reftable: introduce "reftable-system.h" header

We're including a couple of standard headers like <stdint.h> in a bunch
of locations, which makes it hard for a project to plug in their own
logic for making required functionality available. For us this is for
example via "compat/posix.h", which already includes all of the system
headers relevant to us.

Introduce a new "reftable-system.h" header that allows projects to
provide their own headers. This new header is supposed to contain all
the project-specific bits to provide the POSIX-like environment, and some
additional supporting code. With this change, we thus have the following
split in our system-specific code:

  - "reftable/reftable-system.h" is the project-specific header that
    provides a POSIX-like environment. Every project is expected to
    provide their own implementation.

  - "reftable/system.h" contains the project-independent definition of
    the interfaces that a project needs to implement. This file should
    not be touched by a project.

  - "reftable/system.c" contains the project-specific implementation of
    the interfaces defined in "system.h". Again, every project is
    expected to provide their own implementation.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/reftable-basics.h

@@ -9,7 +9,7 @@
 #ifndef REFTABLE_BASICS_H
 #define REFTABLE_BASICS_H
 
-#include <stddef.h>
+#include "reftable-system.h"
 
 /* A buffer that contains arbitrary byte slices. */
 struct reftable_buf {

reftable/reftable-block.h

@@ -9,8 +9,7 @@
 #ifndef REFTABLE_BLOCK_H
 #define REFTABLE_BLOCK_H
 
-#include <stdint.h>
-
+#include "reftable-system.h"
 #include "reftable-basics.h"
 #include "reftable-blocksource.h"
 #include "reftable-iterator.h"

reftable/reftable-blocksource.h

@@ -9,7 +9,7 @@
 #ifndef REFTABLE_BLOCKSOURCE_H
 #define REFTABLE_BLOCKSOURCE_H
 
-#include <stdint.h>
+#include "reftable-system.h"
 
 /*
  * Generic wrapper for a seekable readable file.

reftable/reftable-error.h

@@ -9,6 +9,8 @@
 #ifndef REFTABLE_ERROR_H
 #define REFTABLE_ERROR_H
 
+#include "reftable-system.h"
+
 /*
  * Errors in reftable calls are signaled with negative integer return values. 0
  * means success.

reftable/reftable-fsck.h

@@ -1,6 +1,7 @@
 #ifndef REFTABLE_FSCK_H
 #define REFTABLE_FSCK_H
 
+#include "reftable-system.h"
 #include "reftable-stack.h"
 
 enum reftable_fsck_error {

reftable/reftable-iterator.h

@@ -9,6 +9,7 @@
 #ifndef REFTABLE_ITERATOR_H
 #define REFTABLE_ITERATOR_H
 
+#include "reftable-system.h"
 #include "reftable-record.h"
 
 struct reftable_iterator_vtable;

reftable/reftable-merged.h

@@ -9,6 +9,7 @@
 #ifndef REFTABLE_MERGED_H
 #define REFTABLE_MERGED_H
 
+#include "reftable-system.h"
 #include "reftable-iterator.h"
 
 /*

reftable/reftable-record.h

@@ -9,8 +9,8 @@
 #ifndef REFTABLE_RECORD_H
 #define REFTABLE_RECORD_H
 
+#include "reftable-system.h"
 #include "reftable-basics.h"
-#include <stdint.h>
 
 /*
  * Basic data types

reftable/reftable-stack.h

@@ -9,6 +9,7 @@
 #ifndef REFTABLE_STACK_H
 #define REFTABLE_STACK_H
 
+#include "reftable-system.h"
 #include "reftable-writer.h"
 
 /*

reftable/reftable-system.h

@@ -0,0 +1,15 @@
+#ifndef REFTABLE_SYSTEM_H
+#define REFTABLE_SYSTEM_H
+
+/*
+ * This header defines the platform-specific bits required to compile the
+ * reftable library. It should provide an environment that bridges over the
+ * gaps between POSIX and your system, as well as the zlib interfaces. This
+ * header is expected to be changed by the individual project.
+ */
+
+#define MINGW_DONT_HANDLE_IN_USE_ERROR
+#include "compat/posix.h"
+#include "compat/zlib-compat.h"
+
+#endif

reftable/reftable-table.h

@@ -9,6 +9,7 @@
 #ifndef REFTABLE_TABLE_H
 #define REFTABLE_TABLE_H
 
+#include "reftable-system.h"
 #include "reftable-iterator.h"
 #include "reftable-block.h"
 #include "reftable-blocksource.h"

reftable/reftable-writer.h

@@ -9,11 +9,9 @@
 #ifndef REFTABLE_WRITER_H
 #define REFTABLE_WRITER_H
 
+#include "reftable-system.h"
 #include "reftable-record.h"
 
-#include <stdint.h>
-#include <unistd.h> /* ssize_t */
-
 /* Writing single reftables */
 
 /* reftable_write_options sets options for writing a single reftable. */

reftable/system.h

@@ -9,11 +9,14 @@
 #ifndef SYSTEM_H
 #define SYSTEM_H
 
-/* This header glues the reftable library to the rest of Git */
+/*
+ * This header defines the platform-agnostic interface that is to be
+ * implemented by the project to make it work on their respective supported
+ * systems, and to integrate it into the project itself. This header is not
+ * expected to be changed by the individual project.
+ */
 
-#define MINGW_DONT_HANDLE_IN_USE_ERROR
-#include "compat/posix.h"
-#include "compat/zlib-compat.h"
+#include "reftable-system.h"
 
 /*
  * Return a random 32 bit integer. This function is expected to return