thodg/libgit2/src/libgit2/apply.c

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• Hash : ef4ab298
  Author :
  Date : 2021-11-14T08:47:40
  

  refactor: `src` is now `src/libgit2`
  

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• src/libgit2/apply.c

• /*
   * Copyright (C) the libgit2 contributors. All rights reserved.
   *
   * This file is part of libgit2, distributed under the GNU GPL v2 with
   * a Linking Exception. For full terms see the included COPYING file.
   */
  
  #include "git2/apply.h"
  #include "git2/patch.h"
  #include "git2/filter.h"
  #include "git2/blob.h"
  #include "git2/index.h"
  #include "git2/checkout.h"
  #include "git2/repository.h"
  #include "array.h"
  #include "patch.h"
  #include "futils.h"
  #include "delta.h"
  #include "zstream.h"
  #include "reader.h"
  #include "index.h"
  #include "apply.h"
  
  typedef struct {
  	/* The lines that we allocate ourself are allocated out of the pool.
  	 * (Lines may have been allocated out of the diff.)
  	 */
  	git_pool pool;
  	git_vector lines;
  } patch_image;
  
  static int apply_err(const char *fmt, ...) GIT_FORMAT_PRINTF(1, 2);
  static int apply_err(const char *fmt, ...)
  {
  	va_list ap;
  
  	va_start(ap, fmt);
  	git_error_vset(GIT_ERROR_PATCH, fmt, ap);
  	va_end(ap);
  
  	return GIT_EAPPLYFAIL;
  }
  
  static void patch_line_init(
  	git_diff_line *out,
  	const char *in,
  	size_t in_len,
  	size_t in_offset)
  {
  	out->content = in;
  	out->content_len = in_len;
  	out->content_offset = in_offset;
  }
  
  #define PATCH_IMAGE_INIT { GIT_POOL_INIT, GIT_VECTOR_INIT }
  
  static int patch_image_init_fromstr(
  	patch_image *out, const char *in, size_t in_len)
  {
  	git_diff_line *line;
  	const char *start, *end;
  
  	memset(out, 0x0, sizeof(patch_image));
  
  	if (git_pool_init(&out->pool, sizeof(git_diff_line)) < 0)
  		return -1;
  
  	if (!in_len)
  		return 0;
  
  	for (start = in; start < in + in_len; start = end) {
  		end = memchr(start, '\n', in_len - (start - in));
  
  		if (end == NULL)
  			end = in + in_len;
  
  		else if (end < in + in_len)
  			end++;
  
  		line = git_pool_mallocz(&out->pool, 1);
  		GIT_ERROR_CHECK_ALLOC(line);
  
  		if (git_vector_insert(&out->lines, line) < 0)
  			return -1;
  
  		patch_line_init(line, start, (end - start), (start - in));
  	}
  
  	return 0;
  }
  
  static void patch_image_free(patch_image *image)
  {
  	if (image == NULL)
  		return;
  
  	git_pool_clear(&image->pool);
  	git_vector_free(&image->lines);
  }
  
  static bool match_hunk(
  	patch_image *image,
  	patch_image *preimage,
  	size_t linenum)
  {
  	bool match = 0;
  	size_t i;
  
  	/* Ensure this hunk is within the image boundaries. */
  	if (git_vector_length(&preimage->lines) + linenum >
  		git_vector_length(&image->lines))
  		return 0;
  
  	match = 1;
  
  	/* Check exact match. */
  	for (i = 0; i < git_vector_length(&preimage->lines); i++) {
  		git_diff_line *preimage_line = git_vector_get(&preimage->lines, i);
  		git_diff_line *image_line = git_vector_get(&image->lines, linenum + i);
  
  		if (preimage_line->content_len != image_line->content_len ||
  			memcmp(preimage_line->content, image_line->content, image_line->content_len) != 0) {
  			match = 0;
  			break;
  		}
  	}
  
  	return match;
  }
  
  static bool find_hunk_linenum(
  	size_t *out,
  	patch_image *image,
  	patch_image *preimage,
  	size_t linenum)
  {
  	size_t max = git_vector_length(&image->lines);
  	bool match;
  
  	if (linenum > max)
  		linenum = max;
  
  	match = match_hunk(image, preimage, linenum);
  
  	*out = linenum;
  	return match;
  }
  
  static int update_hunk(
  	patch_image *image,
  	size_t linenum,
  	patch_image *preimage,
  	patch_image *postimage)
  {
  	size_t postlen = git_vector_length(&postimage->lines);
  	size_t prelen = git_vector_length(&preimage->lines);
  	size_t i;
  	int error = 0;
  
  	if (postlen > prelen)
  		error = git_vector_insert_null(
  			&image->lines, linenum, (postlen - prelen));
  	else if (prelen > postlen)
  		error = git_vector_remove_range(
  			&image->lines, linenum, (prelen - postlen));
  
  	if (error) {
  		git_error_set_oom();
  		return -1;
  	}
  
  	for (i = 0; i < git_vector_length(&postimage->lines); i++) {
  		image->lines.contents[linenum + i] =
  			git_vector_get(&postimage->lines, i);
  	}
  
  	return 0;
  }
  
  typedef struct {
  	git_apply_options opts;
  	size_t skipped_new_lines;
  	size_t skipped_old_lines;
  } apply_hunks_ctx;
  
  static int apply_hunk(
  	patch_image *image,
  	git_patch *patch,
  	git_patch_hunk *hunk,
  	apply_hunks_ctx *ctx)
  {
  	patch_image preimage = PATCH_IMAGE_INIT, postimage = PATCH_IMAGE_INIT;
  	size_t line_num, i;
  	int error = 0;
  
  	if (ctx->opts.hunk_cb) {
  		error = ctx->opts.hunk_cb(&hunk->hunk, ctx->opts.payload);
  
  		if (error) {
  			if (error > 0) {
  				ctx->skipped_new_lines += hunk->hunk.new_lines;
  				ctx->skipped_old_lines += hunk->hunk.old_lines;
  				error = 0;
  			}
  
  			goto done;
  		}
  	}
  
  	for (i = 0; i < hunk->line_count; i++) {
  		size_t linenum = hunk->line_start + i;
  		git_diff_line *line = git_array_get(patch->lines, linenum), *prev;
  
  		if (!line) {
  			error = apply_err("preimage does not contain line %"PRIuZ, linenum);
  			goto done;
  		}
  
  		switch (line->origin) {
  			case GIT_DIFF_LINE_CONTEXT_EOFNL:
  			case GIT_DIFF_LINE_DEL_EOFNL:
  			case GIT_DIFF_LINE_ADD_EOFNL:
  				prev = i ? git_array_get(patch->lines, linenum - 1) : NULL;
  				if (prev && prev->content[prev->content_len - 1] == '\n')
  					prev->content_len -= 1;
  				break;
  			case GIT_DIFF_LINE_CONTEXT:
  				if ((error = git_vector_insert(&preimage.lines, line)) < 0 ||
  				    (error = git_vector_insert(&postimage.lines, line)) < 0)
  					goto done;
  				break;
  			case GIT_DIFF_LINE_DELETION:
  				if ((error = git_vector_insert(&preimage.lines, line)) < 0)
  					goto done;
  				break;
  			case GIT_DIFF_LINE_ADDITION:
  				if ((error = git_vector_insert(&postimage.lines, line)) < 0)
  					goto done;
  				break;
  		}
  	}
  
  	if (hunk->hunk.new_start) {
  		line_num = hunk->hunk.new_start -
  			ctx->skipped_new_lines +
  			ctx->skipped_old_lines -
  			1;
  	} else {
  		line_num = 0;
  	}
  
  	if (!find_hunk_linenum(&line_num, image, &preimage, line_num)) {
  		error = apply_err("hunk at line %d did not apply",
  			hunk->hunk.new_start);
  		goto done;
  	}
  
  	error = update_hunk(image, line_num, &preimage, &postimage);
  
  done:
  	patch_image_free(&preimage);
  	patch_image_free(&postimage);
  
  	return error;
  }
  
  static int apply_hunks(
  	git_str *out,
  	const char *source,
  	size_t source_len,
  	git_patch *patch,
  	apply_hunks_ctx *ctx)
  {
  	git_patch_hunk *hunk;
  	git_diff_line *line;
  	patch_image image;
  	size_t i;
  	int error = 0;
  
  	if ((error = patch_image_init_fromstr(&image, source, source_len)) < 0)
  		goto done;
  
  	git_array_foreach(patch->hunks, i, hunk) {
  		if ((error = apply_hunk(&image, patch, hunk, ctx)) < 0)
  			goto done;
  	}
  
  	git_vector_foreach(&image.lines, i, line)
  		git_str_put(out, line->content, line->content_len);
  
  done:
  	patch_image_free(&image);
  
  	return error;
  }
  
  static int apply_binary_delta(
  	git_str *out,
  	const char *source,
  	size_t source_len,
  	git_diff_binary_file *binary_file)
  {
  	git_str inflated = GIT_STR_INIT;
  	int error = 0;
  
  	/* no diff means identical contents */
  	if (binary_file->datalen == 0)
  		return git_str_put(out, source, source_len);
  
  	error = git_zstream_inflatebuf(&inflated,
  		binary_file->data, binary_file->datalen);
  
  	if (!error && inflated.size != binary_file->inflatedlen) {
  		error = apply_err("inflated delta does not match expected length");
  		git_str_dispose(out);
  	}
  
  	if (error < 0)
  		goto done;
  
  	if (binary_file->type == GIT_DIFF_BINARY_DELTA) {
  		void *data;
  		size_t data_len;
  
  		error = git_delta_apply(&data, &data_len, (void *)source, source_len,
  			(void *)inflated.ptr, inflated.size);
  
  		out->ptr = data;
  		out->size = data_len;
  		out->asize = data_len;
  	}
  	else if (binary_file->type == GIT_DIFF_BINARY_LITERAL) {
  		git_str_swap(out, &inflated);
  	}
  	else {
  		error = apply_err("unknown binary delta type");
  		goto done;
  	}
  
  done:
  	git_str_dispose(&inflated);
  	return error;
  }
  
  static int apply_binary(
  	git_str *out,
  	const char *source,
  	size_t source_len,
  	git_patch *patch)
  {
  	git_str reverse = GIT_STR_INIT;
  	int error = 0;
  
  	if (!patch->binary.contains_data) {
  		error = apply_err("patch does not contain binary data");
  		goto done;
  	}
  
  	if (!patch->binary.old_file.datalen && !patch->binary.new_file.datalen)
  		goto done;
  
  	/* first, apply the new_file delta to the given source */
  	if ((error = apply_binary_delta(out, source, source_len,
  			&patch->binary.new_file)) < 0)
  		goto done;
  
  	/* second, apply the old_file delta to sanity check the result */
  	if ((error = apply_binary_delta(&reverse, out->ptr, out->size,
  			&patch->binary.old_file)) < 0)
  		goto done;
  
  	/* Verify that the resulting file with the reverse patch applied matches the source file */
  	if (source_len != reverse.size ||
  		(source_len && memcmp(source, reverse.ptr, source_len) != 0)) {
  		error = apply_err("binary patch did not apply cleanly");
  		goto done;
  	}
  
  done:
  	if (error < 0)
  		git_str_dispose(out);
  
  	git_str_dispose(&reverse);
  	return error;
  }
  
  int git_apply__patch(
  	git_str *contents_out,
  	char **filename_out,
  	unsigned int *mode_out,
  	const char *source,
  	size_t source_len,
  	git_patch *patch,
  	const git_apply_options *given_opts)
  {
  	apply_hunks_ctx ctx = { GIT_APPLY_OPTIONS_INIT };
  	char *filename = NULL;
  	unsigned int mode = 0;
  	int error = 0;
  
  	GIT_ASSERT_ARG(contents_out);
  	GIT_ASSERT_ARG(filename_out);
  	GIT_ASSERT_ARG(mode_out);
  	GIT_ASSERT_ARG(source || !source_len);
  	GIT_ASSERT_ARG(patch);
  
  	if (given_opts)
  		memcpy(&ctx.opts, given_opts, sizeof(git_apply_options));
  
  	*filename_out = NULL;
  	*mode_out = 0;
  
  	if (patch->delta->status != GIT_DELTA_DELETED) {
  		const git_diff_file *newfile = &patch->delta->new_file;
  
  		filename = git__strdup(newfile->path);
  		mode = newfile->mode ?
  			newfile->mode : GIT_FILEMODE_BLOB;
  	}
  
  	if (patch->delta->flags & GIT_DIFF_FLAG_BINARY)
  		error = apply_binary(contents_out, source, source_len, patch);
  	else if (patch->hunks.size)
  		error = apply_hunks(contents_out, source, source_len, patch, &ctx);
  	else
  		error = git_str_put(contents_out, source, source_len);
  
  	if (error)
  		goto done;
  
  	if (patch->delta->status == GIT_DELTA_DELETED &&
  		git_str_len(contents_out) > 0) {
  		error = apply_err("removal patch leaves file contents");
  		goto done;
  	}
  
  	*filename_out = filename;
  	*mode_out = mode;
  
  done:
  	if (error < 0)
  		git__free(filename);
  
  	return error;
  }
  
  static int apply_one(
  	git_repository *repo,
  	git_reader *preimage_reader,
  	git_index *preimage,
  	git_reader *postimage_reader,
  	git_index *postimage,
  	git_diff *diff,
  	git_strmap *removed_paths,
  	size_t i,
  	const git_apply_options *opts)
  {
  	git_patch *patch = NULL;
  	git_str pre_contents = GIT_STR_INIT, post_contents = GIT_STR_INIT;
  	const git_diff_delta *delta;
  	char *filename = NULL;
  	unsigned int mode;
  	git_oid pre_id, post_id;
  	git_filemode_t pre_filemode;
  	git_index_entry pre_entry, post_entry;
  	bool skip_preimage = false;
  	int error;
  
  	if ((error = git_patch_from_diff(&patch, diff, i)) < 0)
  		goto done;
  
  	delta = git_patch_get_delta(patch);
  
  	if (opts->delta_cb) {
  		error = opts->delta_cb(delta, opts->payload);
  
  		if (error) {
  			if (error > 0)
  				error = 0;
  
  			goto done;
  		}
  	}
  
  	/*
  	 * Ensure that the file has not been deleted or renamed if we're
  	 * applying a modification delta.
  	 */
  	if (delta->status != GIT_DELTA_RENAMED &&
  	    delta->status != GIT_DELTA_ADDED) {
  		if (git_strmap_exists(removed_paths, delta->old_file.path)) {
  			error = apply_err("path '%s' has been renamed or deleted", delta->old_file.path);
  			goto done;
  		}
  	}
  
  	/*
  	 * We may be applying a second delta to an already seen file.  If so,
  	 * use the already modified data in the postimage instead of the
  	 * content from the index or working directory.  (Don't do this in
  	 * the case of a rename, which must be specified before additional
  	 * deltas since we apply deltas to the target filename.)
  	 */
  	if (delta->status != GIT_DELTA_RENAMED) {
  		if ((error = git_reader_read(&pre_contents, &pre_id, &pre_filemode,
  		    postimage_reader, delta->old_file.path)) == 0) {
  			skip_preimage = true;
  		} else if (error == GIT_ENOTFOUND) {
  			git_error_clear();
  			error = 0;
  		} else {
  			goto done;
  		}
  	}
  
  	if (!skip_preimage && delta->status != GIT_DELTA_ADDED) {
  		error = git_reader_read(&pre_contents, &pre_id, &pre_filemode,
  			preimage_reader, delta->old_file.path);
  
  		/* ENOTFOUND means the preimage was not found; apply failed. */
  		if (error == GIT_ENOTFOUND)
  			error = GIT_EAPPLYFAIL;
  
  		/* When applying to BOTH, the index did not match the workdir. */
  		if (error == GIT_READER_MISMATCH)
  			error = apply_err("%s: does not match index", delta->old_file.path);
  
  		if (error < 0)
  			goto done;
  
  		/*
  		 * We need to populate the preimage data structure with the
  		 * contents that we are using as the preimage for this file.
  		 * This allows us to apply patches to files that have been
  		 * modified in the working directory.  During checkout,
  		 * we will use this expected preimage as the baseline, and
  		 * limit checkout to only the paths affected by patch
  		 * application.  (Without this, we would fail to write the
  		 * postimage contents to any file that had been modified
  		 * from HEAD on-disk, even if the patch application succeeded.)
  		 * Use the contents from the delta where available - some
  		 * fields may not be available, like the old file mode (eg in
  		 * an exact rename situation) so trust the patch parsing to
  		 * validate and use the preimage data in that case.
  		 */
  		if (preimage) {
  			memset(&pre_entry, 0, sizeof(git_index_entry));
  			pre_entry.path = delta->old_file.path;
  			pre_entry.mode = delta->old_file.mode ? delta->old_file.mode : pre_filemode;
  			git_oid_cpy(&pre_entry.id, &pre_id);
  
  			if ((error = git_index_add(preimage, &pre_entry)) < 0)
  				goto done;
  		}
  	}
  
  	if (delta->status != GIT_DELTA_DELETED) {
  		if ((error = git_apply__patch(&post_contents, &filename, &mode,
  				pre_contents.ptr, pre_contents.size, patch, opts)) < 0 ||
  			(error = git_blob_create_from_buffer(&post_id, repo,
  				post_contents.ptr, post_contents.size)) < 0)
  			goto done;
  
  		memset(&post_entry, 0, sizeof(git_index_entry));
  		post_entry.path = filename;
  		post_entry.mode = mode;
  		git_oid_cpy(&post_entry.id, &post_id);
  
  		if ((error = git_index_add(postimage, &post_entry)) < 0)
  			goto done;
  	}
  
  	if (delta->status == GIT_DELTA_RENAMED ||
  	    delta->status == GIT_DELTA_DELETED)
  		error = git_strmap_set(removed_paths, delta->old_file.path, (char *) delta->old_file.path);
  
  	if (delta->status == GIT_DELTA_RENAMED ||
  	    delta->status == GIT_DELTA_ADDED)
  		git_strmap_delete(removed_paths, delta->new_file.path);
  
  done:
  	git_str_dispose(&pre_contents);
  	git_str_dispose(&post_contents);
  	git__free(filename);
  	git_patch_free(patch);
  
  	return error;
  }
  
  static int apply_deltas(
  	git_repository *repo,
  	git_reader *pre_reader,
  	git_index *preimage,
  	git_reader *post_reader,
  	git_index *postimage,
  	git_diff *diff,
  	const git_apply_options *opts)
  {
  	git_strmap *removed_paths;
  	size_t i;
  	int error = 0;
  
  	if (git_strmap_new(&removed_paths) < 0)
  		return -1;
  
  	for (i = 0; i < git_diff_num_deltas(diff); i++) {
  		if ((error = apply_one(repo, pre_reader, preimage, post_reader, postimage, diff, removed_paths, i, opts)) < 0)
  			goto done;
  	}
  
  done:
  	git_strmap_free(removed_paths);
  	return error;
  }
  
  int git_apply_to_tree(
  	git_index **out,
  	git_repository *repo,
  	git_tree *preimage,
  	git_diff *diff,
  	const git_apply_options *given_opts)
  {
  	git_index *postimage = NULL;
  	git_reader *pre_reader = NULL, *post_reader = NULL;
  	git_apply_options opts = GIT_APPLY_OPTIONS_INIT;
  	const git_diff_delta *delta;
  	size_t i;
  	int error = 0;
  
  	GIT_ASSERT_ARG(out);
  	GIT_ASSERT_ARG(repo);
  	GIT_ASSERT_ARG(preimage);
  	GIT_ASSERT_ARG(diff);
  
  	*out = NULL;
  
  	if (given_opts)
  		memcpy(&opts, given_opts, sizeof(git_apply_options));
  
  	if ((error = git_reader_for_tree(&pre_reader, preimage)) < 0)
  		goto done;
  
  	/*
  	 * put the current tree into the postimage as-is - the diff will
  	 * replace any entries contained therein
  	 */
  	if ((error = git_index_new(&postimage)) < 0 ||
  		(error = git_index_read_tree(postimage, preimage)) < 0 ||
  		(error = git_reader_for_index(&post_reader, repo, postimage)) < 0)
  		goto done;
  
  	/*
  	 * Remove the old paths from the index before applying diffs -
  	 * we need to do a full pass to remove them before adding deltas,
  	 * in order to handle rename situations.
  	 */
  	for (i = 0; i < git_diff_num_deltas(diff); i++) {
  		delta = git_diff_get_delta(diff, i);
  
  		if (delta->status == GIT_DELTA_DELETED ||
  			delta->status == GIT_DELTA_RENAMED) {
  			if ((error = git_index_remove(postimage,
  					delta->old_file.path, 0)) < 0)
  				goto done;
  		}
  	}
  
  	if ((error = apply_deltas(repo, pre_reader, NULL, post_reader, postimage, diff, &opts)) < 0)
  		goto done;
  
  	*out = postimage;
  
  done:
  	if (error < 0)
  		git_index_free(postimage);
  
  	git_reader_free(pre_reader);
  	git_reader_free(post_reader);
  
  	return error;
  }
  
  static int git_apply__to_workdir(
  	git_repository *repo,
  	git_diff *diff,
  	git_index *preimage,
  	git_index *postimage,
  	git_apply_location_t location,
  	git_apply_options *opts)
  {
  	git_vector paths = GIT_VECTOR_INIT;
  	git_checkout_options checkout_opts = GIT_CHECKOUT_OPTIONS_INIT;
  	const git_diff_delta *delta;
  	size_t i;
  	int error;
  
  	GIT_UNUSED(opts);
  
  	/*
  	 * Limit checkout to the paths affected by the diff; this ensures
  	 * that other modifications in the working directory are unaffected.
  	 */
  	if ((error = git_vector_init(&paths, git_diff_num_deltas(diff), NULL)) < 0)
  		goto done;
  
  	for (i = 0; i < git_diff_num_deltas(diff); i++) {
  		delta = git_diff_get_delta(diff, i);
  
  		if ((error = git_vector_insert(&paths, (void *)delta->old_file.path)) < 0)
  			goto done;
  
  		if (strcmp(delta->old_file.path, delta->new_file.path) &&
  		    (error = git_vector_insert(&paths, (void *)delta->new_file.path)) < 0)
  			goto done;
  	}
  
  	checkout_opts.checkout_strategy |= GIT_CHECKOUT_SAFE;
  	checkout_opts.checkout_strategy |= GIT_CHECKOUT_DISABLE_PATHSPEC_MATCH;
  	checkout_opts.checkout_strategy |= GIT_CHECKOUT_DONT_WRITE_INDEX;
  
  	if (location == GIT_APPLY_LOCATION_WORKDIR)
  		checkout_opts.checkout_strategy |= GIT_CHECKOUT_DONT_UPDATE_INDEX;
  
  	checkout_opts.paths.strings = (char **)paths.contents;
  	checkout_opts.paths.count = paths.length;
  
  	checkout_opts.baseline_index = preimage;
  
  	error = git_checkout_index(repo, postimage, &checkout_opts);
  
  done:
  	git_vector_free(&paths);
  	return error;
  }
  
  static int git_apply__to_index(
  	git_repository *repo,
  	git_diff *diff,
  	git_index *preimage,
  	git_index *postimage,
  	git_apply_options *opts)
  {
  	git_index *index = NULL;
  	const git_diff_delta *delta;
  	const git_index_entry *entry;
  	size_t i;
  	int error;
  
  	GIT_UNUSED(preimage);
  	GIT_UNUSED(opts);
  
  	if ((error = git_repository_index(&index, repo)) < 0)
  		goto done;
  
  	/* Remove deleted (or renamed) paths from the index. */
  	for (i = 0; i < git_diff_num_deltas(diff); i++) {
  		delta = git_diff_get_delta(diff, i);
  
  		if (delta->status == GIT_DELTA_DELETED ||
  		    delta->status == GIT_DELTA_RENAMED) {
  			if ((error = git_index_remove(index, delta->old_file.path, 0)) < 0)
  				goto done;
  		}
  	}
  
  	/* Then add the changes back to the index. */
  	for (i = 0; i < git_index_entrycount(postimage); i++) {
  		entry = git_index_get_byindex(postimage, i);
  
  		if ((error = git_index_add(index, entry)) < 0)
  			goto done;
  	}
  
  done:
  	git_index_free(index);
  	return error;
  }
  
  int git_apply_options_init(git_apply_options *opts, unsigned int version)
  {
  	GIT_ASSERT_ARG(opts);
  
  	GIT_INIT_STRUCTURE_FROM_TEMPLATE(
  		opts, version, git_apply_options, GIT_APPLY_OPTIONS_INIT);
  	return 0;
  }
  
  /*
   * Handle the three application options ("locations"):
   *
   * GIT_APPLY_LOCATION_WORKDIR: the default, emulates `git apply`.
   * Applies the diff only to the workdir items and ignores the index
   * entirely.
   *
   * GIT_APPLY_LOCATION_INDEX: emulates `git apply --cached`.
   * Applies the diff only to the index items and ignores the workdir
   * completely.
   *
   * GIT_APPLY_LOCATION_BOTH: emulates `git apply --index`.
   * Applies the diff to both the index items and the working directory
   * items.
   */
  
  int git_apply(
  	git_repository *repo,
  	git_diff *diff,
  	git_apply_location_t location,
  	const git_apply_options *given_opts)
  {
  	git_indexwriter indexwriter = GIT_INDEXWRITER_INIT;
  	git_index *index = NULL, *preimage = NULL, *postimage = NULL;
  	git_reader *pre_reader = NULL, *post_reader = NULL;
  	git_apply_options opts = GIT_APPLY_OPTIONS_INIT;
  	int error = GIT_EINVALID;
  
  	GIT_ASSERT_ARG(repo);
  	GIT_ASSERT_ARG(diff);
  
  	GIT_ERROR_CHECK_VERSION(
  		given_opts, GIT_APPLY_OPTIONS_VERSION, "git_apply_options");
  
  	if (given_opts)
  		memcpy(&opts, given_opts, sizeof(git_apply_options));
  
  	/*
  	 * by default, we apply a patch directly to the working directory;
  	 * in `--cached` or `--index` mode, we apply to the contents already
  	 * in the index.
  	 */
  	switch (location) {
  	case GIT_APPLY_LOCATION_BOTH:
  		error = git_reader_for_workdir(&pre_reader, repo, true);
  		break;
  	case GIT_APPLY_LOCATION_INDEX:
  		error = git_reader_for_index(&pre_reader, repo, NULL);
  		break;
  	case GIT_APPLY_LOCATION_WORKDIR:
  		error = git_reader_for_workdir(&pre_reader, repo, false);
  		break;
  	default:
  		GIT_ASSERT(false);
  	}
  
  	if (error < 0)
  		goto done;
  
  	/*
  	 * Build the preimage and postimage (differences).  Note that
  	 * this is not the complete preimage or postimage, it only
  	 * contains the files affected by the patch.  We want to avoid
  	 * having the full repo index, so we will limit our checkout
  	 * to only write these files that were affected by the diff.
  	 */
  	if ((error = git_index_new(&preimage)) < 0 ||
  	    (error = git_index_new(&postimage)) < 0 ||
  	    (error = git_reader_for_index(&post_reader, repo, postimage)) < 0)
  		goto done;
  
  	if (!(opts.flags & GIT_APPLY_CHECK))
  		if ((error = git_repository_index(&index, repo)) < 0 ||
  		    (error = git_indexwriter_init(&indexwriter, index)) < 0)
  			goto done;
  
  	if ((error = apply_deltas(repo, pre_reader, preimage, post_reader, postimage, diff, &opts)) < 0)
  		goto done;
  
  	if ((opts.flags & GIT_APPLY_CHECK))
  		goto done;
  
  	switch (location) {
  	case GIT_APPLY_LOCATION_BOTH:
  		error = git_apply__to_workdir(repo, diff, preimage, postimage, location, &opts);
  		break;
  	case GIT_APPLY_LOCATION_INDEX:
  		error = git_apply__to_index(repo, diff, preimage, postimage, &opts);
  		break;
  	case GIT_APPLY_LOCATION_WORKDIR:
  		error = git_apply__to_workdir(repo, diff, preimage, postimage, location, &opts);
  		break;
  	default:
  		GIT_ASSERT(false);
  	}
  
  	if (error < 0)
  		goto done;
  
  	error = git_indexwriter_commit(&indexwriter);
  
  done:
  	git_indexwriter_cleanup(&indexwriter);
  	git_index_free(postimage);
  	git_index_free(preimage);
  	git_index_free(index);
  	git_reader_free(pre_reader);
  	git_reader_free(post_reader);
  
  	return error;
  }
  

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