Provide pg_pread() and pg_pwrite() for random I/O.

Forward to POSIX pread() and pwrite(), or emulate them if unavailable.
The emulation is not perfect as the file position is changed, so
we'll put pg_ prefixes on the names to minimize the risk of confusion
in future patches that might inadvertently try to mix pread() and read()
on the same file descriptor.

Author: Thomas Munro
Reviewed-by: Tom Lane, Jesper Pedersen
Discussion: https://postgr.es/m/CAEepm=02rapCpPR3ZGF2vW=SBHSdFYO_bz_f-wwWJonmA3APgw@mail.gmail.com

src/port/pread.c

@@ -0,0 +1,55 @@
+/*-------------------------------------------------------------------------
+ *
+ * pread.c
+ *	  Implementation of pread(2) for platforms that lack one.
+ *
+ * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *	  src/port/pread.c
+ *
+ * Note that this implementation changes the current file position, unlike
+ * the POSIX function, so we use the name pg_pread().
+ *
+ *-------------------------------------------------------------------------
+ */
+
+
+#include "postgres.h"
+
+#ifdef WIN32
+#include <windows.h>
+#else
+#include <unistd.h>
+#endif
+
+ssize_t
+pg_pread(int fd, void *buf, size_t size, off_t offset)
+{
+#ifdef WIN32
+	OVERLAPPED	overlapped = {0};
+	HANDLE		handle;
+	DWORD		result;
+
+	handle = (HANDLE) _get_osfhandle(fd);
+	if (handle == INVALID_HANDLE_VALUE)
+	{
+		errno = EBADF;
+		return -1;
+	}
+
+	overlapped.Offset = offset;
+	if (!ReadFile(handle, buf, size, &result, &overlapped))
+	{
+		_dosmaperr(GetLastError());
+		return -1;
+	}
+
+	return result;
+#else
+	if (lseek(fd, offset, SEEK_SET) < 0)
+		return -1;
+
+	return read(fd, buf, size);
+#endif
+}