approximate-picks: new API to see if commits are cherry-picks/reverts

We want some functions to check if commits are cherry-picks or reverts,
based on information recorded in the commit message.  Namely, substrings
of the form
  This reverts commit ... [reversing\nchanges made to ...]
  cherry picked from commit ...
When these strings are found, get_message_pick() can return that the
specified commit is a cherry-pick or revert and what other commit the
specified commit is a cherry-pick or revert of.

We will use this later to implement --remerge-diff-only, where normal
non-merge commits that are not cherry-picks or reverts are omitted from
log/show output, and cherry-picks or reverts are shown as a diff against
an automatic re-pick/re-revert.  Thus, the new flag will be a way to see
what changes the creator of the commit made in order to complete the
cherry-pick or revert, such as removing conflict markers.

Signed-off-by: Elijah Newren <[email protected]>

Makefile

@@ -955,6 +955,7 @@ LIB_OBJS += advice.o
 LIB_OBJS += alias.o
 LIB_OBJS += alloc.o
 LIB_OBJS += apply.o
+LIB_OBJS += approximate-picks.o
 LIB_OBJS += archive-tar.o
 LIB_OBJS += archive-zip.o
 LIB_OBJS += archive.o

approximate-picks.c

@@ -0,0 +1,77 @@
+#include "git-compat-util.h"
+#include "approximate-picks.h"
+#include "commit.h"
+#include "environment.h"
+#include "hex.h"
+#include "repository.h"
+
+void get_message_pick(struct commit *commit, int *is_revert,
+		      struct commit **pick_of, struct commit **base)
+{
+	const char *message;
+	const char *revert_msg = "This reverts commit ";
+	const char *cherry_msg = "cherry picked from commit ";
+	const char *revert_msg_base = "reversing\nchanges made to ";
+	char *loc = NULL;
+	struct object_id picked_oid;
+	int find_base = 1;
+
+	message = repo_logmsg_reencode(the_repository, commit, NULL,
+				       get_log_output_encoding());
+	if ((loc = strstr(message, cherry_msg))) {
+		/* cherry-pick */
+		*is_revert = 0;
+		if (get_oid_hex(loc + strlen(cherry_msg), &picked_oid))
+			/* Couldn't parse next text as object id */
+			goto not_a_pick;
+	} else if ((loc = strstr(message, revert_msg))) {
+		/* revert */
+		*is_revert = 1;
+		if (get_oid_hex(loc + strlen(revert_msg), &picked_oid))
+			/* Couldn't parse next text as object id */
+			goto not_a_pick;
+		if ((loc = strstr(message, revert_msg_base))) {
+			/* revert relative to specified base */
+			struct object_id tmpid;
+			if (get_oid_hex(loc + strlen(revert_msg_base), &tmpid))
+				/* Couldn't parse next text as object id */
+				goto not_a_pick;
+			*base = lookup_commit(the_repository, &tmpid);
+			find_base = 0;
+		}
+	} else
+		/* neither */
+		goto not_a_pick;
+
+	*pick_of = lookup_commit(the_repository, &picked_oid);
+	if (repo_parse_commit_gently(the_repository, *pick_of, 1))
+		/*
+		 * Getting here means commit referenced a nonexistent commit
+		 * for what it is a cherry-pick or revert of.  We treat this
+		 * as not a cherry-pick or revert.
+		 */
+		goto not_a_pick;
+	if (find_base) {
+		struct commit_list *parents = (*pick_of)->parents;
+		if (!parents) {
+			*base = NULL;
+			return;
+		}
+		if (parents->next != NULL)
+			/*
+			 * Getting here means that either a commit said it was
+			 * a revert of a merge without telling us which parent,
+			 * or it claimed it was a cherry-pick of a merge which
+			 * makes no sense.
+			 */
+			goto not_a_pick;
+		*base = parents->item;
+	}
+	parse_commit_or_die(*base);
+	return;
+
+ not_a_pick:
+	*is_revert = -1;
+	*pick_of = NULL;
+	*base = NULL;
+}

approximate-picks.h

@@ -0,0 +1,27 @@
+#ifndef APPROXIMATE_PICKS_H
+#define APPROXIMATE_PICKS_H
+
+struct commit;
+
+/*
+ * Determine if the commit message for commit claims this is a cherry-pick or
+ * revert of another commit, which exists in this repo.  Possible return
+ * values:
+ *
+ * If commit message claims it is a cherry-pick:
+ *   *is_revert: 0
+ *   *pick_of:   points to what commit is a cherry-pick of
+ *   *base:      points to parent of *pick_of (possibly NULL)
+ * If commit message claims it is a revert:
+ *   *is_revert: 1
+ *   *pick_of:   points to what commit is a revert of
+ *   *base:      points to appropriate parent of *pick_of (possibly NULL)
+ * Otherwise:
+ *   *is_revert: -1
+ *   *pick_of:   NULL
+ *   *base:      NULL
+ */
+void get_message_pick(struct commit *commit, int *is_revert,
+		      struct commit **pick_of, struct commit **base);
+
+#endif /* APPROXIMATE_PICKS_H */