[binutils-gdb] gdb: Handle requests to print source lines backward

[Andrew Burgess]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=ec98a4ad5bfcba33deb5cb786c023082adbbfb46

commit ec98a4ad5bfcba33deb5cb786c023082adbbfb46
Author: Andrew Burgess <andrew.burgess@embecosm.com>
Date:   Sun Jan 6 23:53:22 2019 +0000

    gdb: Handle requests to print source lines backward
    
    ...by which I mean from high line number to low, not, actually
    backward character by character!
    
    Commit:
    
      commit 62f29fda90cf1d5a1899f57ef78452471c707fd6
      Date:   Tue Oct 9 22:21:05 2018 -0600
    
          Highlight source code using GNU Source Highlight
    
    introduced a regression in the test gdb.linespec/explicit.exp, in
    which a request is made to GDB to print a reverse sequence of lines,
    from +10 to -10 from the current line number.  The expected behaviour
    is that GDB prints nothing.  The above commit changed this so that GDB
    now prints:
    
      Line number 32 out of range; /path/to/gdb/testsuite/gdb.linespec/explicit.c has 71 lines.
    
    which is a little confusing.
    
    This commit fixes the regression, and restores the behaviour that GDB
    prints nothing.
    
    While I was passing I noticed a call to `back` on a std::string that I
    was concerned could be empty if the request for source lines returns
    an empty string.  I don't know if it would be possible for a request
    for lines to return an empty string, I guess it should be impossible,
    in which case, maybe this should be an assertion, but adding a `empty`
    check, seems like an easy and cheap safety net.
    
    gdb/ChangeLog:
    
    	* source.c (print_source_lines_base): Handle requests to print
    	reverse line number sequences, and guard against empty lines
    	string.

Diff:
---
 gdb/ChangeLog |  6 ++++++
 gdb/source.c  | 12 +++++++++++-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 3ea7576..2ee19f3 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,11 @@
 2019-01-08  Andrew Burgess  <andrew.burgess@embecosm.com>
 
+	* source.c (print_source_lines_base): Handle requests to print
+	reverse line number sequences, and guard against empty lines
+	string.
+
+2019-01-08  Andrew Burgess  <andrew.burgess@embecosm.com>
+
 	* source.c (print_source_lines_base): Fix skip of '\r' if next
 	character is '\n'.
 
diff --git a/gdb/source.c b/gdb/source.c
index e77789c..71da396 100644
--- a/gdb/source.c
+++ b/gdb/source.c
@@ -1346,6 +1346,16 @@ print_source_lines_base (struct symtab *s, int line, int stopline,
 
   last_source_error = 0;
 
+  /* If the user requested a sequence of lines that seems to go backward
+     (from high to low line numbers) then we don't print anything.
+     The use of '- 1' here instead of '<=' is currently critical, we rely
+     on the undefined wrap around behaviour of 'int' for stopline.  When
+     the use has done: 'set listsize unlimited' then stopline can overflow
+     and appear as MIN_INT.  This is a long-standing bug that needs
+     fixing.  */
+  if (stopline - 1 < line)
+    return;
+
   std::string lines;
   if (!g_source_cache.get_source_lines (s, line, stopline - 1, &lines))
     error (_("Line number %d out of range; %s has %d lines."),
@@ -1392,7 +1402,7 @@ print_source_lines_base (struct symtab *s, int line, int stopline,
       if (c == '\0')
 	break;
     }
-  if (lines.back () != '\n')
+  if (!lines.empty() && lines.back () != '\n')
     uiout->text ("\n");
 }