If one of the blocks which are subordinate to a @(trailer)

happen to request a successful termination by invoking @(accept)
the position must not advance into the trailer material.

* match.c (v_trailer): Added an unwind protect which
detects that an accept is taking place and adjusts the return value to
restrict the input position at the point given to trailer.
(accept_fail): Use uw_block_return_proto instead of uw_block_return
and pass the symbol as the protocol identifier.

* unwind.c (uw_current_exit_point): New function.
(uw_block_return): Function renamed to uw_block_return_proto;
takes new parameter which is stored in the block structure.

* unwind.h (struct uw_block): New member, protocol.
(uw_block_return): Becomes an inline wrapper for uw_block_return_proto.
(uw_block_return_proto, uw_current_exit_point): Declared.

* txr.1: Interaction between @(trailer) and @(accept) documented.

1 files changed, 30 insertions, 0 deletions

diff --git a/txr.1 b/txr.1
index 533bc464..89792af8 100644
--- a/txr.1
+++ b/txr.1
@@ -3040,6 +3040,36 @@ The second clause grabs four lines, which is the longest match.
 And so, the next line of input available for matching is 5, which goes
 to the @second variable.
 
+.SS Interaction between Trailer and Accept Directives
+
+If one of the clauses which follow a @(trailer) request a successful
+termination to an outer block via @(accept), then @(trailer) intercepts
+the transfer and adjusts the data extent to the position that it was given.
+
+Example:
+
+  Query:        @(block)
+                @(trailer)
+                @line1
+                @line2
+                @(accept)
+                @(end)
+                @line3
+
+  Data:         1
+                2
+                3
+
+  Output:       line1="1"
+                line2="2"
+                line3="1"
+
+The variable line3 is bound to 1 because although the @(accept) yields a data
+position which is advanced to the third line, this is intercepted by @(trailer)
+and adjusted back to the first line.
+
+Directives other than @(trailer) have no such special interaction with accept.
+
 .SH FUNCTIONS
 
 .SS Introduction