Re: [PATCH v2 15/20] merge-ort: step 3 of tree writing -- handling subdirectories as we go

From: Jonathan Tan <jonathantanmy@google.com>
Date: Nov 12, 2020, 20:15 UTC
Message-ID: <20201112201524.3438029-1-jonathantanmy@google.com>
In-Reply-To: <20201102204344.342633-16-newren@gmail.com>

Firstly, from [1]:

> Thanks for the reviews!  I was hoping to see some comments on patch
> 15, as it's possibly the gnarliest.  It's a relatively straightforward
> algorithm, just lots of bookkeeping.

I was planning to send this out yesterday, but couldn't finish it. :-P Indeed, a lot of things to think about.

[1] https://lore.kernel.org/git/CABPp-BFgQX6Ash03x7z+RfE3ytbw3x0DzDSBrGddgMr_soODoA@mail.gmail.com/

[snip commit message]

Thanks for the thorough explanation.

Show 5 quoted lines

> @@ -353,6 +353,9 @@ static int string_list_df_name_compare(const char *one, const char *two)
>  
>  struct directory_versions {
>  	struct string_list versions;
> +	struct string_list offsets;

Looking below (and at the explanation in the commit message), this is a mapping from full paths to integers casted to the pointer type.

> +	const char *last_directory;
> +	unsigned last_directory_len;

Is this just the last entry in "versions"?

Show 18 quoted lines

>  static void write_tree(struct object_id *result_oid,
> @@ -409,12 +412,100 @@ static void record_entry_for_tree(struct directory_versions *dir_metadata,
>  		/* nothing to record */
>  		return;
>  
> +	/*
> +	 * Note: write_completed_directories() already added
> +	 * entries for directories to dir_metadata->versions,
> +	 * so no need to handle ci->filemask == 0 again.
> +	 */
> +	if (!ci->merged.clean && !ci->filemask)
> +		return;
> +
>  	basename = path + ci->merged.basename_offset;
>  	assert(strchr(basename, '/') == NULL);
>  	string_list_append(&dir_metadata->versions,
>  			   basename)->util = &ci->merged.result;
>  }

Conceptually, I can see how the algorithm below inserts directories, but I don't understand the significance of "!ci->merged.clean" in the change above.

Show 11 quoted lines

> +static void write_completed_directories(struct merge_options *opt,
> +					const char *new_directory_name,
> +					struct directory_versions *info)
> +{
> +	const char *prev_dir;
> +	struct merged_info *dir_info = NULL;
> +	unsigned int offset;
> +	int wrote_a_new_tree = 0;
> +
> +	if (new_directory_name == info->last_directory)
> +		return;

Pointer equality is OK here presumably because of the string interning of directory names.

I'm starting to think that it might be too difficult to keep track of where strings are interned. Maybe there should be a data structure containing all interned strings, and make the path a struct or something like that (to clearly indicate that the string inside comes from the interned string data structure).

Show 8 quoted lines

> +	/*
> +	 * If we are just starting (last_directory is NULL), or last_directory
> +	 * is a prefix of the current directory, then we can just update
> +	 * last_directory and record the offset where we started this directory.
> +	 */
> +	if (info->last_directory == NULL ||
> +	    !strncmp(new_directory_name, info->last_directory,
> +		     info->last_directory_len)) {

Git has starts_with() for prefix checking. (May not be as optimized as this one, though.)

Show 8 quoted lines

> +		uintptr_t offset = info->versions.nr;
> +
> +		info->last_directory = new_directory_name;
> +		info->last_directory_len = strlen(info->last_directory);
> +		string_list_append(&info->offsets,
> +				   info->last_directory)->util = (void*)offset;
> +		return;
> +	}

Due to the way this is sorted, there might be a jump of 2 or more directories. (The commit message also provides such an example - from "" to "src/moduleB", without going through "src".)

Show 5 quoted lines

> +	/*
> +	 * At this point, ne (next entry) is within a different directory
> +	 * than the last entry, so we need to create a tree object for all
> +	 * the entries in info->versions that are under info->last_directory.
> +	 */

There's no "ne" below.

Show 10 quoted lines

> +	dir_info = strmap_get(&opt->priv->paths, info->last_directory);
> +	assert(dir_info);
> +	offset = (uintptr_t)info->offsets.items[info->offsets.nr-1].util;
> +	if (offset == info->versions.nr) {
> +		dir_info->is_null = 1;
> +	} else {
> +		dir_info->result.mode = S_IFDIR;
> +		write_tree(&dir_info->result.oid, &info->versions, offset);
> +		wrote_a_new_tree = 1;
> +	}

I was trying to figure out the cases in which offset would be info->versions.nr - if such a case existed, and if yes, would it be incorrect to skip creating such a tree because presumably this offset exists in info->offsets for a reason. Do you know in which situation offset would equal info->versions.nr?

Show 6 quoted lines

> +	/*
> +	 * We've now used several entries from info->versions and one entry
> +	 * from info->offsets, so we get rid of those values.
> +	 */
> +	info->offsets.nr--;
> +	info->versions.nr = offset;

OK.

Show 14 quoted lines

> +	/*
> +	 * Now we've got an OID for last_directory in dir_info.  We need to
> +	 * add it to info->versions for it to be part of the computation of
> +	 * its parent directories' OID.  But first, we have to find out what
> +	 * its' parent name was and whether that matches the previous
> +	 * info->offsets or we need to set up a new one.
> +	 */
> +	prev_dir = info->offsets.nr == 0 ? NULL :
> +		   info->offsets.items[info->offsets.nr-1].string;
> +	if (new_directory_name != prev_dir) {
> +		uintptr_t c = info->versions.nr;
> +		string_list_append(&info->offsets,
> +				   new_directory_name)->util = (void*)c;
> +	}

Because of the possible jump of 2 or more directories that I mentioned earlier, there may be gaps in the offsets. So it makes sense that we sometimes need to insert an intermediate one.

I wonder if the code would be clearer if we had explicit "begin tree" and "end tree" steps just like in list-objects-filter.c (LOFS_BEGIN_TREE and LOFS_END_TREE). Here we have "end tree" (because of the way the entries were sorted) but not "begin tree". If we had "begin tree", we probably would be able to create the necessary offsets in a loop at that stage, and the reasoning about the contents of the offsets would not be so complicated.

If we really only want one side (i.e. you don't want to introduce a synthetic entry just to mark the end or the beginning), then my personal experience is that having the "begin" side is easier to understand, as the state is more natural and easier to reason about. (Unlike here, where there could be gaps in the offsets and the reader has to understand that the gaps will be filled just in time.) But that may just be my own experience.

Show 12 quoted lines

> +	/*
> +	 * Okay, finally record OID for last_directory in info->versions,
> +	 * and update last_directory.
> +	 */
> +	if (wrote_a_new_tree) {
> +		const char *dir_name = strrchr(info->last_directory, '/');
> +		dir_name = dir_name ? dir_name+1 : info->last_directory;
> +		string_list_append(&info->versions, dir_name)->util = dir_info;
> +	}
> +	info->last_directory = new_directory_name;
> +	info->last_directory_len = strlen(info->last_directory);
> +}

OK - several entries in info->versions were deleted earlier (through info->versions.nr = offset), and we add one here to represent the tree containing all those deleted versions.

Show 11 quoted lines

> @@ -541,22 +635,27 @@ static void process_entries(struct merge_options *opt,
>  		 */
>  		struct conflict_info *ci = entry->util;
>  
> +		write_completed_directories(opt, ci->merged.directory_name,
> +					    &dir_metadata);
>  		if (ci->merged.clean)
>  			record_entry_for_tree(&dir_metadata, path, ci);
>  		else
>  			process_entry(opt, path, ci, &dir_metadata);
>  	}

Trying to make sense of this: we pass in the directory name of the current entry so that if the last directory is *not* a prefix of the current directory (so we either went up a directory or went sideways), then we write a tree (unless offset == info->versions.nr, which as I stated above, I still don't fully understand - I thought we would always have to write a tree). So maybe the name of the function should be "write_completed_directory" (and document it as "write a tree if ???"), since we write at most one.

In this kind of algorithm (where intermediate accumulated results are being written), there needs to be a last write after the loop that writes whatever's left in the accumulation buffer. I do see it below ("write_tree"), so that's good.

Show 16 quoted lines

> -	/*
> -	 * TODO: We can't actually write a tree yet, because dir_metadata just
> -	 * contains all basenames of all files throughout the tree with their
> -	 * mode and hash.  Not only is that a nonsensical tree, it will have
> -	 * lots of duplicates for paths such as "Makefile" or ".gitignore".
> -	 */
> -	die("Not yet implemented; need to process subtrees separately");
> +	if (dir_metadata.offsets.nr != 1 ||
> +	    (uintptr_t)dir_metadata.offsets.items[0].util != 0) {
> +		printf("dir_metadata.offsets.nr = %d (should be 1)\n",
> +		       dir_metadata.offsets.nr);
> +		printf("dir_metadata.offsets.items[0].util = %u (should be 0)\n",
> +		       (unsigned)(uintptr_t)dir_metadata.offsets.items[0].util);
> +		fflush(stdout);
> +		BUG("dir_metadata accounting completely off; shouldn't happen");
> +	}

Sanity check, OK.

[snip rest]

In summary, I think that the code would be easier to understand (for everyone) if there were both BEGIN_TREE and END_TREE entries. And for me personally, once the offset == info->versions.nr part is clarified (perhaps there is something obvious that I'm missing).