Provide pg_preadv() and pg_pwritev().

Provide synchronous vectored file I/O routines.  These map to preadv()
and pwritev(), with fallback implementations for systems that don't have
them.  Also provide a wrapper pg_pwritev_with_retry() that automatically
retries on short writes.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKGJA%2Bu-220VONeoREBXJ9P3S94Y7J%2BkqCnTYmahvZJwM%3Dg%40mail.gmail.com

3 files changed, 76 insertions, 0 deletions

diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index ddaa9e8e182..f4d9f3b408d 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -412,6 +412,9 @@
 /* Define to 1 if you have the `pread' function. */
 #undef HAVE_PREAD
 
+/* Define to 1 if you have the `preadv' function. */
+#undef HAVE_PREADV
+
 /* Define to 1 if you have the `pstat' function. */
 #undef HAVE_PSTAT
 
@@ -430,6 +433,9 @@
 /* Define to 1 if you have the `pwrite' function. */
 #undef HAVE_PWRITE
 
+/* Define to 1 if you have the `pwritev' function. */
+#undef HAVE_PWRITEV
+
 /* Define to 1 if you have the `random' function. */
 #undef HAVE_RANDOM
 
@@ -445,6 +451,9 @@
 /* Define to 1 if you have the `readlink' function. */
 #undef HAVE_READLINK
 
+/* Define to 1 if you have the `readv' function. */
+#undef HAVE_READV
+
 /* Define to 1 if you have the global variable
    'rl_completion_append_character'. */
 #undef HAVE_RL_COMPLETION_APPEND_CHARACTER
@@ -629,6 +638,9 @@
 /* Define to 1 if you have the <sys/ucred.h> header file. */
 #undef HAVE_SYS_UCRED_H
 
+/* Define to 1 if you have the <sys/uio.h> header file. */
+#undef HAVE_SYS_UIO_H
+
 /* Define to 1 if you have the <sys/un.h> header file. */
 #undef HAVE_SYS_UN_H
 
@@ -683,6 +695,9 @@
 /* Define to 1 if you have the <winldap.h> header file. */
 #undef HAVE_WINLDAP_H
 
+/* Define to 1 if you have the `writev' function. */
+#undef HAVE_WRITEV
+
 /* Define to 1 if you have the `X509_get_signature_nid' function. */
 #undef HAVE_X509_GET_SIGNATURE_NID
 
diff --git a/src/include/port.h b/src/include/port.h
index 3e9d4fcd376..6486db9fdde 100644
--- a/src/include/port.h
+++ b/src/include/port.h
@@ -431,6 +431,8 @@ extern ssize_t pg_pread(int fd, void *buf, size_t nbyte, off_t offset);
 extern ssize_t pg_pwrite(int fd, const void *buf, size_t nbyte, off_t offset);
 #endif
 
+/* For pg_pwritev() and pg_preadv(), see port/pg_iovec.h. */
+
 #if !HAVE_DECL_STRLCAT
 extern size_t strlcat(char *dst, const char *src, size_t siz);
 #endif
diff --git a/src/include/port/pg_iovec.h b/src/include/port/pg_iovec.h
new file mode 100644
index 00000000000..335f35bf0eb
--- /dev/null
+++ b/src/include/port/pg_iovec.h
@@ -0,0 +1,59 @@
+/*-------------------------------------------------------------------------
+ *
+ * pg_iovec.h
+ *	  Header for the vectored I/O functions in src/port/p{read,write}.c.
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/port/pg_iovec.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PG_IOVEC_H
+#define PG_IOVEC_H
+
+#include <limits.h>
+
+#ifdef HAVE_SYS_UIO_H
+#include <sys/uio.h>
+#endif
+
+/* If <sys/uio.h> is missing, define our own POSIX-compatible iovec struct. */
+#ifndef HAVE_SYS_UIO_H
+struct iovec
+{
+	void	   *iov_base;
+	size_t		iov_len;
+};
+#endif
+
+/*
+ * If <limits.h> didn't define IOV_MAX, define our own.  POSIX requires at
+ * least 16.
+ */
+#ifndef IOV_MAX
+#define IOV_MAX 16
+#endif
+
+/* Define a reasonable maximum that is safe to use on the stack. */
+#define PG_IOV_MAX Min(IOV_MAX, 32)
+
+#ifdef HAVE_PREADV
+#define pg_preadv preadv
+#else
+extern ssize_t pg_preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset);
+#endif
+
+#ifdef HAVE_PWRITEV
+#define pg_pwritev pwritev
+#else
+extern ssize_t pg_pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset);
+#endif
+
+extern ssize_t pg_pwritev_with_retry(int fd,
+									 const struct iovec *iov,
+									 int iovcnt,
+									 off_t offset);
+
+#endif							/* PG_IOVEC_H */