difftool: refactor dir-diff to write files using helper functions

Add a helpers function to handle the unlinking and writing
of the dir-diff submodule and symlink stand-in files.

Use the helpers to implement the guts of the hashmap loops.
This eliminate duplicate code and safeguards the submodules
hashmap loop against the symlink-chasing behavior that 5bafb35
(difftool: fix symlink-file writing in dir-diff mode, 2021-09-22)
addressed.

The submodules loop should not strictly require the unlink() call that
this is introducing to them, but it does not necessarily hurt them
either beyond the cost of the extra unlink().

Signed-off-by: David Aguilar <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: davvid

builtin/difftool.c

@@ -320,6 +320,31 @@ static int checkout_path(unsigned mode, struct object_id *oid,
 	return ret;
 }
 
+static void write_file_in_directory(struct strbuf *dir, size_t dir_len,
+			const char *path, const char *content)
+{
+	add_path(dir, dir_len, path);
+	ensure_leading_directories(dir->buf);
+	unlink(dir->buf);
+	write_file(dir->buf, "%s", content);
+}
+
+/* Write the file contents for the left and right sides of the difftool
+ * dir-diff representation for submodules and symlinks. Symlinks and submodules
+ * are written as regular text files so that external diff tools can diff them
+ * as text files, resulting in behavior that is analogous to to what "git diff"
+ * displays for symlink and submodule diffs.
+ */
+static void write_standin_files(struct pair_entry *entry,
+			struct strbuf *ldir, size_t ldir_len,
+			struct strbuf *rdir, size_t rdir_len)
+{
+	if (*entry->left)
+		write_file_in_directory(ldir, ldir_len, entry->path, entry->left);
+	if (*entry->right)
+		write_file_in_directory(rdir, rdir_len, entry->path, entry->right);
+}
+
 static int run_dir_diff(const char *extcmd, int symlinks, const char *prefix,
 			struct child_process *child)
 {
@@ -529,16 +554,7 @@ static int run_dir_diff(const char *extcmd, int symlinks, const char *prefix,
 	 */
 	hashmap_for_each_entry(&submodules, &iter, entry,
 				entry /* member name */) {
-		if (*entry->left) {
-			add_path(&ldir, ldir_len, entry->path);
-			ensure_leading_directories(ldir.buf);
-			write_file(ldir.buf, "%s", entry->left);
-		}
-		if (*entry->right) {
-			add_path(&rdir, rdir_len, entry->path);
-			ensure_leading_directories(rdir.buf);
-			write_file(rdir.buf, "%s", entry->right);
-		}
+		write_standin_files(entry, &ldir, ldir_len, &rdir, rdir_len);
 	}
 
 	/*
@@ -548,18 +564,8 @@ static int run_dir_diff(const char *extcmd, int symlinks, const char *prefix,
 	 */
 	hashmap_for_each_entry(&symlinks2, &iter, entry,
 				entry /* member name */) {
-		if (*entry->left) {
-			add_path(&ldir, ldir_len, entry->path);
-			ensure_leading_directories(ldir.buf);
-			unlink(ldir.buf);
-			write_file(ldir.buf, "%s", entry->left);
-		}
-		if (*entry->right) {
-			add_path(&rdir, rdir_len, entry->path);
-			ensure_leading_directories(rdir.buf);
-			unlink(rdir.buf);
-			write_file(rdir.buf, "%s", entry->right);
-		}
+
+		write_standin_files(entry, &ldir, ldir_len, &rdir, rdir_len);
 	}
 
 	strbuf_release(&buf);