strbuf: add a helper function to call the editor "on an strbuf"

This helper supports the scenario where Git has a populated `strbuf` and
wants to let the user edit it interactively.

In `git add -p`, we will use this to allow interactive hunk editing: the
diff hunks are already in memory, but we need to write them out to a
file so that an editor can be launched, then read everything back once
the user is done editing.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.c

@@ -1125,3 +1125,31 @@ int strbuf_normalize_path(struct strbuf *src)
 	strbuf_release(&dst);
 	return 0;
 }
+
+int strbuf_edit_interactively(struct strbuf *buffer, const char *path,
+			      const char *const *env)
+{
+	char *path2 = NULL;
+	int fd, res = 0;
+
+	if (!is_absolute_path(path))
+		path = path2 = xstrdup(git_path("%s", path));
+
+	fd = open(path, O_WRONLY | O_CREAT | O_TRUNC, 0666);
+	if (fd < 0)
+		res = error_errno(_("could not open '%s' for writing"), path);
+	else if (write_in_full(fd, buffer->buf, buffer->len) < 0) {
+		res = error_errno(_("could not write to '%s'"), path);
+		close(fd);
+	} else if (close(fd) < 0)
+		res = error_errno(_("could not close '%s'"), path);
+	else {
+		strbuf_reset(buffer);
+		if (launch_editor(path, buffer, env) < 0)
+			res = error_errno(_("could not edit '%s'"), path);
+		unlink(path);
+	}
+
+	free(path2);
+	return res;
+}

strbuf.h

@@ -621,6 +621,17 @@ int launch_editor(const char *path, struct strbuf *buffer,
 int launch_sequence_editor(const char *path, struct strbuf *buffer,
 			   const char *const *env);
 
+/*
+ * In contrast to `launch_editor()`, this function writes out the contents
+ * of the specified file first, then clears the `buffer`, then launches
+ * the editor and reads back in the file contents into the `buffer`.
+ * Finally, it deletes the temporary file.
+ *
+ * If `path` is relative, it refers to a file in the `.git` directory.
+ */
+int strbuf_edit_interactively(struct strbuf *buffer, const char *path,
+			      const char *const *env);
+
 void strbuf_add_lines(struct strbuf *sb,
 		      const char *prefix,
 		      const char *buf,