diff --no-index: support symlinks and pipes

index 7f91f6d226..09e646060e 100644
@@ -360,6 +360,8 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 	if (nongit)
 		die(_("Not a git repository"));
 	argc = setup_revisions(argc, argv, &rev, NULL);
+	if (DIFF_OPT_TST(&rev.diffopt, DEREFERENCE))
+		die(_("--dereference can only be used together with --no-index"));
 	if (!rev.diffopt.output_format) {
 		rev.diffopt.output_format = DIFF_FORMAT_PATCH;
 		diff_setup_done(&rev.diffopt);

index f420786039..fe48f32ddd 100644
@@ -40,7 +40,7 @@ static int read_directory_contents(const char *path, struct string_list *list)
  */
 static const char file_from_standard_input[] = "-";
 
-static int get_mode(const char *path, int *mode)
+static int get_mode(const char *path, int *mode, int dereference)
 {
 	struct stat st;
 

@@ -52,7 +52,7 @@ static int get_mode(const char *path, int *mode)
 #endif
 	else if (path == file_from_standard_input)
 		*mode = create_ce_mode(0666);
-	else if (lstat(path, &st))
+	else if (dereference ? stat(path, &st) : lstat(path, &st))
 		return error("Could not access '%s'", path);
 	else
 		*mode = st.st_mode;

@@ -93,7 +93,8 @@ static int queue_diff(struct diff_options *o,
 {
 	int mode1 = 0, mode2 = 0;
 
-	if (get_mode(name1, &mode1) || get_mode(name2, &mode2))
+	if (get_mode(name1, &mode1, DIFF_OPT_TST(o, DEREFERENCE)) ||
+		get_mode(name2, &mode2, DIFF_OPT_TST(o, DEREFERENCE)))
 		return -1;
 
 	if (mode1 && mode2 && S_ISDIR(mode1) != S_ISDIR(mode2)) {

index be11e4ef2b..2afecfb939 100644
@@ -2815,7 +2815,7 @@ int diff_populate_filespec(struct diff_filespec *s, unsigned int flags)
 		s->size = xsize_t(st.st_size);
 		if (!s->size)
 			goto empty;
-		if (S_ISLNK(st.st_mode)) {
+		if (S_ISLNK(s->mode)) {
 			struct strbuf sb = STRBUF_INIT;
 
 			if (strbuf_readlink(&sb, s->path, s->size))

@@ -2825,6 +2825,10 @@ int diff_populate_filespec(struct diff_filespec *s, unsigned int flags)
 			s->should_free = 1;
 			return 0;
 		}
+		if (S_ISLNK(st.st_mode)) {
+			stat(s->path, &st);
+			s->size = xsize_t(st.st_size);
+		}
 		if (size_only)
 			return 0;
 		if ((flags & CHECK_BINARY) &&

@@ -3884,7 +3888,11 @@ int diff_opt_parse(struct diff_options *options,
 	else if (!strcmp(arg, "--no-follow")) {
 		DIFF_OPT_CLR(options, FOLLOW_RENAMES);
 		DIFF_OPT_CLR(options, DEFAULT_FOLLOW_RENAMES);
-	} else if (!strcmp(arg, "--color"))
+	} else if (!strcmp(arg, "--dereference"))
+		DIFF_OPT_SET(options, DEREFERENCE);
+	else if (!strcmp(arg, "--no-dereference"))
+		DIFF_OPT_CLR(options, DEREFERENCE);
+	else if (!strcmp(arg, "--color"))
 		options->use_color = 1;
 	else if (skip_prefix(arg, "--color=", &arg)) {
 		int value = git_config_colorbool(NULL, arg);

index 25ae60d5ff..db33dc67f6 100644
@@ -69,7 +69,7 @@ typedef struct strbuf *(*diff_prefix_fn_t)(struct diff_options *opt, void *data)
 #define DIFF_OPT_FIND_COPIES_HARDER  (1 <<  6)
 #define DIFF_OPT_FOLLOW_RENAMES      (1 <<  7)
 #define DIFF_OPT_RENAME_EMPTY        (1 <<  8)
-/* (1 <<  9) unused */
+#define DIFF_OPT_DEREFERENCE         (1 <<  9)
 #define DIFF_OPT_HAS_CHANGES         (1 << 10)
 #define DIFF_OPT_QUICK               (1 << 11)
 #define DIFF_OPT_NO_INDEX            (1 << 12)

index fe48f32ddd..1262a587e5 100644
@@ -83,6 +83,15 @@ static struct diff_filespec *noindex_filespec(const char *name, int mode)
 		name = "/dev/null";
 	s = alloc_filespec(name);
 	fill_filespec(s, null_sha1, 0, mode);
+	/*
+	 * In --no-index mode, we support reading from pipes. canon_mode, called by
+	 * fill_filespec, gets confused by this and thinks we now have subprojects.
+	 * To help the rest of the diff machinery along, we now override what
+	 * canon_mode says. This is done here instead of in canon_mode, because the
+	 * rest of git does not (and should not) support pipes.
+	 */
+	if (S_ISFIFO(mode))
+		s->mode = S_IFREG | ce_permissions(mode);
 	if (name == file_from_standard_input)
 		populate_from_stdin(s);
 	return s;

index 2afecfb939..4f74a54d74 100644
@@ -2765,6 +2765,11 @@ static int diff_populate_gitlink(struct diff_filespec *s, int size_only)
 	return 0;
 }
 
+static int should_mmap_file_contents(struct stat *st)
+{
+	return S_ISREG(st->st_mode);
+}
+
 /*
  * While doing rename detection and pickaxe operation, we may need to
  * grab the data for the blob (or file) for our own in-core comparison.

@@ -2839,9 +2844,18 @@ int diff_populate_filespec(struct diff_filespec *s, unsigned int flags)
 		fd = open(s->path, O_RDONLY);
 		if (fd < 0)
 			goto err_empty;
-		s->data = xmmap(NULL, s->size, PROT_READ, MAP_PRIVATE, fd, 0);
+		if (!should_mmap_file_contents(&st)) {
+			struct strbuf sb = STRBUF_INIT;
+			strbuf_read(&sb, fd, 0);
+			s->size = sb.len;
+			s->data = strbuf_detach(&sb, NULL);
+			s->should_free = 1;
+		}
+		else {
+			s->data = xmmap(NULL, s->size, PROT_READ, MAP_PRIVATE, fd, 0);
+			s->should_munmap = 1;
+		}
 		close(fd);
-		s->should_munmap = 1;
 
 		/*
 		 * Convert from working tree format to canonical git format