Switch to reading content streams on the fly during interpretation.

Previously, before interpreting a pages content stream we would
load it entirely into a buffer. Then we would interpret that
buffer. This has a cost in memory use.

Here, we update the code to read from a stream on the fly.

This has required changes in various different parts of the code.

Firstly, we have removed all use of the FILE lock - as stream
reads can now safely be interrupted by resource (or object) reads
from elsewhere in the file, the file lock becomes a very hard
thing to maintain, and doesn't actually benefit us at all. The
choices were to either use a recursive lock, or to remove it
entirely; I opted for the latter.

The file lock enum value remains as a placeholder for future use in
extendable data streams.

Secondly, we add a new 'concat' filter that concatenates a series of
streams together into one, optionally putting whitespace between each
stream (as the pdf parser requires this).

Finally, we change page/xobject/pattern content streams to work
on the fly, but we leave type3 glyphs using buffers (as presumably
these will be run repeatedly).

11 files changed, 138 insertions, 184 deletions

diff --git a/pdf/base_object.c b/pdf/base_object.c
index 97cb97cc..2a5e6d78 100644
--- a/pdf/base_object.c
+++ b/pdf/base_object.c
@@ -145,8 +145,8 @@ pdf_new_indirect(fz_context *ctx, int num, int gen, void *xref)
 pdf_obj *
 pdf_keep_obj(pdf_obj *obj)
 {
-	assert(obj);
-	obj->refs ++;
+	if (obj)
+		obj->refs ++;
 	return obj;
 }
 
@@ -159,7 +159,6 @@ int pdf_is_indirect(pdf_obj *obj)
 	do { \
 		if (obj && obj->kind == PDF_INDIRECT) \
 		{\
-			fz_assert_lock_not_held(obj->ctx, FZ_LOCK_FILE); \
 			obj = pdf_resolve_indirect(obj); \
 		} \
 	} while (0)
diff --git a/pdf/mupdf-internal.h b/pdf/mupdf-internal.h
index 3e4a4729..3396bb8a 100644
--- a/pdf/mupdf-internal.h
+++ b/pdf/mupdf-internal.h
@@ -198,6 +198,7 @@ fz_buffer *pdf_load_image_stream(pdf_document *doc, int num, int gen, pdf_image_
 fz_stream *pdf_open_image_stream(pdf_document *doc, int num, int gen, pdf_image_params *params);
 fz_stream *pdf_open_stream_with_offset(pdf_document *doc, int num, int gen, pdf_obj *dict, int stm_ofs);
 fz_stream *pdf_open_image_decomp_stream(fz_context *ctx, fz_buffer *, pdf_image_params *params, int *factor);
+fz_stream *pdf_open_contents_stream(pdf_document *xref, pdf_obj *obj);
 
 void pdf_repair_xref(pdf_document *doc, pdf_lexbuf *buf);
 void pdf_repair_obj_stms(pdf_document *doc);
@@ -257,7 +258,7 @@ struct pdf_pattern_s
 	fz_matrix matrix;
 	fz_rect bbox;
 	pdf_obj *resources;
-	fz_buffer *contents;
+	pdf_obj *contents;
 };
 
 pdf_pattern *pdf_load_pattern(pdf_document *doc, pdf_obj *obj);
@@ -280,7 +281,7 @@ struct pdf_xobject_s
 	int transparency;
 	fz_colorspace *colorspace;
 	pdf_obj *resources;
-	fz_buffer *contents;
+	pdf_obj *contents;
 	pdf_obj *me;
 };
 
@@ -513,7 +514,7 @@ struct pdf_page_s
 	int rotate;
 	int transparency;
 	pdf_obj *resources;
-	fz_buffer *contents;
+	pdf_obj *contents;
 	fz_link *links;
 	pdf_annot *annots;
 };
diff --git a/pdf/pdf_interpret.c b/pdf/pdf_interpret.c
index a11b8c30..8851be6e 100644
--- a/pdf/pdf_interpret.c
+++ b/pdf/pdf_interpret.c
@@ -104,7 +104,7 @@ struct pdf_csi_s
 	fz_cookie *cookie;
 };
 
-static void pdf_run_buffer(pdf_csi *csi, pdf_obj *rdb, fz_buffer *contents);
+static void pdf_run_contents_object(pdf_csi *csi, pdf_obj *rdb, pdf_obj *contents);
 static void pdf_run_xobject(pdf_csi *csi, pdf_obj *resources, pdf_xobject *xobj, fz_matrix transform);
 static void pdf_show_pattern(pdf_csi *csi, pdf_pattern *pat, fz_rect area, int what);
 
@@ -1291,7 +1291,7 @@ pdf_show_pattern(pdf_csi *csi, pdf_pattern *pat, fz_rect area, int what)
 		gstate->ctm = ptm;
 		csi->top_ctm = gstate->ctm;
 		pdf_gsave(csi);
-		pdf_run_buffer(csi, pat->resources, pat->contents);
+		pdf_run_contents_object(csi, pat->resources, pat->contents);
 		/* RJW: "cannot render pattern tile" */
 		pdf_grestore(csi);
 		while (oldtop < csi->gtop)
@@ -1310,7 +1310,7 @@ pdf_show_pattern(pdf_csi *csi, pdf_pattern *pat, fz_rect area, int what)
 				pdf_gsave(csi);
 				fz_try(ctx)
 				{
-					pdf_run_buffer(csi, pat->resources, pat->contents);
+					pdf_run_contents_object(csi, pat->resources, pat->contents);
 				}
 				fz_catch(ctx)
 				{
@@ -1407,7 +1407,7 @@ pdf_run_xobject(pdf_csi *csi, pdf_obj *resources, pdf_xobject *xobj, fz_matrix t
 		if (xobj->resources)
 			resources = xobj->resources;
 
-		pdf_run_buffer(csi, resources, xobj->contents);
+		pdf_run_contents_object(csi, resources, xobj->contents);
 		/* RJW: "cannot interpret XObject stream" */
 	}
 	fz_always(ctx)
@@ -2528,7 +2528,6 @@ pdf_run_keyword(pdf_csi *csi, pdf_obj *rdb, fz_stream *file, char *buf)
 			fz_warn(ctx, "unknown keyword: '%s'", buf);
 		break;
 	}
-	fz_assert_lock_not_held(ctx, FZ_LOCK_FILE);
 }
 
 static void
@@ -2662,44 +2661,78 @@ pdf_run_stream(pdf_csi *csi, pdf_obj *rdb, fz_stream *file, pdf_lexbuf *buf)
  */
 
 static void
-pdf_run_buffer(pdf_csi *csi, pdf_obj *rdb, fz_buffer *contents)
+pdf_run_contents_stream(pdf_csi *csi, pdf_obj *rdb, fz_stream *file)
 {
 	fz_context *ctx = csi->dev->ctx;
 	pdf_lexbuf_large *buf;
-	fz_stream * file = NULL;
 	int save_in_text;
 
 	fz_var(buf);
-	fz_var(file);
+
+	if (file == NULL)
+		return;
+
+	buf = fz_malloc(ctx, sizeof(*buf)); /* we must be re-entrant for type3 fonts */
+	buf->base.size = PDF_LEXBUF_LARGE;
+	save_in_text = csi->in_text;
+	csi->in_text = 0;
+	fz_try(ctx)
+	{
+		pdf_run_stream(csi, rdb, file, &buf->base);
+	}
+	fz_catch(ctx)
+	{
+		fz_warn(ctx, "Content stream parsing error - rendering truncated");
+	}
+	csi->in_text = save_in_text;
+	fz_free(ctx, buf);
+}
+
+static void
+pdf_run_contents_object(pdf_csi *csi, pdf_obj *rdb, pdf_obj *contents)
+{
+	fz_context *ctx = csi->dev->ctx;
+	fz_stream *file = NULL;
 
 	if (contents == NULL)
 		return;
 
+	file = pdf_open_contents_stream(csi->xref, contents);
 	fz_try(ctx)
 	{
-		buf = fz_malloc(ctx, sizeof(*buf)); /* we must be re-entrant for type3 fonts */
-		buf->base.size = PDF_LEXBUF_LARGE;
-		file = fz_open_buffer(ctx, contents);
-		save_in_text = csi->in_text;
-		csi->in_text = 0;
-		fz_try(ctx)
-		{
-			pdf_run_stream(csi, rdb, file, &buf->base);
-		}
-		fz_catch(ctx)
-		{
-			fz_warn(ctx, "Content stream parsing error - rendering truncated");
-		}
-		csi->in_text = save_in_text;
+		pdf_run_contents_stream(csi, rdb, file);
 	}
 	fz_always(ctx)
 	{
 		fz_close(file);
-		fz_free(ctx, buf);
 	}
 	fz_catch(ctx)
 	{
-		fz_throw(ctx, "cannot parse context stream");
+		fz_rethrow(ctx);
+	}
+}
+
+static void
+pdf_run_contents_buffer(pdf_csi *csi, pdf_obj *rdb, fz_buffer *contents)
+{
+	fz_context *ctx = csi->dev->ctx;
+	fz_stream *file = NULL;
+
+	if (contents == NULL)
+		return;
+
+	file = fz_open_buffer(ctx, contents);
+	fz_try(ctx)
+	{
+		pdf_run_contents_stream(csi, rdb, file);
+	}
+	fz_always(ctx)
+	{
+		fz_close(file);
+	}
+	fz_catch(ctx)
+	{
+		fz_rethrow(ctx);
 	}
 }
 
@@ -2719,14 +2752,16 @@ pdf_run_page_with_usage(pdf_document *xref, pdf_page *page, fz_device *dev, fz_m
 	csi = pdf_new_csi(xref, dev, ctm, event, cookie, NULL);
 	fz_try(ctx)
 	{
-		pdf_run_buffer(csi, page->resources, page->contents);
+		pdf_run_contents_object(csi, page->resources, page->contents);
 	}
-	fz_catch(ctx)
+	fz_always(ctx)
 	{
 		pdf_free_csi(csi);
+	}
+	fz_catch(ctx)
+	{
 		fz_throw(ctx, "cannot parse page content stream");
 	}
-	pdf_free_csi(csi);
 
 	if (cookie && cookie->progress_max != -1)
 	{
@@ -2792,7 +2827,7 @@ pdf_run_glyph(pdf_document *xref, pdf_obj *resources, fz_buffer *contents, fz_de
 
 	fz_try(ctx)
 	{
-		pdf_run_buffer(csi, resources, contents);
+		pdf_run_contents_buffer(csi, resources, contents);
 	}
 	fz_catch(ctx)
 	{
diff --git a/pdf/pdf_outline.c b/pdf/pdf_outline.c
index d4bea75a..e88d613d 100644
--- a/pdf/pdf_outline.c
+++ b/pdf/pdf_outline.c
@@ -65,7 +65,7 @@ pdf_load_outline(pdf_document *xref)
 	obj = pdf_dict_gets(root, "Outlines");
 	first = pdf_dict_gets(obj, "First");
 	if (first)
-		return pdf_load_outline_imp(xref, first);
+		return  pdf_load_outline_imp(xref, first);
 
 	return NULL;
 }
diff --git a/pdf/pdf_page.c b/pdf/pdf_page.c
index f5fbc0b0..42e830da 100644
--- a/pdf/pdf_page.c
+++ b/pdf/pdf_page.c
@@ -281,72 +281,6 @@ found:
 	return useBM;
 }
 
-/* we need to combine all sub-streams into one for the content stream interpreter */
-
-static fz_buffer *
-pdf_load_page_contents_array(pdf_document *xref, pdf_obj *list)
-{
-	fz_buffer *big;
-	fz_buffer *one;
-	int i, n;
-	fz_context *ctx = xref->ctx;
-
-	big = fz_new_buffer(ctx, 32 * 1024);
-
-	n = pdf_array_len(list);
-	fz_var(i); /* Workaround Mac compiler bug */
-	for (i = 0; i < n; i++)
-	{
-		pdf_obj *stm = pdf_array_get(list, i);
-		fz_try(ctx)
-		{
-			one = pdf_load_stream(xref, pdf_to_num(stm), pdf_to_gen(stm));
-		}
-		fz_catch(ctx)
-		{
-			fz_warn(ctx, "cannot load content stream part %d/%d", i + 1, n);
-			continue;
-		}
-
-		if (big->len + one->len + 1 > big->cap)
-			fz_resize_buffer(ctx, big, big->len + one->len + 1);
-		memcpy(big->data + big->len, one->data, one->len);
-		big->data[big->len + one->len] = ' ';
-		big->len += one->len + 1;
-
-		fz_drop_buffer(ctx, one);
-	}
-
-	if (n > 0 && big->len == 0)
-	{
-		fz_drop_buffer(ctx, big);
-		fz_throw(ctx, "cannot load content stream");
-	}
-	fz_trim_buffer(ctx, big);
-
-	return big;
-}
-
-static fz_buffer *
-pdf_load_page_contents(pdf_document *xref, pdf_obj *obj)
-{
-	fz_context *ctx = xref->ctx;
-
-	if (pdf_is_array(obj))
-	{
-		return pdf_load_page_contents_array(xref, obj);
-		/* RJW: "cannot load content stream array" */
-	}
-	else if (pdf_is_stream(xref, pdf_to_num(obj), pdf_to_gen(obj)))
-	{
-		return pdf_load_stream(xref, pdf_to_num(obj), pdf_to_gen(obj));
-		/* RJW: "cannot load content stream (%d 0 R)", pdf_to_num(obj) */
-	}
-
-	fz_warn(ctx, "page contents missing, leaving page blank");
-	return fz_new_buffer(ctx, 0);
-}
-
 pdf_page *
 pdf_load_page(pdf_document *xref, int number)
 {
@@ -424,7 +358,7 @@ pdf_load_page(pdf_document *xref, int number)
 	obj = pdf_dict_gets(pageobj, "Contents");
 	fz_try(ctx)
 	{
-		page->contents = pdf_load_page_contents(xref, obj);
+		page->contents = pdf_keep_obj(obj);
 
 		if (pdf_resources_use_blending(ctx, page->resources))
 			page->transparency = 1;
@@ -464,7 +398,7 @@ pdf_free_page(pdf_document *xref, pdf_page *page)
 	if (page->resources)
 		pdf_drop_obj(page->resources);
 	if (page->contents)
-		fz_drop_buffer(xref->ctx, page->contents);
+		pdf_drop_obj(page->contents);
 	if (page->links)
 		fz_drop_link(xref->ctx, page->links);
 	if (page->annots)
diff --git a/pdf/pdf_pattern.c b/pdf/pdf_pattern.c
index 14175670..af96c2d5 100644
--- a/pdf/pdf_pattern.c
+++ b/pdf/pdf_pattern.c
@@ -21,7 +21,7 @@ pdf_free_pattern_imp(fz_context *ctx, fz_storable *pat_)
 	if (pat->resources)
 		pdf_drop_obj(pat->resources);
 	if (pat->contents)
-		fz_drop_buffer(ctx, pat->contents);
+		pdf_drop_obj(pat->contents);
 	fz_free(ctx, pat);
 }
 
@@ -30,7 +30,7 @@ pdf_pattern_size(pdf_pattern *pat)
 {
 	if (pat == NULL)
 		return 0;
-	return sizeof(*pat) + (pat->contents ? pat->contents->cap : 0);
+	return sizeof(*pat);
 }
 
 pdf_pattern *
@@ -72,7 +72,7 @@ pdf_load_pattern(pdf_document *xref, pdf_obj *dict)
 
 	fz_try(ctx)
 	{
-		pat->contents = pdf_load_stream(xref, pdf_to_num(dict), pdf_to_gen(dict));
+		pat->contents = pdf_keep_obj(dict);
 	}
 	fz_catch(ctx)
 	{
diff --git a/pdf/pdf_repair.c b/pdf/pdf_repair.c
index a51b9631..27846855 100644
--- a/pdf/pdf_repair.c
+++ b/pdf/pdf_repair.c
@@ -195,6 +195,7 @@ pdf_repair_obj_stm(pdf_document *xref, int num, int gen)
 	}
 }
 
+/* Entered with file locked, remains locked throughout. */
 void
 pdf_repair_xref(pdf_document *xref, pdf_lexbuf *buf)
 {
@@ -389,19 +390,7 @@ pdf_repair_xref(pdf_document *xref, pdf_lexbuf *buf)
 			/* corrected stream length */
 			if (list[i].stm_len >= 0)
 			{
-				fz_unlock(ctx, FZ_LOCK_FILE);
-				fz_try(ctx)
-				{
-					dict = pdf_load_object(xref, list[i].num, list[i].gen);
-				}
-				fz_always(ctx)
-				{
-					fz_lock(ctx, FZ_LOCK_FILE);
-				}
-				fz_catch(ctx)
-				{
-					fz_rethrow(ctx);
-				}
+				dict = pdf_load_object(xref, list[i].num, list[i].gen);
 				/* RJW: "cannot load stream object (%d %d R)", list[i].num, list[i].gen */
 
 				length = pdf_new_int(ctx, list[i].stm_len);
diff --git a/pdf/pdf_stream.c b/pdf/pdf_stream.c
index 5338d81c..3086fbc9 100644
--- a/pdf/pdf_stream.c
+++ b/pdf/pdf_stream.c
@@ -273,7 +273,6 @@ pdf_open_filter(fz_stream *chain, pdf_document *xref, pdf_obj *stmobj, int num,
 	else if (pdf_array_len(filters) > 0)
 		chain = build_filter_chain(chain, xref, filters, params, num, gen, imparams);
 
-	fz_lock_stream(chain);
 	return chain;
 }
 
@@ -309,7 +308,6 @@ fz_stream *
 pdf_open_raw_stream(pdf_document *xref, int num, int gen)
 {
 	pdf_xref_entry *x;
-	fz_stream *stm;
 
 	fz_var(x);
 
@@ -324,9 +322,7 @@ pdf_open_raw_stream(pdf_document *xref, int num, int gen)
 	if (x->stm_ofs == 0)
 		fz_throw(xref->ctx, "object is not a stream");
 
-	stm = pdf_open_raw_filter(xref->file, xref, x->obj, num, gen, x->stm_ofs);
-	fz_lock_stream(stm);
-	return stm;
+	return pdf_open_raw_filter(xref->file, xref, x->obj, num, gen, x->stm_ofs);
 }
 
 /*
@@ -506,3 +502,49 @@ pdf_load_image_stream(pdf_document *xref, int num, int gen, pdf_image_params *pa
 
 	return buf;
 }
+
+static fz_stream *
+pdf_open_object_array(pdf_document *xref, pdf_obj *list)
+{
+	int i, n;
+	fz_context *ctx = xref->ctx;
+	fz_stream *stm;
+
+	n = pdf_array_len(list);
+	stm = fz_open_concat(ctx, n, 1);
+
+	fz_var(i); /* Workaround Mac compiler bug */
+	for (i = 0; i < n; i++)
+	{
+		pdf_obj *obj = pdf_array_get(list, i);
+		fz_try(ctx)
+		{
+			fz_concat_push(stm, pdf_open_stream(xref, pdf_to_num(obj), pdf_to_gen(obj)));
+		}
+		fz_catch(ctx)
+		{
+			fz_warn(ctx, "cannot load content stream part %d/%d", i + 1, n);
+			continue;
+		}
+	}
+
+	return stm;
+}
+
+fz_stream *
+pdf_open_contents_stream(pdf_document *xref, pdf_obj *obj)
+{
+	fz_context *ctx = xref->ctx;
+
+	if (pdf_is_array(obj))
+	{
+		return pdf_open_object_array(xref, obj);
+	}
+	else if (pdf_is_stream(xref, pdf_to_num(obj), pdf_to_gen(obj)))
+	{
+		return pdf_open_image_stream(xref, pdf_to_num(obj), pdf_to_gen(obj), NULL);
+	}
+
+	fz_warn(ctx, "pdf object stream missing (%d %d R)", pdf_to_num(obj), pdf_to_gen(obj));
+	return NULL;
+}
diff --git a/pdf/pdf_type3.c b/pdf/pdf_type3.c
index 6603fc8b..4df46ce3 100644
--- a/pdf/pdf_type3.c
+++ b/pdf/pdf_type3.c
@@ -2,10 +2,9 @@
 #include "mupdf-internal.h"
 
 static void
-pdf_run_glyph_func(void *doc, void *rdb_, fz_buffer *contents, fz_device *dev, fz_matrix ctm, void *gstate)
+pdf_run_glyph_func(void *doc, void *rdb, fz_buffer *contents, fz_device *dev, fz_matrix ctm, void *gstate)
 {
-	pdf_obj *rdb = (pdf_obj *)rdb_;
-	pdf_run_glyph(doc, rdb, contents, dev, ctm, gstate);
+	pdf_run_glyph(doc, (pdf_obj *)rdb, contents, dev, ctm, gstate);
 }
 
 static void
diff --git a/pdf/pdf_xobject.c b/pdf/pdf_xobject.c
index ffa86184..1de0b20e 100644
--- a/pdf/pdf_xobject.c
+++ b/pdf/pdf_xobject.c
@@ -23,7 +23,8 @@ pdf_free_xobject_imp(fz_context *ctx, fz_storable *xobj_)
 	if (xobj->resources)
 		pdf_drop_obj(xobj->resources);
 	if (xobj->contents)
-		fz_drop_buffer(ctx, xobj->contents);
+		//fz_drop_buffer(ctx, xobj->contents);
+		pdf_drop_obj(xobj->contents);
 	pdf_drop_obj(xobj->me);
 	fz_free(ctx, xobj);
 }
@@ -33,7 +34,7 @@ pdf_xobject_size(pdf_xobject *xobj)
 {
 	if (xobj == NULL)
 		return 0;
-	return sizeof(*xobj) + (xobj->colorspace ? xobj->colorspace->size : 0) + (xobj->contents ? xobj->contents->len : 0);
+	return sizeof(*xobj) + (xobj->colorspace ? xobj->colorspace->size : 0);
 }
 
 pdf_xobject *
@@ -98,7 +99,7 @@ pdf_load_xobject(pdf_document *xref, pdf_obj *dict)
 
 	fz_try(ctx)
 	{
-		form->contents = pdf_load_stream(xref, pdf_to_num(dict), pdf_to_gen(dict));
+		form->contents = pdf_keep_obj(dict);
 	}
 	fz_catch(ctx)
 	{
diff --git a/pdf/pdf_xref.c b/pdf/pdf_xref.c
index fb8adc7a..3af2901c 100644
--- a/pdf/pdf_xref.c
+++ b/pdf/pdf_xref.c
@@ -305,8 +305,7 @@ pdf_read_new_xref_section(pdf_document *xref, fz_stream *stm, int i0, int i1, in
 	}
 }
 
-/* Entered with file locked. Drops the lock in the middle, but then picks
- * it up again before exiting. */
+/* Entered with file locked, remains locked throughout. */
 static pdf_obj *
 pdf_read_new_xref(pdf_document *xref, pdf_lexbuf *buf)
 {
@@ -333,7 +332,6 @@ pdf_read_new_xref(pdf_document *xref, pdf_lexbuf *buf)
 
 	fz_try(ctx)
 	{
-		fz_unlock(ctx, FZ_LOCK_FILE);
 		obj = pdf_dict_gets(trailer, "Size");
 		if (!obj)
 			fz_throw(ctx, "xref stream missing Size entry (%d %d R)", num, gen);
@@ -383,7 +381,6 @@ pdf_read_new_xref(pdf_document *xref, pdf_lexbuf *buf)
 		pdf_drop_obj(index);
 		fz_rethrow(ctx);
 	}
-	fz_lock(ctx, FZ_LOCK_FILE);
 
 	return trailer;
 }
@@ -436,22 +433,9 @@ pdf_read_xref_sections(pdf_document *xref, int ofs, pdf_lexbuf *buf)
 		{
 			trailer = pdf_read_xref(xref, ofs, buf);
 
-			/* Unlock file in case XRefStm or Prev are indirect. */
-			fz_unlock(ctx, FZ_LOCK_FILE);
-			fz_try(ctx)
-			{
-				/* FIXME: do we overwrite free entries properly? */
-				xrefstmofs = pdf_to_int(pdf_dict_gets(trailer, "XRefStm"));
-				prevofs = pdf_to_int(pdf_dict_gets(trailer, "Prev"));
-			}
-			fz_always(ctx)
-			{
-				fz_lock(ctx, FZ_LOCK_FILE);
-			}
-			fz_catch(ctx)
-			{
-				fz_rethrow(ctx);
-			}
+			/* FIXME: do we overwrite free entries properly? */
+			xrefstmofs = pdf_to_int(pdf_dict_gets(trailer, "XRefStm"));
+			prevofs = pdf_to_int(pdf_dict_gets(trailer, "Prev"));
 
 			/* We only recurse if we have both xrefstm and prev.
 			 * Hopefully this happens infrequently. */
@@ -476,7 +460,7 @@ pdf_read_xref_sections(pdf_document *xref, int ofs, pdf_lexbuf *buf)
 /*
  * load xref tables from pdf
  *
- * File locked on entry and exit; lock may be dropped in the middle.
+ * File locked on entry, throughout and on exit.
  */
 
 static void
@@ -492,22 +476,9 @@ pdf_load_xref(pdf_document *xref, pdf_lexbuf *buf)
 
 	pdf_read_trailer(xref, buf);
 
-	/* Unlock (and relock) in case Size is indirect. */
-	fz_unlock(ctx, FZ_LOCK_FILE);
-	fz_try(ctx)
-	{
-		size = pdf_to_int(pdf_dict_gets(xref->trailer, "Size"));
-		if (!size)
-			fz_throw(ctx, "trailer missing Size entry");
-	}
-	fz_always(ctx)
-	{
-		fz_lock(ctx, FZ_LOCK_FILE);
-	}
-	fz_catch(ctx)
-	{
-		fz_rethrow(ctx);
-	}
+	size = pdf_to_int(pdf_dict_gets(xref->trailer, "Size"));
+	if (!size)
+		fz_throw(ctx, "trailer missing Size entry");
 
 	pdf_resize_xref(xref, size);
 
@@ -712,12 +683,10 @@ pdf_open_document_with_stream(fz_stream *file)
 	pdf_obj *obj;
 	pdf_obj *nobj = NULL;
 	int i, repaired = 0;
-	int locked;
 	fz_context *ctx = file->ctx;
 
 	fz_var(dict);
 	fz_var(nobj);
-	fz_var(locked);
 
 	xref = fz_malloc_struct(ctx, pdf_document);
 	pdf_init_document(xref);
@@ -726,9 +695,6 @@ pdf_open_document_with_stream(fz_stream *file)
 	xref->file = fz_keep_stream(file);
 	xref->ctx = ctx;
 
-	fz_lock(ctx, FZ_LOCK_FILE);
-	locked = 1;
-
 	fz_try(ctx)
 	{
 		pdf_load_xref(xref, &xref->lexbuf.base);
@@ -757,9 +723,6 @@ pdf_open_document_with_stream(fz_stream *file)
 		if (repaired)
 			pdf_repair_xref(xref, &xref->lexbuf.base);
 
-		fz_unlock(ctx, FZ_LOCK_FILE);
-		locked = 0;
-
 		encrypt = pdf_dict_gets(xref->trailer, "Encrypt");
 		id = pdf_dict_gets(xref->trailer, "ID");
 		if (pdf_is_dict(encrypt))
@@ -818,11 +781,6 @@ pdf_open_document_with_stream(fz_stream *file)
 			}
 		}
 	}
-	fz_always(ctx)
-	{
-		if (locked)
-			fz_unlock(ctx, FZ_LOCK_FILE);
-	}
 	fz_catch(ctx)
 	{
 		pdf_drop_obj(dict);
@@ -1023,7 +981,6 @@ pdf_cache_object(pdf_document *xref, int num, int gen)
 	}
 	else if (x->type == 'n')
 	{
-		fz_lock(ctx, FZ_LOCK_FILE);
 		fz_seek(xref->file, x->ofs, 0);
 
 		fz_try(ctx)
@@ -1033,7 +990,6 @@ pdf_cache_object(pdf_document *xref, int num, int gen)
 		}
 		fz_catch(ctx)
 		{
-			fz_unlock(ctx, FZ_LOCK_FILE);
 			fz_throw(ctx, "cannot parse object (%d %d R)", num, gen);
 		}
 
@@ -1041,13 +997,11 @@ pdf_cache_object(pdf_document *xref, int num, int gen)
 		{
 			pdf_drop_obj(x->obj);
 			x->obj = NULL;
-			fz_unlock(ctx, FZ_LOCK_FILE);
 			fz_throw(ctx, "found object (%d %d R) instead of (%d %d R)", rnum, rgen, num, gen);
 		}
 
 		if (xref->crypt)
 			pdf_crypt_obj(ctx, xref->crypt, x->obj, num, gen);
-		fz_unlock(ctx, FZ_LOCK_FILE);
 	}
 	else if (x->type == 'o')
 	{