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

[Stefan Beller <sbeller@google.com>]

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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 git-compat-util.h |  1 +
 wrapper.c         | 22 ++++++++++++++++++++++
 2 files changed, 23 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 4f720fe..f71237c 100644
--- a/wrapper.c
+++ b/wrapper.c
@@ -252,6 +252,28 @@ ssize_t xread(int fd, void *buf, size_t len)
 }
 
 /*
+ * xread_nonblock() is the same a read(), but it automatically restarts read()
+ * interrupted operations (EINTR). xread_nonblock() DOES NOT GUARANTEE that
+ * "len" bytes is read. EWOULDBLOCK is turned into EAGAIN.
+ */
+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.
-- 
2.6.4.443.ge094245.dirty