range-diff: support reading mbox files

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
currently less helpful.

The Git mailing list is such a scenario: Instead of using Git to
exchange commits, the patches are sent there as plain-text and no commit
range can be specified to let `range-diff` consume those patches.

Instead, the expectation is to download the mails, apply them locally
and then use `range-diff`. This can be quite cumbersome e.g. when a
suitable base revision has to be found first where the patch applies
cleanly.

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.

For extra convenience, interpret the filename `-` as standard input.
This makes it easy to compare contributions on the mailing list with the
actual commits that were integrated into Git's main branch. Example:

	commit=5c4003ca3f0e9ac6d3c8aa3e387ff843bd440411
	mid=bdfa3845b81531863941e6a97c28eb1afa62dd2c.1489435755.git.johannes.schindelin@gmx.de
	curl -s https://lore.kernel.org/git/$mid/raw |
	git range-diff mbox:- $commit^!

This addresses #207

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

Documentation/git-range-diff.txt

@@ -37,7 +37,8 @@ There are three ways to specify the commit ranges:
 
 - `<range1> <range2>`: Either commit range can be of the form
   `<base>..<rev>`, `<rev>^!` or `<rev>^-<n>`. See `SPECIFYING RANGES`
-  in linkgit:gitrevisions[7] for more details.
+  in linkgit:gitrevisions[7] for more details. Alternatively, the
+  patches can be provided as an mbox-formatted file via `mbox:<path>`.
 
 - `<rev1>...<rev2>`. This is equivalent to
   `<rev2>..<rev1> <rev1>..<rev2>`.

range-diff.c

@@ -12,6 +12,7 @@
 #include "userdiff.h"
 #include "apply.h"
 #include "revision.h"
+#include "dir.h"
 
 struct patch_util {
 	/* For the search for an exact match */
@@ -26,6 +27,313 @@ struct patch_util {
 	struct object_id oid;
 };
 
+static inline int strtost(char const *s, size_t *result, const char **end)
+{
+	unsigned long u;
+	char *p;
+
+	errno = 0;
+	/* negative values would be accepted by strtoul */
+	if (!isdigit(*s))
+		return -1;
+	u = strtoul(s, &p, 10);
+	if (errno || p == s)
+		return -1;
+	if (result)
+		*result = u;
+	*end = p;
+
+	return 0;
+}
+
+static int parse_hunk_header(const char *p,
+			     size_t *old_count, size_t *new_count,
+			     const char **end)
+{
+	size_t o = 1, n = 1;
+
+	if (!skip_prefix(p, "@@ -", &p) ||
+	    strtost(p, NULL, &p) ||
+	    /* The range is -<start>[,<count>], defaulting to count = 1 */
+	    !(*p == ' ' || (*p == ',' && !strtost(p + 1, &o, &p))) ||
+	    !skip_prefix(p, " +", &p) ||
+	    strtost(p, NULL, &p) ||
+	    /* The range is +<start>[,<count>], defaulting to count = 1 */
+	    !(*p == ' ' || (*p == ',' && !strtost(p + 1, &n, &p))) ||
+	    !skip_prefix(p, " @@", &p))
+		return -1;
+
+	*old_count = o;
+	*new_count = n;
+	*end = p;
+
+	return 0;
+}
+
+/*
+ * This function finds the end of the line, replaces the newline character with
+ * a NUL, and returns the offset of the start of the next line.
+ *
+ * If no newline character was found, it returns the offset of the trailing NUL
+ * instead.
+ */
+static inline int find_next_line(const char *line, size_t size)
+{
+	char *eol;
+
+	eol = memchr(line, '\n', size);
+	if (!eol)
+		return size;
+
+	*eol = '\0';
+
+	return eol + 1 - line;
+}
+
+static int read_mbox(const char *path, struct string_list *list)
+{
+	struct strbuf buf = STRBUF_INIT, contents = STRBUF_INIT;
+	struct strbuf long_subject = STRBUF_INIT;
+	struct patch_util *util = NULL;
+	enum {
+		MBOX_BEFORE_HEADER,
+		MBOX_IN_HEADER,
+		MBOX_IN_COMMIT_MESSAGE,
+		MBOX_AFTER_TRIPLE_DASH,
+		MBOX_IN_DIFF
+	} state = MBOX_BEFORE_HEADER;
+	char *line, *current_filename = NULL;
+	int len;
+	size_t size, old_count = 0, new_count = 0;
+	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 (; size; size -= len, line += len) {
+		const char *p;
+
+		len = find_next_line(line, size);
+
+		if (state == MBOX_BEFORE_HEADER) {
+parse_from_delimiter:
+			if (!skip_prefix(line, "From ", &p))
+				continue;
+
+			if (util)
+				BUG("util already allocated");
+			util = xcalloc(1, sizeof(*util));
+			if (get_oid_hex(p, &util->oid) < 0)
+				oidcpy(&util->oid, null_oid());
+			util->matching = -1;
+			author = subject = NULL;
+
+			state = MBOX_IN_HEADER;
+			continue;
+		}
+
+		if (starts_with(line, "diff --git ")) {
+			struct patch patch = { 0 };
+			struct strbuf root = STRBUF_INIT;
+			int linenr = 0;
+			int orig_len;
+
+			state = MBOX_IN_DIFF;
+			old_count = new_count = 0;
+			strbuf_addch(&buf, '\n');
+			if (!util->diff_offset)
+				util->diff_offset = buf.len;
+
+			orig_len = len;
+			/* `find_next_line()`'s replaced the LF with a NUL */
+			line[len - 1] = '\n';
+			len = len > 1 && line[len - 2] == '\r' ?
+				error(_("cannot handle diff headers with "
+					"CR/LF line endings")) :
+				parse_git_diff_header(&root, &linenr, 1, line,
+						      len, size, &patch);
+			if (len < 0) {
+				error(_("could not parse git header '%.*s'"),
+				      orig_len, line);
+				release_patch(&patch);
+				free(util);
+				free(current_filename);
+				string_list_clear(list, 1);
+				strbuf_release(&buf);
+				strbuf_release(&contents);
+				strbuf_release(&long_subject);
+				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)
+				strbuf_addf(&buf, "%s (new)", patch.new_name);
+			else if (patch.is_delete)
+				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)
+				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++;
+			release_patch(&patch);
+		} else if (state == MBOX_IN_HEADER) {
+			if (!line[0]) {
+				state = MBOX_IN_COMMIT_MESSAGE;
+				/* Look for an in-body From: */
+				if (skip_prefix(line + 1, "From: ", &p)) {
+					size -= p - line;
+					line += p - line;
+					len = find_next_line(line, size);
+
+					while (isspace(*p))
+						p++;
+					author = p;
+				}
+				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\n", subject);
+			} else if (skip_prefix(line, "From: ", &p)) {
+				while (isspace(*p))
+					p++;
+				author = p;
+			} else if (skip_prefix(line, "Subject: ", &p)) {
+				const char *q;
+
+				while (isspace(*p))
+					p++;
+				subject = p;
+
+				if (starts_with(p, "[PATCH") &&
+				    (q = strchr(p, ']'))) {
+					q++;
+					while (isspace(*q))
+						q++;
+					subject = q;
+				}
+
+				if (len < size && line[len] == ' ') {
+					/* handle long subject */
+					strbuf_reset(&long_subject);
+					strbuf_addstr(&long_subject, subject);
+					while (len < size && line[len] == ' ') {
+						line += len;
+						size -= len;
+						len = find_next_line(line, size);
+						strbuf_addstr(&long_subject, line);
+					}
+					subject = long_subject.buf;
+				}
+			}
+		} else if (state == MBOX_IN_COMMIT_MESSAGE) {
+			if (!line[0]) {
+				strbuf_addch(&buf, '\n');
+			} else if (strcmp(line, "---")) {
+				int tabs = 0;
+
+				/* simulate tab expansion */
+				while (line[tabs] == '\t')
+					tabs++;
+				strbuf_addf(&buf, "%*s%s\n",
+					    4 + 8 * tabs, "", line + tabs);
+			} else {
+				/*
+				 * Trim the trailing newline that is added
+				 * by `format-patch`.
+				 */
+				strbuf_trim_trailing_newline(&buf);
+				state = MBOX_AFTER_TRIPLE_DASH;
+			}
+		} else if (state == MBOX_IN_DIFF) {
+			switch (line[0]) {
+			case '\0': /* newer GNU diff, an empty context line */
+			case '+':
+			case '-':
+			case ' ':
+				/* A `-- ` line indicates the end of a diff */
+				if (!old_count && !new_count)
+					break;
+				if (old_count && line[0] != '+')
+					old_count--;
+				if (new_count && line[0] != '-')
+					new_count--;
+				/* fallthrough */
+			case '\\':
+				strbuf_addstr(&buf, line);
+				strbuf_addch(&buf, '\n');
+				util->diffsize++;
+				continue;
+			case '@':
+				if (parse_hunk_header(line, &old_count,
+						      &new_count, &p))
+					break;
+
+				strbuf_addstr(&buf, "@@");
+				if (current_filename && *p)
+					strbuf_addf(&buf, " %s:",
+						    current_filename);
+				strbuf_addstr(&buf, p);
+				strbuf_addch(&buf, '\n');
+				util->diffsize++;
+				continue;
+			default:
+				if (old_count || new_count)
+					warning(_("diff ended prematurely (-%d/+%d)"),
+						(int)old_count, (int)new_count);
+				break;
+			}
+
+			if (util) {
+				string_list_append(list, buf.buf)->util = util;
+				util = NULL;
+				strbuf_reset(&buf);
+			}
+			state = MBOX_BEFORE_HEADER;
+			goto parse_from_delimiter;
+		}
+	}
+	strbuf_release(&contents);
+
+	if (util) {
+		if (state == MBOX_IN_DIFF)
+			string_list_append(list, buf.buf)->util = util;
+		else
+			free(util);
+	}
+	strbuf_release(&buf);
+	strbuf_release(&long_subject);
+	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).
@@ -41,6 +349,10 @@ static int read_patches(const char *range, struct string_list *list,
 	ssize_t len;
 	size_t size;
 	int ret = -1;
+	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",
@@ -424,6 +736,19 @@ static void output_pair_header(struct diff_options *diffopt,
 
 		strbuf_addch(buf, ' ');
 		pp_commit_easy(CMIT_FMT_ONELINE, commit, buf);
+	} else {
+		struct patch_util *util = b_util ? b_util : a_util;
+		const char *needle = "\n ## Commit message ##\n";
+		const char *p = !util || !util->patch ?
+			NULL : strstr(util->patch, needle);
+		if (p) {
+			if (status == '!')
+				strbuf_addf(buf, "%s%s", color_reset, color);
+
+			strbuf_addch(buf, ' ');
+			p += strlen(needle);
+			strbuf_add(buf, p, strchrnul(p, '\n') - p);
+		}
 	}
 	strbuf_addf(buf, "%s\n", color_reset);
 
@@ -554,6 +879,9 @@ int show_range_diff(const char *range1, const char *range2,
 	if (range_diff_opts->left_only && range_diff_opts->right_only)
 		res = error(_("options '%s' and '%s' cannot be used together"), "--left-only", "--right-only");
 
+	if (!strcmp(range1, "mbox:-") && !strcmp(range2, "mbox:-"))
+		res = error(_("only one mbox can be read from stdin"));
+
 	if (!res && read_patches(range1, &branch1, range_diff_opts->other_arg))
 		res = error(_("could not parse log for '%s'"), range1);
 	if (!res && read_patches(range2, &branch2, range_diff_opts->other_arg))
@@ -575,10 +903,18 @@ int show_range_diff(const char *range1, const char *range2,
 int is_range_diff_range(const char *arg)
 {
 	char *copy = xstrdup(arg); /* setup_revisions() modifies it */
-	const char *argv[] = { "", copy, "--", NULL };
+	const char *argv[] = { "", copy, "--", NULL }, *path;
 	int i, positive = 0, negative = 0;
 	struct rev_info revs;
 
+	if (skip_prefix(arg, "mbox:", &path)) {
+		free(copy);
+		if (!strcmp(path, "-") || file_exists(path))
+			return 1;
+		error_errno(_("not an mbox: '%s'"), path);
+		return 0;
+	}
+
 	init_revisions(&revs, NULL);
 	if (setup_revisions(3, argv, &revs, NULL) == 1) {
 		for (i = 0; i < revs.pending.nr; i++)