sequencer: truncate labels to accommodate loose refs

Some commits may have unusually long subject lines. When those subject
lines are used as labels in the `--rebase-merges` mode of `git rebase`,
they can cause errors when writing the corresponding loose refs because
most file systems have a maximal file name length of 255 (`NAME_MAX`).
The symptom looks like this:

	$ git rebase --continue
	error: cannot lock ref 'refs/rewritten/SANITIZED-SUBJECT': Unable to create '.git/refs/rewritten/SANITIZED-SUBJECT.lock': File name too long - where SANITIZED-SUBJECT is very long

Let's accommodate this situation by truncating the labels.

Care must be taken in case the subject line contains multi-byte
characters so as not to truncate in the middle of a character.

Signed-off-by: Mark Ruvald Pedersen <mped@demant.com>
Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -422,6 +422,10 @@ char *gitdirname(char *);
 #define PATH_MAX 4096
 #endif
 
+#ifndef NAME_MAX
+#define NAME_MAX 255
+#endif
+
 typedef uintmax_t timestamp_t;
 #define PRItime PRIuMAX
 #define parse_timestamp strtoumax

sequencer.c

@@ -50,6 +50,15 @@
 
 #define GIT_REFLOG_ACTION "GIT_REFLOG_ACTION"
 
+/*
+ * To accommodate common filesystem limitations, where the loose refs' file
+ * names must not exceed `NAME_MAX`, the labels generated by `git rebase
+ * --rebase-merges` need to be truncated if the corresponding commit subjects
+ * are too long.
+ * Add some margin to stay clear from reaching `NAME_MAX`.
+ */
+#define GIT_MAX_LABEL_LENGTH ((NAME_MAX) - (LOCK_SUFFIX_LEN) - 16)
+
 static const char sign_off_header[] = "Signed-off-by: ";
 static const char cherry_picked_prefix[] = "(cherry picked from commit ";
 
@@ -5386,6 +5395,8 @@ static const char *label_oid(struct object_id *oid, const char *label,
 		}
 	} else {
 		struct strbuf *buf = &state->buf;
+		int label_is_utf8 = 1; /* start with this assumption */
+		size_t max_len = buf->len + GIT_MAX_LABEL_LENGTH;
 
 		/*
 		 * Sanitize labels by replacing non-alpha-numeric characters
@@ -5394,14 +5405,34 @@ static const char *label_oid(struct object_id *oid, const char *label,
 		 *
 		 * Note that we retain non-ASCII UTF-8 characters (identified
 		 * via the most significant bit). They should be all acceptable
-		 * in file names. We do not validate the UTF-8 here, that's not
+		 * in file names.
-		 * the job of this function.
+		 *
+		 * As we will use the labels as names of (loose) refs, it is
+		 * vital that the name not be longer than the maximum component
+		 * size of the file system (`NAME_MAX`). We are careful to
+		 * truncate the label accordingly, allowing for the `.lock`
+		 * suffix and for the label to be UTF-8 encoded (i.e. we avoid
+		 * truncating in the middle of a character).
 		 */
-		for (; *label; label++)
+		for (; *label && buf->len + 1 < max_len; label++)
-			if ((*label & 0x80) || isalnum(*label))
+			if (isalnum(*label) ||
+			    (!label_is_utf8 && (*label & 0x80)))
 				strbuf_addch(buf, *label);
+			else if (*label & 0x80) {
+				const char *p = label;
+
+				utf8_width(&p, NULL);
+				if (p) {
+					if (buf->len + (p - label) > max_len)
+						break;
+					strbuf_add(buf, label, p - label);
+					label = p - 1;
+				} else {
+					label_is_utf8 = 0;
+					strbuf_addch(buf, *label);
+				}
 			/* avoid leading dash and double-dashes */
-			else if (buf->len && buf->buf[buf->len - 1] != '-')
+			} else if (buf->len && buf->buf[buf->len - 1] != '-')
 				strbuf_addch(buf, '-');
 		if (!buf->len) {
 			strbuf_addstr(buf, "rev-");