diff --git a/contrib/xz/src/liblzma/api/lzma/container.h b/contrib/xz/src/liblzma/api/lzma/container.h
index 2849fbfd3c51..4cbb1b7bf7b4 100644
--- a/contrib/xz/src/liblzma/api/lzma/container.h
+++ b/contrib/xz/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)
  *
diff --git a/contrib/xz/src/liblzma/common/filter_encoder.c b/contrib/xz/src/liblzma/common/filter_encoder.c
index 46fe8af1c153..0699bcee62a4 100644
--- a/contrib/xz/src/liblzma/common/filter_encoder.c
+++ b/contrib/xz/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;
 }
 
 
diff --git a/contrib/xz/src/liblzma/common/filter_encoder.h b/contrib/xz/src/liblzma/common/filter_encoder.h
index f1d5683fe793..da92be8b34da 100644
--- a/contrib/xz/src/liblzma/common/filter_encoder.h
+++ b/contrib/xz/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);
diff --git a/contrib/xz/src/liblzma/common/stream_encoder_mt.c b/contrib/xz/src/liblzma/common/stream_encoder_mt.c
index f64de9bdbc57..64de526b6a17 100644
--- a/contrib/xz/src/liblzma/common/stream_encoder_mt.c
+++ b/contrib/xz/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
diff --git a/contrib/xz/src/liblzma/liblzma_generic.map b/contrib/xz/src/liblzma/liblzma_generic.map
index bb82167ed57a..b251d366e15c 100644
--- a/contrib/xz/src/liblzma/liblzma_generic.map
+++ b/contrib/xz/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;
diff --git a/contrib/xz/src/liblzma/liblzma_linux.map b/contrib/xz/src/liblzma/liblzma_linux.map
index 449f5fd682db..25b393883693 100644
--- a/contrib/xz/src/liblzma/liblzma_linux.map
+++ b/contrib/xz/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;
diff --git a/contrib/xz/src/liblzma/lz/lz_encoder.c b/contrib/xz/src/liblzma/lz/lz_encoder.c
index 5489085a0860..8e724a035a13 100644
--- a/contrib/xz/src/liblzma/lz/lz_encoder.c
+++ b/contrib/xz/src/liblzma/lz/lz_encoder.c
@@ -196,9 +196,7 @@ lz_encoder_prepare(lzma_mf *mf, const lzma_allocator *allocator,
 	// For now, the dictionary size is limited to 1.5 GiB. This may grow
 	// in the future if needed, but it needs a little more work than just
 	// changing this check.
-	if (lz_options->dict_size < LZMA_DICT_SIZE_MIN
-			|| lz_options->dict_size
-				> (UINT32_C(1) << 30) + (UINT32_C(1) << 29)
+	if (!IS_ENC_DICT_SIZE_VALID(lz_options->dict_size)
 			|| lz_options->nice_len > lz_options->match_len_max)
 		return true;
 
diff --git a/contrib/xz/src/liblzma/lz/lz_encoder.h b/contrib/xz/src/liblzma/lz/lz_encoder.h
index ffcba02ce931..b71f11805e50 100644
--- a/contrib/xz/src/liblzma/lz/lz_encoder.h
+++ b/contrib/xz/src/liblzma/lz/lz_encoder.h
@@ -17,6 +17,14 @@
 #include "common.h"
 
 
+// For now, the dictionary size is limited to 1.5 GiB. This may grow
+// in the future if needed, but it needs a little more work than just
+// changing this check.
+#define IS_ENC_DICT_SIZE_VALID(size) \
+	((size) >= LZMA_DICT_SIZE_MIN \
+			&&  (size) <= (UINT32_C(1) << 30) + (UINT32_C(1) << 29))
+
+
 /// A table of these is used by the LZ-based encoder to hold
 /// the length-distance pairs found by the match finder.
 typedef struct {
diff --git a/contrib/xz/src/liblzma/lzma/lzma2_encoder.c b/contrib/xz/src/liblzma/lzma/lzma2_encoder.c
index 4b6b23118d70..5043a07e0fdf 100644
--- a/contrib/xz/src/liblzma/lzma/lzma2_encoder.c
+++ b/contrib/xz/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);
 }
diff --git a/lib/liblzma/Symbol.map b/lib/liblzma/Symbol.map
index 9532da0a5bde..938b6191b4f4 100644
--- a/lib/liblzma/Symbol.map
+++ b/lib/liblzma/Symbol.map
@@ -114,6 +114,10 @@ XZ_5.4 {
 	lzma_str_to_filters;
 };
 
+XZ_5.6 {
+	lzma_mt_block_size;
+};
+
 XZprivate_1.0 {
 	lzma_alloc;
 	lzma_alloc_zero;
@@ -173,7 +177,6 @@ XZprivate_1.0 {
 	lzma_mf_hc3_skip;
 	lzma_mf_hc4_find;
 	lzma_mf_hc4_skip;
-	lzma_mt_block_size;
 	lzma_next_end;
 	lzma_next_filter_init;
 	lzma_next_filter_update;
diff --git a/lib/liblzma/Versions.def b/lib/liblzma/Versions.def
index dea89514beda..25064d9d50e7 100644
--- a/lib/liblzma/Versions.def
+++ b/lib/liblzma/Versions.def
@@ -8,6 +8,9 @@ XZ_5.2 {
 XZ_5.4 {
 } XZ_5.2;
 
-XZprivate_1.0 {
+XZ_5.6 {
 } XZ_5.4;
 
+XZprivate_1.0 {
+} XZ_5.6;
+