liblzma: Exports lzma_mt_block_size() as an API function.

The lzma_mt_block_size() was previously just an internal function for
the multithreaded .xz encoder. It is used to provide a recommended Block
size for a given filter chain.

This function is helpful to determine the maximum Block size for the
multithreaded .xz encoder when one wants to change the filters between
blocks. Then, this determined Block size can be provided to
lzma_stream_encoder_mt() in the lzma_mt options parameter when
intializing the coder. This requires one to know all the filter chains
they are using before starting to encode (or at least the filter chain
that will need the largest Block size), but that isn't a bad limitation.

src/liblzma/api/lzma/container.h

@@ -435,6 +435,34 @@ extern LZMA_API(lzma_ret) lzma_stream_encoder_mt(
 		lzma_nothrow lzma_attr_warn_unused_result;
 
 
+/**
+ * \brief       Calculate recommended Block size for multithreaded .xz encoder
+ *
+ * This calculates a recommended Block size for multithreaded encoding given
+ * a filter chain. This is used internally by lzma_stream_encoder_mt() to
+ * determine the Block size if the block_size member is not set to the
+ * special value of 0 in the lzma_mt options struct.
+ *
+ * If one wishes to change the filters between Blocks, this function is
+ * helpful to set the block_size member of the lzma_mt struct before calling
+ * lzma_stream_encoder_mt(). Since the block_size member represents the
+ * maximum possible Block size for the multithreaded .xz encoder, one can
+ * use this function to find the maximum recommended Block size based on
+ * all planned filter chains. Otherwise, the multithreaded encoder will
+ * base its maximum Block size on the first filter chain used (if the
+ * block_size member is not set), which may unnecessarily limit the Block
+ * size for a later filter chain.
+ *
+ * \param       filters   Array of filters terminated with
+ *                        .id == LZMA_VLI_UNKNOWN.
+ *
+ * \return      Recommended Block size in bytes, or UINT64_MAX if
+ *              an error occurred.
+ */
+extern LZMA_API(uint64_t) lzma_mt_block_size(const lzma_filter *filters)
+		lzma_nothrow;
+
+
 /**
  * \brief       Initialize .lzma encoder (legacy file format)
  *

src/liblzma/common/filter_encoder.c

@@ -33,7 +33,8 @@ typedef struct {
 	/// Calculates the recommended Uncompressed Size for .xz Blocks to
 	/// which the input data can be split to make multithreaded
 	/// encoding possible. If this is NULL, it is assumed that
-	/// the encoder is fast enough with single thread.
+	/// the encoder is fast enough with single thread. If the options
+	/// are invalid, UINT64_MAX is returned.
 	uint64_t (*block_size)(const void *options);
 
 	/// Tells the size of the Filter Properties field. If options are
@@ -248,26 +249,29 @@ lzma_raw_encoder_memusage(const lzma_filter *filters)
 }
 
 
-extern uint64_t
+extern LZMA_API(uint64_t)
 lzma_mt_block_size(const lzma_filter *filters)
 {
+	if (filters == NULL)
+		return UINT64_MAX;
+
 	uint64_t max = 0;
 
 	for (size_t i = 0; filters[i].id != LZMA_VLI_UNKNOWN; ++i) {
 		const lzma_filter_encoder *const fe
 				= encoder_find(filters[i].id);
+		if (fe == NULL)
+			return UINT64_MAX;
+
 		if (fe->block_size != NULL) {
 			const uint64_t size
 					= fe->block_size(filters[i].options);
-			if (size == 0)
-				return 0;
-
 			if (size > max)
 				max = size;
 		}
 	}
 
-	return max;
+	return max == 0 ? UINT64_MAX : max;
 }
 
 

src/liblzma/common/filter_encoder.h

@@ -1,6 +1,6 @@
 ///////////////////////////////////////////////////////////////////////////////
 //
-/// \file       filter_encoder.c
+/// \file       filter_encoder.h
 /// \brief      Filter ID mapping to filter-specific functions
 //
 //  Author:     Lasse Collin
@@ -16,10 +16,6 @@
 #include "common.h"
 
 
-// FIXME: Might become a part of the public API.
-extern uint64_t lzma_mt_block_size(const lzma_filter *filters);
-
-
 extern lzma_ret lzma_raw_encoder_init(
 		lzma_next_coder *next, const lzma_allocator *allocator,
 		const lzma_filter *filters);

src/liblzma/common/stream_encoder_mt.c

@@ -979,20 +979,18 @@ get_options(const lzma_mt *options, lzma_options_easy *opt_easy,
 		*filters = opt_easy->filters;
 	}
 
-	// Block size
-	if (options->block_size > 0) {
-		if (options->block_size > BLOCK_SIZE_MAX)
-			return LZMA_OPTIONS_ERROR;
-
+	// If the Block size is not set, determine it from the filter chain.
+	if (options->block_size > 0)
 		*block_size = options->block_size;
-	} else {
-		// Determine the Block size from the filter chain.
+	else
 		*block_size = lzma_mt_block_size(*filters);
-		if (*block_size == 0)
-			return LZMA_OPTIONS_ERROR;
 
-		assert(*block_size <= BLOCK_SIZE_MAX);
-	}
+	// UINT64_MAX > BLOCK_SIZE_MAX, so the second condition
+	// should be optimized out by any reasonable compiler.
+	// The second condition should be there in the unlikely event that
+	// the macros change and UINT64_MAX < BLOCK_SIZE_MAX.
+	if (*block_size > BLOCK_SIZE_MAX || *block_size == UINT64_MAX)
+		return LZMA_OPTIONS_ERROR;
 
 	// Calculate the maximum amount output that a single output buffer
 	// may need to hold. This is the same as the maximum total size of

src/liblzma/liblzma_generic.map

@@ -119,3 +119,8 @@ global:
 	lzma_str_list_filters;
 	lzma_str_to_filters;
 } XZ_5.2;
+
+XZ_5.5.0alpha {
+global:
+	lzma_mt_block_size;
+} XZ_5.4;

src/liblzma/liblzma_linux.map

@@ -134,3 +134,8 @@ global:
 	lzma_str_list_filters;
 	lzma_str_to_filters;
 } XZ_5.2;
+
+XZ_5.5.0alpha {
+global:
+	lzma_mt_block_size;
+} XZ_5.4;

src/liblzma/lzma/lzma2_encoder.c

@@ -409,6 +409,9 @@ lzma_lzma2_block_size(const void *options)
 {
 	const lzma_options_lzma *const opt = options;
 
+	if (!IS_ENC_DICT_SIZE_VALID(opt->dict_size))
+		return UINT64_MAX;
+
 	// Use at least 1 MiB to keep compression ratio better.
 	return my_max((uint64_t)(opt->dict_size) * 3, UINT64_C(1) << 20);
 }