over some form of malloc()+read(), as the
+ * data access will be random and is not likely to touch the
+ * majority of the region requested.
+ *
+ * @param out buffer to populate with the mapping information.
+ * @param fd open descriptor to configure the mapping from.
+ * @param begin first byte to map, this should be page aligned.
+ * @param end number of bytes to map.
+ * @return
+ * - GIT_SUCCESS on success;
+ * - GIT_EOSERR on an unspecified OS related error.
+ */
+extern int gitfo_map_ro(
+	gitfo_map *out,
+	git_file fd,
+	off_t begin,
+	size_t len);
+
+/**
+ * Release the memory associated with a previous memory mapping.
+ * @param map the mapping description previously configured.
+ */
+extern void gitfo_free_map(gitfo_map *map);
+
+/**
  * Walk each directory entry, except '.' and '..', calling fn(state).
  *
  * @param pathbuf buffer the function reads the initial directory