cdatapack: add some windows portability

Summary:
This is a first set of changes to help `cdatapack` compile on Windows. Second
set will include adding some way of using `mman` on Windows.

Test Plan:
- `make local` on Linux, `rt`
- with some intermediary solution for `mman` this also builds on Windows 10,
  I was able to produce `cdatapack_get.exe` and `cdatapack_dump.exe`. Here's an
  example:
```
PS C:\Code\fb-hg-rpms\fb-hgext\cdatapack> .\cdatapack_get.exe 3ba0b10b8d251743a2692e042b114c1204b19d74 88dadb363234ec4fec3df85810810d6073288350

xplat/third-party/yarn/offline-mirror/smoothscroll-polyfill-0.3.5.tgz
Node                                      Delta Base                                Delta SHA1                                Delta Length
88dadb363234ec4fec3df85810810d6073288350  0000000000000000000000000000000000000000  466e6039b51cb525d70e1a5077ef81e064678eae  26057
```

Reviewers: durham, #fbhgext

Differential Revision: https://phab.mercurial-scm.org/D106

cdatapack/cdatapack.c

@@ -18,8 +18,6 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <memory.h>
-#include <unistd.h>
-#include <arpa/inet.h>
 #include <sys/mman.h>
 
 #if defined(__linux__)
@@ -38,6 +36,8 @@
 #endif /* #if defined(__APPLE__) */
 
 #include <lz4.h>
+#include "portability/inet.h"
+#include "portability/unistd.h"
 
 #include "cdatapack.h"
 #include "buffer.h"
@@ -47,11 +47,8 @@
 /**
  * This is an exact representation of an index entry on disk.  Do not consume
  * the fields directly, as they may need processing.
- *
- * NOTE: this uses gcc's __attribute__((packed)) syntax to indicate a packed
- * data structure, which obviously has potential portability issues.
  */
-typedef struct _disk_index_entry_t {
+PACKEDSTRUCT(typedef struct _disk_index_entry_t {
   uint8_t node[NODE_SZ];
 
   // offset of the next element in the delta chain in the index file
@@ -61,7 +58,7 @@ typedef struct _disk_index_entry_t {
   // file.
   data_offset_t data_offset;
   data_offset_t data_sz;
-} __attribute__((packed)) disk_index_entry_t;
+}) disk_index_entry_t;
 
 /**
  * This represents offsets into the index indicating the range of a fanout
@@ -91,17 +88,14 @@ typedef struct _pack_chain_t {
 /**
  * This is an exact representation of an index file's header on disk.  Do not
  * consume the fields directly, as they may need processing.
- *
- * NOTE: this uses gcc's __attribute__((packed)) syntax to indicate a packed
- * data structure, which obviously has potential portability issues.
  */
-typedef struct _disk_index_header_t {
+PACKEDSTRUCT(typedef struct _disk_index_header_t {
 #define VERSION 0
   uint8_t version;
 
 #define LARGE_FANOUT 0x80
   uint8_t config;
-} __attribute__((packed)) disk_index_header_t;
+}) disk_index_header_t;
 
 static void unpack_disk_deltachunk(
     const disk_index_entry_t *disk_deltachunk,

clib/portability/inet.h

@@ -0,0 +1,13 @@
+#ifndef PORTABILITY_INET_H
+#define PORTABILITY_INET_H
+
+#if defined(_MSC_VER)
+	#include <winsock2.h>
+	#pragma comment(lib, "Ws2_32.lib")
+	/* See https://fburl.com/7hd350j8 for more details about Ws2_32.lib */
+#else
+	#include <arpa/inet.h>
+#endif
+
+#endif /* PORTABILITY_INET_H */
+

clib/portability/portability.h

@@ -13,4 +13,11 @@
 	#define COMPOUND_LITERAL(typename_) (typename_)
 #endif /* #if defined(_MSC_VER) */
 
+#if defined(_MSC_VER)
+	#define PACKEDSTRUCT(__Declaration__) __pragma(pack(push, 1)) \
+	                                      __Declaration__ __pragma(pack(pop))
+#else
+	#define PACKEDSTRUCT(__Declaration__) __Declaration__ __attribute__((packed))
+#endif
+
 #endif /* #ifndef PORTABILITY_PORTABILITY_H */

clib/portability/unistd.h

@@ -0,0 +1,16 @@
+#ifndef PORTABILITY_UNISTD_H
+#define PORTABILITY_UNISTD_H
+
+#if defined(_MSC_VER)
+	#include <io.h>
+	/* MSVC's io.h header shows deprecation
+	warnings on these without underscore */
+	#define lseek _lseek
+	#define open _open
+	#define close _close
+#else
+	#include <unistd.h>
+#endif
+
+#endif /* PORTABILITY_UNISTD_H */
+