Change the handling of non-anchored global substitutions of the empty

string from a silent implicit non-global substitution to a non-silent
explicit fatal error. Archored substitutions are those containing '^'
or '$'.
The problem with changing the substitution to prevent an infinite
number of matches is that it doesn't provide the necessary feedback
to the user that there's a bug in the/a makefile. Reporting the bug
without making the condition fatal makes the feedback mostly useless
due to the way that make fails to prefix the error with program name,
makefile file name and line number information.
Note that global substitutions of the empty string anchored with '^'
(start of string) or '$' (end of string) do not cause an infinite
number of matches and are therefore not reported and hence are non-
fatal.

Suggested by: bde
Tested with: buildworld

1 files changed, 9 insertions, 8 deletions

diff --git a/usr.bin/make/var.c b/usr.bin/make/var.c
index 32da4e8..3221bdd 100644
--- a/usr.bin/make/var.c
+++ b/usr.bin/make/var.c
@@ -1350,15 +1350,16 @@ Var_Parse(char *str, GNode *ctxt, Boolean err, int *lengthPtr, Boolean *freePtr)
 		    }
 
 		    /*
-		     * Replacing the empty string for something else when
-		     * done globally causes an infinite loop. The only
-		     * meaningful substitution of the empty string would
-		     * be those anchored by '^' or '$'. Thus, we can
-		     * safely turn the substitution into a non-global one
-		     * if the LHS is the empty string.
+		     * Global substitution of the empty string causes an
+		     * infinite number of matches, unless anchored by '^'
+		     * (start of string) or '$' (end of string). Catch the
+		     * infinite substitution here.
+		     * Note that flags can only contain the 3 bits we're
+		     * interested in so we don't have to mask unrelated
+		     * bits. We can test for equality.
 		     */
-		    if (pattern.leftLen == 0)
-			pattern.flags &= ~VAR_SUB_GLOBAL;
+		    if (!pattern.leftLen && pattern.flags == VAR_SUB_GLOBAL)
+			Fatal("Global substitution of the empty string");
 
 		    termc = *cp;
 		    newStr = VarModify(str, VarSubstitute,