[binutils-gdb] Use noncapturing subpattern/parens in gdb_test implementation

[Kevin Buettner <kevinb at sourceware dot org>]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=75312ae3ab577c3a72ada54512533fec8bfae7bb

commit 75312ae3ab577c3a72ada54512533fec8bfae7bb
Author: Kevin Buettner <kevinb@redhat.com>
Date:   Fri Jun 2 17:58:12 2017 -0700

    Use noncapturing subpattern/parens in gdb_test implementation
    
    This is the portion of gdb_test which performs the match against
    the RE (regular expression) passed to it:
    
        return [gdb_test_multiple $command $message {
            -re "\[\r\n\]*($pattern)\[\r\n\]+$gdb_prompt $" {
                if ![string match "" $message] then {
                    pass "$message"
                }
            }
    
    In a test that I've been working on recently, I wanted to use
    a backreference - that's the \1 in the the RE below:
    
    gdb_test "info threads"  \
    	{.*[\r\n]+\* +([0-9]+) +Thread[^\r\n]* do_something \(n=\1\) at.*}
    
    Put into English, I wanted to make sure that the value of n passed to
    do_something() is the same as the thread number shown in the "info
    threads" Id column.  (I've structured the test case so that this
    *should* be the case.)
    
    It didn't work though.  It turned out that ($pattern) in the RE
    noted above is capturing the attempted backreference.  So, in this
    case, the backreference does not refer to ([0-9]+) as intended, but
    instead refers to ($pattern).  This is wrong because it's not what I
    intended, but is also wrong because, if allowed, it could only match a
    string of infinite length.
    
    This problem can be fixed by using parens for a "noncapturing
    subpattern".  The way that this is done, syntactically, is to use
    (?:$pattern) instead of ($pattern).
    
    My research shows that this feature has been present since tcl8.1 which
    was released in 1999.
    
    The current tcl version is 8.6 - at least that's what I have on my
    machine.  It appears to me that mingw uses some subversion of tcl8.4
    which will also have this feature (since 8.4 > 8.1).
    
    So it seems to me that any platform upon which we might wish to test
    GDB will have a version of tcl which has this feature.  That being the
    case, my hope is that there won't be any objections to its use.
    
    When I looked at the implementation of gdb_test, I wondered whether
    the parens were needed at all.  I've concluded that they are.  In the
    event that $pattern is an RE which uses alternation at the top level,
    e.g. a|b, we need to make $pattern a subpattern (via parens) to limit
    the extend of the alternation.  I.e, we don't want the alternation to
    extend to the other portions of the RE which gdb_test uses to match
    potential blank lines at the beginning of the pattern or the gdb
    prompt at the end.
    
    gdb/testsuite/ChangeLog:
    
    	* gdb.exp (gdb_test): Using noncapturing parens for the $pattern
    	subpattern.

Diff:
---
 gdb/testsuite/ChangeLog   | 5 +++++
 gdb/testsuite/lib/gdb.exp | 2 +-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 5b71fec..253e229 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2017-06-21  Kevin Buettner  <kevinb@redhat.com>
+
+	* gdb.exp (gdb_test): Using noncapturing parens for the $pattern
+	subpattern.
+
 2017-06-19  Peter Bergner  <bergner@vnet.ibm.com>
 
 	* gdb.arch/powerpc-power9.exp: Update test case for new lnia
diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index a74f974..3d3eaab 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -1000,7 +1000,7 @@ proc gdb_test { args } {
     }
 
     return [gdb_test_multiple $command $message {
-	-re "\[\r\n\]*($pattern)\[\r\n\]+$gdb_prompt $" {
+	-re "\[\r\n\]*(?:$pattern)\[\r\n\]+$gdb_prompt $" {
 	    if ![string match "" $message] then {
 		pass "$message"
             }