Stop emitting references to inexistant #n= forms in the pretty printer

 * Special logic was introduced in 2003 to avoid pprinting backquote-comma
   forms as ", foo" when ",foo" is unambiguous; the bug has likely been
   around since then.

 * Reported by Douglas Katzman on launchpad, and reduced by James M. Lawrence
   (lp#1161218)

diff --git a/NEWS b/NEWS
index 0357268..0450bf6 100644 (file)
--- a/NEWS
+++ b/NEWS
@@ -2,6 +2,8 @@
 changes relative to sbcl-1.1.6
   * bug fix: svref/(setf svref) on symbol macros don't crash the compiler
     anymore. (Minimal test case provided by James M. Lawrence on sbcl-devel)
+  * bug fix: no more bogus ## references when pretty printing backquoted
+    forms with non-trivial structure sharing. (lp#1161218)
 
 changes in sbcl-1.1.6 relative to sbcl-1.1.5:
   * enhancement: the continuable error when defknown-ing over extant 

diff --git a/src/code/pp-backq.lisp b/src/code/pp-backq.lisp
index c1268d0..fd1fb13 100644 (file)
--- a/src/code/pp-backq.lisp
+++ b/src/code/pp-backq.lisp
@@ -96,14 +96,24 @@
   ;; stream, possibly with a space prepended.  However, this doesn't
   ;; work for pretty streams which need to do margin calculations.  Oh
   ;; well.  It was good while it lasted.  -- CSR, 2003-12-15
-  (let ((output (with-output-to-string (s)
-                  (write (cadr form) :stream s))))
-    (unless (= (length output) 0)
-      (when (and (eql (car form) 'backq-comma)
+  ;;
+  ;; This is an evil hack. If we print to a string and then print again,
+  ;; the circularity detection logic behaves as though it's already
+  ;; printed that data... and it has, to a string stream that we send
+  ;; to the bitbucket in the sky.  -- PK, 2013-03-30
+  (when (eql (car form) 'backq-comma)
+    (let ((output (with-output-to-string (s)
+                    ;; Patching evil with more evil.  The next step is
+                    ;; likely to stop the madness and unconditionally
+                    ;; insert a space.
+                    (let (*circularity-hash-table*
+                          *circularity-counter*)
+                      (write (cadr form) :stream s)))))
+      (when (and (plusp (length output))
                  (or (char= (char output 0) #\.)
                      (char= (char output 0) #\@)))
-        (write-char #\Space stream))
-      (write (cadr form) :stream stream))))
+        (write-char #\Space stream))))
+  (write (cadr form) :stream stream))
 
 ;;; This is called by !PPRINT-COLD-INIT, fairly late, because
 ;;; SET-PPRINT-DISPATCH doesn't work until the compiler works.

diff --git a/tests/pprint.impure.lisp b/tests/pprint.impure.lisp
index 4e6d8a3..39884e0 100644 (file)
--- a/tests/pprint.impure.lisp
+++ b/tests/pprint.impure.lisp
@@ -291,5 +291,11 @@
     (when errors
       (error "Can't PPRINT imporper lists: ~a" errors))))
 
+(with-test (:name :pprint-circular-backq-comma)
+  ;; LP 1161218 reported by James M. Lawrence
+  (let ((string (write-to-string '(let ((#1=#:var '(99)))
+                                   `(progn ,@(identity #1#)))
+                                 :circle t :pretty t)))
+    (assert (not (search "#2#" string)))))
 \f
 ;;; success