strbuf: add strbuf_read_once to read without blocking

The new call will read from a file descriptor into a strbuf once. The
underlying call xread is just run once. xread only reattempts
reading in case of EINTR, which makes it suitable to use for a
nonblocking read.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 19 insertions, 0 deletions

diff --git a/strbuf.c b/strbuf.c
index d76f0aed85..38686ffb65 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(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 7123fca7af..2bf90e70fc 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -367,6 +367,14 @@ extern size_t strbuf_fread(struct strbuf *, size_t, FILE *);
 extern ssize_t strbuf_read(struct strbuf *, int fd, size_t hint);
 
 /**
+ * Read the contents of a given file descriptor partially by using only one
+ * attempt of xread. The third argument can be used to give a hint about the
+ * file size, to avoid reallocs. Returns the number of new bytes appended to
+ * the sb.
+ */
+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.
  */