write-tree: --prefix=<path>

The "bind" commit can express an aggregation of multiple
projects into a single commit.

In such an organization, there would be one project, root of
whose tree object is at the same level of the root of the
aggregated projects, and other projects have their toplevel in
separate subdirectories.  Let's call that root level project the
"primary project", and call other ones just "subprojects".

You would first read-tree the primary project, and then graft
the subprojects under their appropriate location using read-tree
--prefix=<subdir>/ repeatedly.

To write out a tree object from such an index for a subproject,
write-tree --prefix=<subdir>/ is used.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-write-tree.txt

@@ -8,7 +8,7 @@ git-write-tree - Creates a tree object from the current index
 
 SYNOPSIS
 --------
-'git-write-tree' [--missing-ok]
+'git-write-tree' [--missing-ok] [--prefix=<prefix>/]
 
 DESCRIPTION
 -----------
@@ -30,6 +30,12 @@ OPTIONS
 	directory exist in the object database.  This option disables this
 	check.
 
+--prefix=<prefix>/::
+	Writes a tree object that represents a subdirectory
+	`<prefix>`.  This can be used to write the tree object
+	for a subproject that is in the named subdirectory.
+
+
 Author
 ------
 Written by Linus Torvalds <torvalds@osdl.org>

cache-tree.c

@@ -525,3 +525,29 @@ struct cache_tree *cache_tree_read(const char *buffer, unsigned long size)
 		return NULL; /* not the whole tree */
 	return read_one(&buffer, &size);
 }
+
+struct cache_tree *cache_tree_find(struct cache_tree *it, const char *path)
+{
+	while (*path) {
+		const char *slash;
+		struct cache_tree_sub *sub;
+
+		slash = strchr(path, '/');
+		if (!slash)
+			slash = path + strlen(path);
+		/* between path and slash is the name of the
+		 * subtree to look for.
+		 */
+		sub = find_subtree(it, path, slash - path, 0);
+		if (!sub)
+			return NULL;
+		it = sub->cache_tree;
+		if (slash)
+			while (*slash && *slash == '/')
+				slash++;
+		if (!slash || !*slash)
+			return it; /* prefix ended with slashes */
+		path = slash;
+	}
+	return it;
+}

cache-tree.h

@@ -28,4 +28,6 @@ struct cache_tree *cache_tree_read(const char *buffer, unsigned long size);
 int cache_tree_fully_valid(struct cache_tree *);
 int cache_tree_update(struct cache_tree *, struct cache_entry **, int, int, int);
 
+struct cache_tree *cache_tree_find(struct cache_tree *, const char *);
+
 #endif

t/t0000-basic.sh

@@ -195,6 +195,20 @@ test_expect_success \
     'git-ls-tree -r output for a known tree.' \
     'diff current expected'
 
+test_expect_success \
+    'writing partial tree out with git-write-tree --prefix.' \
+    'ptree=$(git-write-tree --prefix=path3)'
+test_expect_success \
+    'validate object ID for a known tree.' \
+    'test "$ptree" = 21ae8269cacbe57ae09138dcc3a2887f904d02b3'
+
+test_expect_success \
+    'writing partial tree out with git-write-tree --prefix.' \
+    'ptree=$(git-write-tree --prefix=path3/subp3)'
+test_expect_success \
+    'validate object ID for a known tree.' \
+    'test "$ptree" = 3c5e5399f3a333eddecce7a9b9465b63f65f51e2'
+
 ################################################################
 rm .git/index
 test_expect_success \

write-tree.c

@@ -8,8 +8,10 @@
 #include "cache-tree.h"
 
 static int missing_ok = 0;
+static char *prefix = NULL;
 
-static const char write_tree_usage[] = "git-write-tree [--missing-ok]";
+static const char write_tree_usage[] =
+"git-write-tree [--missing-ok] [--prefix=<prefix>/]";
 
 static struct cache_file cache_file;
 
@@ -21,13 +23,18 @@ int main(int argc, char **argv)
 
 	newfd = hold_index_file_for_update(&cache_file, get_index_file());
 	entries = read_cache();
-	if (argc == 2) {
-		if (!strcmp(argv[1], "--missing-ok"))
+
+	while (1 < argc) {
+		char *arg = argv[1];
+		if (!strcmp(arg, "--missing-ok"))
 			missing_ok = 1;
+		else if (!strncmp(arg, "--prefix=", 9))
+			prefix = arg + 9;
 		else
 			die(write_tree_usage);
+		argc--; argv++;
 	}
-	
+
 	if (argc > 2)
 		die("too many options");
 
@@ -54,6 +61,12 @@ int main(int argc, char **argv)
 		 * performance penalty and not a big deal.
 		 */
 	}
-	printf("%s\n", sha1_to_hex(active_cache_tree->sha1));
+	if (prefix) {
+		struct cache_tree *subtree =
+			cache_tree_find(active_cache_tree, prefix);
+		printf("%s\n", sha1_to_hex(subtree->sha1));
+	}
+	else
+		printf("%s\n", sha1_to_hex(active_cache_tree->sha1));
 	return 0;
 }