Re: [PATCH 3/8] xread_nonblock: add functionality to read from fds without blocking

[Junio C Hamano <gitster@pobox.com>]

Eric Sunshine <ericsunshine@gmail.com> writes:

> The last sentence is confusing. From the commit message, we learn that
> this function doesn't care about EAGAIN or EWOULDBLOCK, yet the above
> comment seems to imply that it does. What it really ought to be saying
> is that "as a convenience, errno is transformed from EWOULDBLOCK to
> EAGAIN so that the caller only has to check for EAGAIN".

Let's do this for now, then.

-- >8 --
From: Stefan Beller <sbeller@google.com>
Date: Mon, 14 Dec 2015 11:37:13 -0800
Subject: [PATCH] xread_nonblock: add functionality to read from fds without blocking

Provide a wrapper to read(), similar to xread(), that restarts on
EINTR but not EAGAIN (or EWOULDBLOCK). This enables the caller to
handle polling itself, possibly polling multiple sockets or performing
some other action.

Helped-by: Jacob Keller <jacob.keller@gmail.com>
Helped-by: Jeff King <peff@peff.net>,
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 git-compat-util.h |  1 +
 wrapper.c         | 24 ++++++++++++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/git-compat-util.h b/git-compat-util.h
index 8e39867..87456a3 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -723,6 +723,7 @@ extern void *xmmap(void *start, size_t length, int prot, int flags, int fd, off_
 extern void *xmmap_gently(void *start, size_t length, int prot, int flags, int fd, off_t offset);
 extern int xopen(const char *path, int flags, ...);
 extern ssize_t xread(int fd, void *buf, size_t len);
+extern ssize_t xread_nonblock(int fd, void *buf, size_t len);
 extern ssize_t xwrite(int fd, const void *buf, size_t len);
 extern ssize_t xpread(int fd, void *buf, size_t len, off_t offset);
 extern int xdup(int fd);
diff --git a/wrapper.c b/wrapper.c
index 1770efa..0b5b03d 100644
--- a/wrapper.c
+++ b/wrapper.c
@@ -259,6 +259,30 @@ ssize_t xread(int fd, void *buf, size_t len)
 }
 
 /*
+ * xread_nonblock() automatically restarts interrupted operations
+ * (EINTR). xread_nonblock() DOES NOT GUARANTEE that "len" bytes is
+ * read.  For convenience to callers that mark the file descriptor
+ * non-blocking, EWOULDBLOCK is turned into EAGAIN to allow them to
+ * check only for EAGAIN (POSIX.1 allows either to be returned).
+ */
+ssize_t xread_nonblock(int fd, void *buf, size_t len)
+{
+	ssize_t nr;
+	if (len > MAX_IO_SIZE)
+		len = MAX_IO_SIZE;
+	while (1) {
+		nr = read(fd, buf, len);
+		if (nr < 0) {
+			if (errno == EINTR)
+				continue;
+			if (errno == EWOULDBLOCK)
+				errno = EAGAIN;
+		}
+		return nr;
+	}
+}
+
+/*
  * xwrite() is the same a write(), but it automatically restarts write()
  * operations with a recoverable error (EAGAIN and EINTR). xwrite() DOES NOT
  * GUARANTEE that "len" bytes is written even if the operation is successful.
-- 
2.7.0-rc0-109-gb762328