[PATCHv4 04/14] strbuf: add strbuf_read_once to read without blocking

[Stefan Beller <sbeller@google.com>]

The new call will read from a file descriptor into a strbuf once. The
underlying call xread_nonblock is meant to execute without blocking if
the file descriptor is set to O_NONBLOCK. It is a bug to call
strbuf_read_once on a file descriptor which would block.

Signed-off-by: Stefan Beller <sbeller@google.com>
---
 strbuf.c | 11 +++++++++++
 strbuf.h |  9 +++++++++
 2 files changed, 20 insertions(+)

diff --git a/strbuf.c b/strbuf.c
index cce5eed..35e71b8 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -384,6 +384,17 @@ ssize_t strbuf_read(struct strbuf *sb, int fd, size_t hint)
 	return sb->len - oldlen;
 }
 
+ssize_t strbuf_read_once(struct strbuf *sb, int fd, size_t hint)
+{
+	ssize_t cnt;
+
+	strbuf_grow(sb, hint ? hint : 8192);
+	cnt = xread_nonblock(fd, sb->buf + sb->len, sb->alloc - sb->len - 1);
+	if (cnt > 0)
+		strbuf_setlen(sb, sb->len + cnt);
+	return cnt;
+}
+
 #define STRBUF_MAXLINK (2*PATH_MAX)
 
 int strbuf_readlink(struct strbuf *sb, const char *path, size_t hint)
diff --git a/strbuf.h b/strbuf.h
index aef2794..ea69665 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -367,6 +367,15 @@ extern size_t strbuf_fread(struct strbuf *, size_t, FILE *);
 extern ssize_t strbuf_read(struct strbuf *, int fd, size_t hint);
 
 /**
+ * Read from a file descriptor that is marked as O_NONBLOCK without
+ * blocking.  Returns the number of new bytes appended to the sb.
+ * Negative return value signals there was an error returned from
+ * underlying read(2), in which case the caller should check errno.
+ * e.g. errno == EAGAIN when the read may have blocked.
+ */
+extern ssize_t strbuf_read_once(struct strbuf *, int fd, size_t hint);
+
+/**
  * Read the contents of a file, specified by its path. The third argument
  * can be used to give a hint about the file size, to avoid reallocs.
  */
-- 
2.5.0.272.ga84127c.dirty