Lay the foundations for pkt-line parsing

This are the types I intend to use for pkt-line parsing and (later)
creation. git_pkt serves as a base pointer type and once you know what
type it is you can use the real one (command, tip list, etc.)

Signed-off-by: Carlos Martín Nieto <carlos@cmartin.tk>

include/git2/pkt.h

+/*
+ * This file is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License, version 2,
+ * as published by the Free Software Foundation.
+ *
+ * In addition to the permissions in the GNU General Public License,
+ * the authors give you unlimited permission to link the compiled
+ * version of this file into combinations with other programs,
+ * and to distribute those combinations without any restriction
+ * coming from the use of this file.  (The General Public License
+ * restrictions do apply in other respects; for example, they cover
+ * modification of the file, and distribution when not linked into
+ * a combined executable.)
+ *
+ * This file is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; see the file COPYING.  If not, write to
+ * the Free Software Foundation, 51 Franklin Street, Fifth Floor,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#include "git2/net.h"
+
+enum git_pkt_type {
+	GIT_PKT_CMD,
+	GIT_PKT_FLUSH,
+	GIT_PKT_HEAD,
+	GIT_PKT_HAVE,
+};
+
+/* This would be a flush pkt */
+struct git_pkt {
+	enum git_pkt_type type;
+};
+
+struct git_pkt_cmd {
+	enum git_pkt_type type;
+	char *cmd;
+	char *path;
+	char *host;
+};
+
+/* This is a pkt-line with some info in it */
+struct git_pkt_head {
+	enum git_pkt_type type;
+	git_remote_head head;
+};

include/git2/types.h

@@ -182,6 +182,12 @@ typedef int (*git_transport_cb)(git_transport *transport);
 typedef struct git_remote_head git_remote_head;
 typedef struct git_headarray git_headarray;
 
+/* Several types of packets */
+typedef enum git_pkt_type git_pkt_type;
+typedef struct git_pkt git_pkt;
+typedef struct git_pkt_cmd git_pkt_cmd;
+typedef struct git_pkt_head git_pkt_head;
+
 /** @} */
 GIT_END_DECL
 

src/pkt.c

+/*
+ * This file is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License, version 2,
+ * as published by the Free Software Foundation.
+ *
+ * In addition to the permissions in the GNU General Public License,
+ * the authors give you unlimited permission to link the compiled
+ * version of this file into combinations with other programs,
+ * and to distribute those combinations without any restriction
+ * coming from the use of this file.  (The General Public License
+ * restrictions do apply in other respects; for example, they cover
+ * modification of the file, and distribution when not linked into
+ * a combined executable.)
+ *
+ * This file is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; see the file COPYING.  If not, write to
+ * the Free Software Foundation, 51 Franklin Street, Fifth Floor,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#include "git2/pkt.h"
+#include "git2/types.h"
+#include "git2/errors.h"
+
+#include "common.h"
+#include "util.h"
+
+/*
+ * As per the documentation, the syntax is:
+ *
+ * pkt-line    = data-pkt / flush-pkt
+ * data-pkt    = pkt-len pkt-payload
+ * pkt-len     = 4*(HEXDIG)
+ * pkt-payload = (pkt-len -4)*(OCTET)
+ * flush-pkt   = "0000"
+ *
+ * Which means that the first four bytes are the length of the line,
+ * in ASCII hexadecimal (including itself)
+ */
+
+int git_pkt_parse_line(git_pkt **head, const char *line, const char **out)
+{
+	int error = GIT_SUCCESS;
+	long int len;
+	const char *num_end;
+	git_pkt *pkt;
+
+	error = git__strtol32(&len, line, &num_end, 16);
+	if (error < GIT_SUCCESS)
+		return error;
+
+	/*
+	 * TODO: How do we deal with empty lines? Try again? with the next
+	 * line?
+	 */
+	if (len == 4) {
+		*out = num_end;
+		return GIT_SUCCESS;
+	}
+
+	if (len == 0) { /* Flush, should go into its own fn */
+		pkt = git__malloc(sizeof(git_pkt));
+		if (pkt == NULL)
+			return GIT_ENOMEM;
+
+		pkt->type = GIT_PKT_FLUSH;
+		*head = pkt;
+		*out = num_end;
+		return GIT_SUCCESS;
+	}
+
+	/* TODO: Write the rest of this thing */
+
+	return GIT_SUCCESS;
+}