commit: Add --base argument

I was trying to followup the `--selinux-policy-from-base` work
to add a `cosa build --fast=overlay` for coreos-assembler,
but hit on the fact that using e.g. `--owner-uid` disables
commit optimizations.

A while ago, https://github.com/ostreedev/ostree/pull/1643 landed
which optimized this for the case where no modifications are provided.
But, we really need the SELinux policy bits, and it's super convenient
to run `ostree commit` as non-root.

It's fairly surprising actually that it's taken us so long to
iterate on a good interface for this "commit changes on top of a base"
model.  In practice, many nontrivial cases really end up needing
to do a (hardlink) checkout, and that case is optimized.

But for this coreos-assembler work I want to directly overlay onto
a commit object another commit object.

That previous PR above added exactly the API we need, so let's
expose it in the CLI.

What you can see happening in the test is that we provide
`--owner-uid 42`, but that only applies to directories/files
that were added in the commit.

And now that I look at this, I think what we really want here
is to avoid changing directories that exist in the base, but
eh; in practice the main use here is for `--owner-uid 0` while
committing as non-root; and that works fine with this since
the baseline uid will be zero as well.

man/ostree-commit.xml

@@ -106,6 +106,15 @@ Boston, MA 02111-1307, USA.
                 </para></listitem>
             </varlistentry>
 
+            <varlistentry>
+                <term><option>--base</option>="REV"</term>
+
+                <listitem><para>
+                    Start from the content in a commit.  This differs from <literal>--tree=ref=REV</literal> in that no commit modifiers are applied.  This is usually what you want when
+                    creating a derived commit.  This is also used for <literal>--selinux-policy-from-base</literal>.
+                </para></listitem>
+            </varlistentry>
+
             <varlistentry>
                 <term><option>--add-metadata-string</option>="KEY=VALUE"</term>
 

src/ostree/ot-builtin-commit.c

@@ -58,6 +58,7 @@ static gboolean opt_selinux_policy_from_base;
 static gboolean opt_canonical_permissions;
 static gboolean opt_consume;
 static gboolean opt_devino_canonical;
+static char *opt_base;
 static char **opt_trees;
 static gint opt_owner_uid = -1;
 static gint opt_owner_gid = -1;
@@ -101,6 +102,7 @@ static GOptionEntry options[] = {
   { "orphan", 0, 0, G_OPTION_ARG_NONE, &opt_orphan, "Create a commit without writing a ref", NULL },
   { "no-bindings", 0, 0, G_OPTION_ARG_NONE, &opt_no_bindings, "Do not write any ref bindings", NULL },
   { "bind-ref", 0, 0, G_OPTION_ARG_STRING_ARRAY, &opt_bind_refs, "Add a ref to ref binding commit metadata", "BRANCH" },
+  { "base", 0, 0, G_OPTION_ARG_STRING, &opt_base, "Start from the given commit as a base (no modifiers apply)" },
   { "tree", 0, 0, G_OPTION_ARG_STRING_ARRAY, &opt_trees, "Overlay the given argument as a tree", "dir=PATH or tar=TARFILE or ref=COMMIT" },
   { "add-metadata-string", 0, 0, G_OPTION_ARG_STRING_ARRAY, &opt_metadata_strings, "Add a key/value pair to metadata", "KEY=VALUE" },
   { "add-metadata", 0, 0, G_OPTION_ARG_STRING_ARRAY, &opt_metadata_variants, "Add a key/value pair to metadata, where the KEY is a string, an VALUE is g_variant_parse() formatted", "KEY=VALUE" },
@@ -600,7 +602,32 @@ ostree_builtin_commit (int argc, char **argv, OstreeCommandInvocation *invocatio
   if (opt_link_checkout_speedup && !ostree_repo_scan_hardlinks (repo, cancellable, error))
     goto out;
 
-  mtree = ostree_mutable_tree_new ();
+  if (opt_base)
+    {
+      g_autofree char *base_commit = NULL;
+      g_autoptr(GFile) root = NULL;
+      if (!ostree_repo_read_commit (repo, opt_base, &root, &base_commit, cancellable, error))
+        goto out;
+      OstreeRepoFile *rootf = (OstreeRepoFile*) root;
+
+      mtree = ostree_mutable_tree_new_from_checksum (repo,
+                                                     ostree_repo_file_tree_get_contents_checksum (rootf),
+                                                     ostree_repo_file_tree_get_metadata_checksum (rootf));
+
+      if (opt_selinux_policy_from_base)
+        {
+          g_assert (modifier);
+          if (!ostree_repo_commit_modifier_set_sepolicy_from_commit (modifier, repo, base_commit, cancellable, error))
+            goto out;
+          /* Don't try to handle it twice */
+          opt_selinux_policy_from_base = FALSE;
+        }
+    }
+  else
+    {
+      mtree = ostree_mutable_tree_new ();
+    }
+
 
   /* Convert implicit . or explicit path via argv into
    * --tree=dir= so that we only have one primary code path below.

tests/archive-test.sh

@@ -68,3 +68,11 @@ echo "ok cat-file"
 cd ${test_tmpdir}
 $OSTREE fsck
 echo "ok fsck"
+
+mkdir -p test-overlays
+date > test-overlays/overlaid-file
+$OSTREE commit ${COMMIT_ARGS} -b test-base --base test2 --owner-uid 42 --owner-gid 42 test-overlays/
+$OSTREE ls -R test-base > ls.txt
+assert_streq "$(wc -l < ls.txt)" 14
+assert_streq "$(grep '42.*42' ls.txt | wc -l)" 2
+echo "ok commit overlay base"

tests/test-archivez.sh

@@ -23,7 +23,7 @@ set -euo pipefail
 
 . $(dirname $0)/libtest.sh
 
-echo '1..12'
+echo '1..13'
 
 setup_test_repository "archive"