WIP range-diff: support reading mbox files

TODO:
	- document
	- split out the `mbox:-` code
	- add a test (probably by using `git format-patch --stdout`)

Internally, the `git range-diff` command spawns a `git log` process and
parses its output for the given commit ranges.

This works well when the patches that need to be compared are present in
the local repository in the form of commits.

In scenarios where that is not the case, the `range-diff` command is
unhelpful.

For example, if one desired to compare commits in a local repository to
patches that were sent to the Git mailing list, it would be necessary to
apply the patches locally first, which requires finding an appropriate
base commit or adjusting the patches manually so that they apply. This
is a lot of effort given that `range-diff` would then go and internally
re-generate the patches, anyway.

Let's offer a way to read those patches from pre-prepared MBox files
instead when an argument "mbox:<filename>" is passed instead of a commit
range.

This addresses gitgitgadget#207

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

builtin/range-diff.c

@@ -16,7 +16,7 @@ static int is_range(const char *range)
 	size_t i;
 	char c;
 
-	if (strstr(range, ".."))
+	if (starts_with(range, "mbox:") || strstr(range, ".."))
 		return 1;
 
 	i = strlen(range);

range-diff.c

@@ -36,6 +36,171 @@ static size_t find_end_of_line(char *buffer, unsigned long size)
 	return eol + 1 - buffer;
 }
 
+static int read_mbox(const char *path, struct string_list *list)
+{
+	struct strbuf buf = STRBUF_INIT, contents = STRBUF_INIT;
+	struct patch_util *util = NULL;
+	int in_header = 1, in_diff = 0;
+	char *line, *current_filename = NULL;
+	int offset, len;
+	size_t size;
+	const char *author = NULL, *subject = NULL;
+
+	if (!strcmp(path, "-")) {
+		if (strbuf_read(&contents, STDIN_FILENO, 0) < 0)
+			return error_errno(_("could not read stdin"));
+	} else if (strbuf_read_file(&contents, path, 0) < 0)
+		return error_errno(_("could not read '%s'"), path);
+
+	line = contents.buf;
+	size = contents.len;
+	for (offset = 0; size > 0; offset += len, size -= len, line += len) {
+		const char *p;
+
+		len = find_end_of_line(line, size);
+		line[len - 1] = '\0';
+
+		if (starts_with(line, "diff --git")) {
+			struct patch patch = { 0 };
+			struct strbuf root = STRBUF_INIT;
+			int linenr = 0;
+			int orig_len;
+
+			in_diff = 1;
+			strbuf_addch(&buf, '\n');
+			if (!util) {
+				util = xcalloc(sizeof(*util), 1);
+				oidcpy(&util->oid, &null_oid);
+				util->matching = -1;
+				author = subject = NULL;
+			}
+			if (!util->diff_offset)
+				util->diff_offset = buf.len;
+			line[len - 1] = '\n';
+			orig_len = len;
+			len = parse_git_diff_header(&root, &linenr, 0, line,
+						    len, size, &patch);
+			if (len < 0) {
+				error(_("could not parse git header '%.*s'"),
+				      orig_len, line);
+				free(util);
+				free(current_filename);
+				string_list_clear(list, 1);
+				strbuf_release(&buf);
+				strbuf_release(&contents);
+				return -1;
+			}
+
+			if (patch.old_name)
+				skip_prefix(patch.old_name, "a/",
+					    (const char **)&patch.old_name);
+			if (patch.new_name)
+				skip_prefix(patch.new_name, "b/",
+					    (const char **)&patch.new_name);
+
+			strbuf_addstr(&buf, " ## ");
+			if (patch.is_new > 0)
+				strbuf_addf(&buf, "%s (new)", patch.new_name);
+			else if (patch.is_delete > 0)
+				strbuf_addf(&buf, "%s (deleted)", patch.old_name);
+			else if (patch.is_rename)
+				strbuf_addf(&buf, "%s => %s", patch.old_name, patch.new_name);
+			else
+				strbuf_addstr(&buf, patch.new_name);
+
+			free(current_filename);
+			if (patch.is_delete > 0)
+				current_filename = xstrdup(patch.old_name);
+			else
+				current_filename = xstrdup(patch.new_name);
+
+			if (patch.new_mode && patch.old_mode &&
+			    patch.old_mode != patch.new_mode)
+				strbuf_addf(&buf, " (mode change %06o => %06o)",
+					    patch.old_mode, patch.new_mode);
+
+			strbuf_addstr(&buf, " ##\n");
+			util->diffsize++;
+			continue;
+		} else if (in_diff) {
+			switch (line[0]) {
+			case '\0':
+				continue; /* ignore empty lines after diff */
+			case '+':
+			case '-':
+			case ' ':
+			case '\\':
+				strbuf_addstr(&buf, line);
+				strbuf_addch(&buf, '\n');
+				util->diffsize++;
+				continue;
+			case '@':
+				if (skip_prefix(line, "@@ ", &p)) {
+					p = strstr(p, "@@");
+					strbuf_addstr(&buf, "@@");
+					if (current_filename && p[2])
+						strbuf_addf(&buf, " %s:",
+							    current_filename);
+					if (p)
+						strbuf_addstr(&buf, p + 2);
+
+					strbuf_addch(&buf, '\n');
+					util->diffsize++;
+					continue;
+				}
+				break;
+			}
+
+			if (util) {
+				string_list_append(list, buf.buf)->util = util;
+				strbuf_reset(&buf);
+			}
+			util = xcalloc(sizeof(*util), 1);
+			oidcpy(&util->oid, &null_oid);
+			util->matching = -1;
+			author = subject = NULL;
+			in_header = 1;
+			in_diff = 0;
+		}
+
+		if (in_header) {
+			if (!line[0]) {
+				in_header = 0;
+				strbuf_addstr(&buf, " ## Metadata ##\n");
+				if (author)
+					strbuf_addf(&buf, "Author: %s\n", author);
+				strbuf_addstr(&buf, "\n ## Commit message ##\n");
+				if (subject)
+					strbuf_addf(&buf, "%s\n", subject);
+			} else if (skip_prefix(line, "From: ", &p)) {
+				author = p;
+			} else if (skip_prefix(line, "Subject: ", &p)) {
+				const char *q;
+
+				subject = p;
+
+				if (starts_with(p, "[PATCH") &&
+				    (q = strchr(p, ']'))) {
+					while (isspace(*q))
+						q++;
+					subject = q;
+				}
+			}
+		} else {
+			strbuf_addch(&buf, ' ');
+			strbuf_addstr(&buf, line);
+		}
+	}
+	strbuf_release(&contents);
+
+	if (util)
+		string_list_append(list, buf.buf)->util = util;
+	strbuf_release(&buf);
+	free(current_filename);
+
+	return 0;
+}
+
 /*
  * Reads the patches into a string list, with the `util` field being populated
  * as struct object_id (will need to be free()d).
@@ -50,6 +215,10 @@ static int read_patches(const char *range, struct string_list *list,
 	char *line, *current_filename = NULL;
 	int offset, len;
 	size_t size;
+	const char *path;
+
+	if (skip_prefix(range, "mbox:", &path))
+		return read_mbox(path, list);
 
 	strvec_pushl(&cp.args, "log", "--no-color", "-p", "--no-merges",
 		     "--reverse", "--date-order", "--decorate=no",
@@ -534,6 +703,9 @@ int show_range_diff(const char *range1, const char *range2,
 	struct string_list branch1 = STRING_LIST_INIT_DUP;
 	struct string_list branch2 = STRING_LIST_INIT_DUP;
 
+	if (!strcmp(range1, "mbox:-") && !strcmp(range2, "mbox:-"))
+		res = error(_("only one mbox can be read from stdin"));
+
 	if (read_patches(range1, &branch1, other_arg))
 		res = error(_("could not parse log for '%s'"), range1);
 	if (!res && read_patches(range2, &branch2, other_arg))