Add basic support for handling and serializing hard links

In libfstree, add a function to add a hard link to the fstree. The
hard links stores the target in the data.target field, canonicalizes
the target and sets a sentinel mode. A second function is used to
resolve link, i.e. replacing it with a direct pointer, setting another
sentinel mode and increasing the targets link count.

The post process function tries to resolve unresolved hard links and
only allocates inode numbers for nodes that aren't hard links. If the
target node of a hard link does not have an inode number yet, the two
need to be swapped, since this is also the order in which they are
serialized.

The serialization function in libcommon simply has to skip hard link
nodes and when writing directory entries, use the inode num/ref of
the target node.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>

3 files changed, 179 insertions, 13 deletions

diff --git a/lib/fstree/Makemodule.am b/lib/fstree/Makemodule.am
index 751d7dc..56394a6 100644
--- a/lib/fstree/Makemodule.am
+++ b/lib/fstree/Makemodule.am
@@ -1,5 +1,5 @@
 libfstree_a_SOURCES = lib/fstree/fstree.c lib/fstree/fstree_from_file.c
-libfstree_a_SOURCES += lib/fstree/fstree_sort.c
+libfstree_a_SOURCES += lib/fstree/fstree_sort.c lib/fstree/hardlink.c
 libfstree_a_SOURCES += lib/fstree/post_process.c lib/fstree/get_path.c
 libfstree_a_SOURCES += lib/fstree/mknode.c
 libfstree_a_SOURCES += lib/fstree/add_by_path.c lib/fstree/get_by_path.c
diff --git a/lib/fstree/hardlink.c b/lib/fstree/hardlink.c
new file mode 100644
index 0000000..8a79d46
--- /dev/null
+++ b/lib/fstree/hardlink.c
@@ -0,0 +1,65 @@
+/* SPDX-License-Identifier: GPL-3.0-or-later */
+/*
+ * hardlink.c
+ *
+ * Copyright (C) 2019 David Oberhollenzer <goliath@infraroot.at>
+ */
+#include "config.h"
+
+#include "fstree.h"
+
+#include <string.h>
+#include <stdlib.h>
+#include <errno.h>
+
+tree_node_t *fstree_add_hard_link(fstree_t *fs, const char *path,
+				  const char *target)
+{
+	struct stat sb;
+	tree_node_t *n;
+
+	memset(&sb, 0, sizeof(sb));
+	sb.st_mode = S_IFLNK | 0777;
+
+	n = fstree_add_generic(fs, path, &sb, target);
+	if (n != NULL) {
+		if (canonicalize_name(n->data.target)) {
+			free(n);
+			errno = EINVAL;
+			return NULL;
+		}
+
+		n->mode = FSTREE_MODE_HARD_LINK;
+	}
+
+	return n;
+}
+
+int fstree_resolve_hard_link(fstree_t *fs, tree_node_t *node)
+{
+	tree_node_t *start = node;
+
+	while (node->mode == FSTREE_MODE_HARD_LINK ||
+	       node->mode == FSTREE_MODE_HARD_LINK_RESOLVED) {
+		if (node->mode == FSTREE_MODE_HARD_LINK_RESOLVED) {
+			node = node->data.target_node;
+		} else {
+			node = fstree_get_node_by_path(fs, fs->root,
+						       node->data.target,
+						       false, false);
+			if (node == NULL)
+				return -1;
+		}
+
+		if (node == start) {
+			errno = EMLINK;
+			return -1;
+		}
+	}
+
+	start->mode = FSTREE_MODE_HARD_LINK_RESOLVED;
+	start->data.target_node = node;
+
+	node->link_count += 1;
+	return 0;
+}
diff --git a/lib/fstree/post_process.c b/lib/fstree/post_process.c
index ea52e4f..e6bcb3f 100644
--- a/lib/fstree/post_process.c
+++ b/lib/fstree/post_process.c
@@ -7,33 +7,129 @@
 #include "internal.h"
 
 #include <stdlib.h>
+#include <string.h>
+#include <assert.h>
 #include <stdio.h>
+#include <errno.h>
 
-static void map_child_nodes(fstree_t *fs, tree_node_t *root, size_t *counter)
+static void swap_link_with_target(tree_node_t *node)
+{
+	tree_node_t *tgt, *it;
+
+	tgt = node->data.target_node;
+
+	node->xattr_idx = tgt->xattr_idx;
+	node->uid = tgt->uid;
+	node->gid = tgt->gid;
+	node->inode_num = tgt->inode_num;
+	node->mod_time = tgt->mod_time;
+	node->mode = tgt->mode;
+	node->link_count = tgt->link_count;
+	node->inode_ref = tgt->inode_ref;
+
+	/* FIXME: data pointers now point to foreign node! */
+	node->data = tgt->data;
+
+	tgt->mode = FSTREE_MODE_HARD_LINK_RESOLVED;
+	tgt->data.target_node = node;
+
+	if (S_ISDIR(node->mode)) {
+		for (it = node->data.dir.children; it != NULL; it = it->next)
+			it->parent = node;
+	}
+}
+
+static void hard_link_snap(tree_node_t *n)
+{
+	/* XXX: the hard-link-vs-target swap may create hard links
+	   pointing to hard links, making this necessary */
+	while (n->data.target_node->mode == FSTREE_MODE_HARD_LINK_RESOLVED)
+		n->data.target_node = n->data.target_node->data.target_node;
+}
+
+static int map_child_nodes(fstree_t *fs, tree_node_t *root, size_t *counter)
 {
 	bool has_subdirs = false;
-	tree_node_t *it;
+	tree_node_t *it, *tgt;
 
 	for (it = root->data.dir.children; it != NULL; it = it->next) {
-		if (S_ISDIR(it->mode)) {
-			has_subdirs = true;
-			break;
+		if (it->mode == FSTREE_MODE_HARD_LINK_RESOLVED) {
+			hard_link_snap(it);
+			tgt = it->data.target_node;
+
+			if (tgt->inode_num == 0 && tgt->parent != root)
+				swap_link_with_target(it);
 		}
+
+		if (S_ISDIR(it->mode))
+			has_subdirs = true;
 	}
 
 	if (has_subdirs) {
 		for (it = root->data.dir.children; it != NULL; it = it->next) {
-			if (S_ISDIR(it->mode))
-				map_child_nodes(fs, it, counter);
+			if (S_ISDIR(it->mode)) {
+				if (map_child_nodes(fs, it, counter))
+					return -1;
+			}
 		}
 	}
 
 	for (it = root->data.dir.children; it != NULL; it = it->next) {
-		fs->unique_inode_count += 1;
+		if (it->mode == FSTREE_MODE_HARD_LINK_RESOLVED) {
+			hard_link_snap(it);
+		} else {
+			fs->unique_inode_count += 1;
 
-		it->inode_num = *counter;
-		*counter += 1;
+			it->inode_num = *counter;
+			*counter += 1;
+		}
 	}
+	return 0;
+}
+
+static int resolve_hard_links_dfs(fstree_t *fs, tree_node_t *n)
+{
+	tree_node_t *it;
+
+	if (n->mode == FSTREE_MODE_HARD_LINK) {
+		if (fstree_resolve_hard_link(fs, n))
+			goto fail_link;
+
+		assert(n->mode == FSTREE_MODE_HARD_LINK_RESOLVED);
+		it = n->data.target_node;
+
+		if (S_ISDIR(it->mode) && it->data.dir.visited)
+			goto fail_link_loop;
+	} else if (S_ISDIR(n->mode)) {
+		n->data.dir.visited = true;
+
+		for (it = n->data.dir.children; it != NULL; it = it->next) {
+			if (resolve_hard_links_dfs(fs, it))
+				return -1;
+		}
+
+		n->data.dir.visited = false;
+	}
+
+	return 0;
+fail_link: {
+	char *path = fstree_get_path(n);
+	fprintf(stderr, "Resolving hard link '%s' -> '%s': %s\n",
+		path == NULL ? n->name : path, n->data.target,
+		strerror(errno));
+	free(path);
+}
+	return -1;
+fail_link_loop: {
+	char *npath = fstree_get_path(n);
+	char *tpath = fstree_get_path(it);
+	fprintf(stderr, "Hard link loop detected in '%s' -> '%s'\n",
+		npath == NULL ? n->name : npath,
+		tpath == NULL ? it->name : tpath);
+	free(npath);
+	free(tpath);
+}
+	return -1;
 }
 
 static void sort_recursive(tree_node_t *n)
@@ -76,16 +172,21 @@ static file_info_t *file_list_dfs(tree_node_t *n)
 	return NULL;
 }
 
-void fstree_post_process(fstree_t *fs)
+int fstree_post_process(fstree_t *fs)
 {
 	size_t inum = 1;
 
 	sort_recursive(fs->root);
 
+	if (resolve_hard_links_dfs(fs, fs->root))
+		return -1;
+
 	fs->unique_inode_count = 0;
-	map_child_nodes(fs, fs->root, &inum);
+	if (map_child_nodes(fs, fs->root, &inum))
+		return -1;
 	fs->root->inode_num = inum;
 	fs->unique_inode_count += 1;
 
 	fs->files = file_list_dfs(fs->root);
+	return 0;
 }