[RFC PATCH 7/7] repack-promisor: record dropped objects in a drop log

@@ -702,6 +702,10 @@ int cmd_repack(int argc,
 		write_midx_file(files->packed, NULL, NULL, flags);
 	}
 
+	if (drop_filtered && !dry_run)
+		append_drop_log(repo, &drop_oids,
+			expand_list_objects_filter_spec(&po_args.filter_options));
+
 cleanup:
 	string_list_clear(&keep_pack_list, 0);
 	string_list_clear(&names, 1);

@@ -7,6 +7,97 @@
 #include "repository.h"
 #include "run-command.h"
 #include "oidset.h"
+#include "date.h"
+#include "promisor-remote.h"
+#include "strbuf.h"
+
+/*
+ * Append the drop-log entries to the already-computed path.
+ * Returns -1 on any I/O failure so the caller can warn once.
+ * Keeping this in a separate helper avoids goto-based cleanup
+ * in append_drop_log();
+ */
+static int write_to_drop_log(struct repository *repo,
+			     const char *path,
+			     const struct oidset *dropped,
+			     const char *stamp,
+			     const char *filter_spec,
+			     const char *remotes)
+{
+	struct oidset_iter iter;
+	const struct object_id *oid;
+	FILE *fp;
+
+	if (safe_create_leading_directories(repo, (char *)path)) {
+		warning(_("could not create leading directories for '%s'"), path);
+		return -1;
+	}
+
+	fp = fopen(path, "a");
+	if (!fp) {
+		warning_errno(_("could not open '%s'"), path);
+		return -1;
+	}
+
+	oidset_iter_init(dropped, &iter);
+	while ((oid = oidset_iter_next(&iter))) {
+		if (fprintf(fp, "%s %s filter=%s remote=%s\n",
+				oid_to_hex(oid), stamp,
+				filter_spec ? filter_spec : "",
+				remotes) < 0) {
+			warning(_("could not write to '%s'"), path);
+			fclose(fp);
+			return -1;
+		}
+	}
+
+	if (fclose(fp)) {
+		warning_errno(_("could not close '%s'"), path);
+		return -1;
+	}
+
+	return 0;
+}
+
+void append_drop_log(struct repository *repo,
+		     const struct oidset *dropped,
+		     const char *filter_spec)
+{
+	char *path;
+	struct strbuf stamp = STRBUF_INIT;
+	struct strbuf remotes = STRBUF_INIT;
+	struct promisor_remote *pr;
+
+	if (!oidset_size(dropped))
+		return;
+
+	datestamp(&stamp);
+
+	/*
+	 * NEEDSWORK: we temporarily record all configured promisor remotes rather
+	 * than the specific one a given object is recoverable from because there
+	 * is currently no way to determine that locally. it would require
+	 * asking the remote whether it has the object. A "remote-object-info"
+	 * command is being added to the "git cat-file --batch" protocol for
+	 * this kind of query. Once it is merged in the codebase, this should
+	 * record the exact promisor remote that has each dropped object.
+	 */
+	for (pr = repo_promisor_remote_find(repo, NULL); pr; pr = pr->next) {
+		if (remotes.len)
+			strbuf_addch(&remotes, ',');
+		strbuf_addstr(&remotes, pr->name);
+	}
+
+	path = repo_git_path(repo, "objects/info/promisor-dropped");
+
+	if (write_to_drop_log(repo, path, dropped, stamp.buf,
+			filter_spec, remotes.buf))
+		warning(_("could not record all dropped objects in the drop log"));
+
+	strbuf_release(&stamp);
+	strbuf_release(&remotes);
+	free(path);
+}
 
 struct write_oid_context {
 	struct child_process *cmd;

@@ -171,6 +171,10 @@ int enumerate_promisor_blobs(struct repository *repo,
 			     const struct list_objects_filter_options *filter,
 			     struct oidset *to_drop);
 
+void append_drop_log(struct repository *repo,
+		     const struct oidset *dropped,
+		     const char *filter_spec);
+
 int write_cruft_pack(const struct write_pack_opts *opts,
 		     const char *cruft_expiration,
 		     unsigned long combine_cruft_below_size,