[PATCH 3/6] util: Add write_remainder() helper

[David Gibson <david@gibson.dropbear.id.au>]

We have several places where we want to write(2) a buffer or buffers and we
do (or should) handle sort write()s by retrying until everything is
successfully written.  Add a helper for this in util.c.

This version has some differences from the typical write_all() function.
First, take an IO vector rather than a single buffer, because that will be
useful for some of our cases.  Second, allow it to take an parameter to
skip the first n bytes of the given buffers.  This will be usefl for some
of the cases we want, and also falls out quite naturally from the
implementation.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 util.c | 32 ++++++++++++++++++++++++++++++++
 util.h |  1 +
 2 files changed, 33 insertions(+)

diff --git a/util.c b/util.c
index fb6a0430..a475f2b5 100644
--- a/util.c
+++ b/util.c
@@ -19,6 +19,7 @@
 #include <arpa/inet.h>
 #include <net/ethernet.h>
 #include <sys/epoll.h>
+#include <sys/uio.h>
 #include <fcntl.h>
 #include <string.h>
 #include <time.h>
@@ -597,3 +598,34 @@ size_t iov_offset(const struct iovec *iov, size_t n, size_t *offset)
 
 	return i;
 }
+
+/* write_remainder() - write the tail of an IO vector to an fd
+ * @fd:		File descriptor
+ * @iov:	IO vector
+ * @iovcnt:	Number of entries in @iov
+ * @skip:	Number of bytes of the vector to skip writing
+ *
+ * Return: 0 on success, -1 on error (with errno set)
+ *
+ * #syscalls write writev
+ */
+int write_remainder(int fd, const struct iovec *iov, int iovcnt, size_t skip)
+{
+	int i;
+
+	while ((i = iov_offset(iov, iovcnt, &skip)) < iovcnt) {
+		ssize_t rc;
+
+		if (skip)
+			rc = write(fd, (char *)iov[i].iov_base + skip,
+				   iov[i].iov_len - skip);
+		else
+			rc = writev(fd, &iov[i], iovcnt - i);
+
+		if (rc < 0)
+			return -1;
+		skip += rc;
+	}
+
+	return 0;
+}
diff --git a/util.h b/util.h
index 62fad6fe..ee380f55 100644
--- a/util.h
+++ b/util.h
@@ -230,6 +230,7 @@ int __daemon(int pidfile_fd, int devnull_fd);
 int fls(unsigned long x);
 int write_file(const char *path, const char *buf);
 size_t iov_offset(const struct iovec *iov, size_t n, size_t *offset);
+int write_remainder(int fd, const struct iovec *iov, int iovcnt, size_t skip);
 
 /**
  * mod_sub() - Modular arithmetic subtraction
-- 
@@ -230,6 +230,7 @@ int __daemon(int pidfile_fd, int devnull_fd);
 int fls(unsigned long x);
 int write_file(const char *path, const char *buf);
 size_t iov_offset(const struct iovec *iov, size_t n, size_t *offset);
+int write_remainder(int fd, const struct iovec *iov, int iovcnt, size_t skip);
 
 /**
  * mod_sub() - Modular arithmetic subtraction
-- 
2.43.2