Add cleaner line-by-line reading primitives

Two places in passt need to read files line by line (one parsing
resolv.conf, the other parsing /proc/net/*.  They can't use fgets()
because in glibc that can allocate memory.  Instead they use an
implementation line_read() in util.c.  This has some problems:

 * It has two completely separate modes of operation, one buffering
   and one not, the relation between these and how they're activated
   is subtle and confusing
 * At least in non-buffered mode, it will mishandle an empty line,
   folding them onto the start of the next non-empty line
 * In non-buffered mode it will use lseek() which prevents using this
   on non-regular files (we don't need that at present, but it's a
   surprising limitation)
 * It has a lot of difficult to read pointer mangling

Add a new cleaner implementation of allocation-free line-by-line
reading in lineread.c.  This one always buffers, using a state
structure to keep track of what we need.  This is larger than I'd
like, but it turns out handling all the edge cases of line-by-line
reading in C is surprisingly hard.

This just adds the code, subsequent patches will change the existing
users of line_read() to the new implementation.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

1 files changed, 31 insertions, 0 deletions

diff --git a/lineread.h b/lineread.h
new file mode 100644
index 0000000..b6faff5
--- /dev/null
+++ b/lineread.h
@@ -0,0 +1,31 @@
+/* SPDX-License-Identifier: AGPL-3.0-or-later
+ * Copyright Red Hat
+ * Author: David Gibson <david@gibson.dropbear.id.au>
+ */
+
+#ifndef LINEREAD_H
+#define LINEREAD_H
+
+#define LINEREAD_BUFFER_SIZE	8192
+
+/**
+ * struct lineread - Line reader state
+ * @fd:		File descriptor lines are read from
+ * @next_line:	Offset in @buf of the start of the first line not yet
+ *		returned by lineread_get()
+ * @count:	Number of bytes in @buf read from the file, but not yet
+ *		returned by lineread_get()
+ * @buf:	Buffer storing data read from file.
+ */
+struct lineread {
+	int fd; int next_line;
+	int count;
+
+	/* One extra byte for possible trailing \0 */
+	char buf[LINEREAD_BUFFER_SIZE+1];
+};
+
+void lineread_init(struct lineread *lr, int fd);
+int lineread_get(struct lineread *lr, char **line);
+
+#endif /* _LINEREAD_H */