wip preserve symlinks

lib/private/modify_mtree.awk

@@ -1,5 +1,21 @@
 # Edits mtree files. See the modify_mtree macro in /lib/tar.bzl.
 {
+    if (preserve_symlink != "") {
+        # By default Bazel reports symlinks as regular file/dir therefore mtree_spec has no way of knowing that a file
+        # is a symlink. This is a problem when we want to preserve symlinks especially for symlink sensitive applications
+        # such as nodejs with pnpm. To work around this we need to determine if a file a symlink and if so, we need to
+        # determine where the symlink points to by calling readlink repeatedly until we get the final destination.
+        #
+        # We then need to decide if it's a symlink based on how many times we had to call readlink and where we ended up.
+        #
+        # Unlike Bazels own symlinks, which points out of the sandbox symlinks, symlinks created by ctx.actions.symlink
+        # stays within the bazel sandbox so it's possible to detect those.
+        #
+        # See https://github.com/bazelbuild/rules_pkg/pull/609
+        if ($0 ~ /type=file/) {
+
+        }
+    }
     if (strip_prefix != "") {
         if ($1 == strip_prefix) {
             # this line declares the directory which is now the root. It may be discarded.

lib/tar.bzl

@@ -137,6 +137,8 @@ def tar(name, mtree = "auto", stamp = 0, **kwargs):
 def mtree_mutate(
         name,
         mtree,
+        srcs = None,
+        preserve_symlinks = False,
         strip_prefix = None,
         package_dir = None,
         mtime = None,
@@ -148,13 +150,16 @@ def mtree_mutate(
 
     Args:
         name: name of the target, output will be `[name].mtree`.
+        srcs: source files to be used when resolving symlinks. required if `preserve_symlinks` is set to True.
+        preserve_symlinks: preserve symlinks
         mtree: input mtree file, typically created by `mtree_spec`.
         strip_prefix: prefix to remove from all paths in the tar. Files and directories not under this prefix are dropped.
         package_dir: directory prefix to add to all paths in the tar.
         mtime: new modification time for all entries.
         owner: new uid for all entries.
         ownername: new uname for all entries.
         awk_script: may be overridden to change the script containing the modification logic.
+
         **kwargs: additional named parameters to genrule
     """
     vars = []
@@ -168,6 +173,10 @@ def mtree_mutate(
         vars.append("-v owner='{}'".format(owner))
     if ownername:
         vars.append("-v ownername='{}'".format(ownername))
+    if preserve_symlinks:
+        vars.append("-v preserve_symlinks=1")
+        if not srcs:
+            fail("preserve_symlinks requires srcs to be set in order to resolve symlinks")
 
     native.genrule(
         name = name,

lib/tests/tar/BUILD.bazel

@@ -4,6 +4,7 @@ load("@aspect_bazel_lib//lib:tar.bzl", "mtree_mutate", "mtree_spec", "tar")
 load("@aspect_bazel_lib//lib:testing.bzl", "assert_archive_contains")
 load("@bazel_skylib//rules:write_file.bzl", "write_file")
 load(":asserts.bzl", "assert_tar_listing", "assert_unused_listing")
+load(":node_modules_tree.bzl", "node_modules_tree")
 
 # The examples below work with both source files and generated files.
 # Here we generate a file to use in the examples.
@@ -465,3 +466,22 @@ assert_unused_listing(
         "lib/tests/tar/unused/space in name.txt",
     ],
 )
+
+#############
+# Example 16: mtree_mutate preserves symlinks
+node_modules_tree(
+    name = "e16_node_modules",
+)
+
+mtree_spec(
+    name = "mtree16",
+    srcs = [
+        ":e16_node_modules",
+    ],
+)
+
+assert_tar_listing(
+    name = "test_16_before_processing",
+    actual = ":mtree16",
+    expected = [],
+)

lib/tests/tar/node_modules_tree.bzl

@@ -0,0 +1,51 @@
+# https://github.com/bazelbuild/rules_pkg/pull/609
+def impl(ctx):
+    # packages
+    # - a
+    # - b depends on a
+    store_a = ctx.actions.declare_directory("node_modules/.pnpm/[email protected]/node_modules/a")
+    store_b = ctx.actions.declare_directory("node_modules/.pnpm/[email protected]/node_modules/b")
+
+    ctx.actions.run_shell(
+        outputs = [store_a, store_b],
+        command = "echo 'test' > %s/package.json" % store_a.path,
+    )
+
+    dep_symlink_b_to_a = ctx.actions.declare_directory("node_modules/.pnpm/[email protected]/node_modules/a")
+
+    ctx.actions.symlink(
+        output = dep_symlink_b_to_a,
+        target_file = store_a,
+    )
+
+    node_modules_a = ctx.actions.declare_directory("node_modules/a")
+    ctx.actions.symlink(
+        output = node_modules_a,
+        target_file = store_a,
+    )
+
+    # single file
+    a = ctx.actions.declare_file("dir/a")
+    ctx.actions.run_shell(
+        outputs = [a],
+        command = "echo 'test' > %s" % a.path,
+    )
+
+    b = ctx.actions.declare_file("dir/b")
+    ctx.actions.symlink(
+        output = b,
+        target_file = a,
+    )
+
+    return DefaultInfo(files = depset([
+        store_a,
+        store_b,
+        dep_symlink_b_to_a,
+        node_modules_a,
+        a,
+        b,
+    ]))
+
+node_modules_tree = rule(
+    implementation = impl,
+)