Re: [PATCH v12] gdb: Add source-tracking breakpoints feature

[Andrew Burgess <aburgess@redhat.com>]

Alexandra Hájková <ahajkova@redhat.com> writes:

> When we rerun the executable after changing its source files,
> GDB would re-set all previously set breakpoints. The
> breakpoints set to the function names would remain at their initial
> locations. But the breakpoints which used filename:line notation would
> be silently shifted following the source code changes.
>
> To address this, GDB now optionally captures a small window of source
> code lines around each breakpoint set with filename:line notation,
> when it is first set. When the binary is reloaded, GDB detects the BFD
> change and tries to locate the same source context in the new file,
> and if successful, re-sets the breakpoint to the matched source code
> line.
>
> The breakpoint_source structure stores captured source code lines
> around a breakpoint location, along with a reference to the BFD
> that was current when the source was captured.
>
> When source tracking is enabled (via 'set breakpoint source-tracking
> enabled on'), GDB captures 3 lines of source context
> (BREAKPOINT_SRC_CTX_LINES) along with the current BFD when a
> breakpoint is first set.  On executable reload (detected by comparing
> BFDs), it searches within a 12-line window
> (BREAKPOINT_SRC_CTX_LINES * BREAKPOINT_SRC_SEARCH_MULTIPLIER) for
> the best match and adjusts the breakpoint location if needed.
>
> If source tracking is disabled after breakpoints have been tracked,
> all existing source tracking information is discarded and a message
> is printed.
>
> Tests added:
>   gdb.base/adjust_breakpoint.exp
>   gdb.base/adjust_breakpoint-missing-source.exp
>   gdb.base/source-tracking-inline.exp
>   gdb.base/test_source_tracking.exp
>
> adjust_breakpoint.exp covers four scenarios:
>   - adjust the breakpoint when lines are deleted
>   - adjust the breakpoint when lines are inserted
>   - the tracked line disappears entirely
>   - verify the tracking can be disabled
>
> adjust_breakpoint-missing-source.exp covers the edge case where source
> files are unavailable, verifying GDB falls back to non-tracking breakpoints.
>
> source-tracking-inline.exp covers source tracking with inline functions.
>
> test_source_tracking.exp verifies that source context is correctly captured
> when the breakpoint is on the last line of the file.
>
> Add maintenance command to print tracked source code.
> Add documentation for the new source-tracking breakpoints feature.
>
> Limitations of the current implementation:
>
> Source tracking is not enabled for pending breakpoints that become
> non-pending.  When a breakpoint is created pending (e.g. with 'set
> breakpoint pending on'), source context is not captured at creation
> time since no symtab is available yet.  When the breakpoint later
> resolves to a location, re_set_default() only updates existing tracked
> breakpoints and does not initiate tracking for newly resolved ones.
> This could be fixed in the future by initiating source tracking in
> re_set_default() when a breakpoint transitions from pending to
> non-pending.
>
> Source tracking for ranged breakpoints is not currently supported.
> Ranged breakpoints have a start and end location spec, and tracking
> both independently raises questions about whether to preserve the
> range length or track each end separately.  For now, ranged
> breakpoints will never be source-tracked.
>
> Reviewed-By: Eli Zaretskii <eliz@gnu.org>
> ---
>  gdb/NEWS                                      |  19 +
>  gdb/breakpoint.c                              | 488 ++++++++++++++++++
>  gdb/breakpoint.h                              |  14 +
>  gdb/doc/gdb.texinfo                           |  44 ++
>  .../gdb.base/adjust_breakpoint-2.cpp          |  39 ++
>  .../gdb.base/adjust_breakpoint-3.cpp          |  41 ++
>  .../gdb.base/adjust_breakpoint-4.cpp          |  37 ++
>  .../adjust_breakpoint-missing-source.exp      |  55 ++
>  gdb/testsuite/gdb.base/adjust_breakpoint.cpp  |  40 ++
>  gdb/testsuite/gdb.base/adjust_breakpoint.exp  | 167 ++++++
>  .../gdb.base/source-tracking-inline-1.c       |  50 ++
>  .../gdb.base/source-tracking-inline-2.c       |  50 ++
>  .../gdb.base/source-tracking-inline.exp       |  80 +++
>  gdb/testsuite/gdb.base/test_source_tracking.c |  18 +
>  .../gdb.base/test_source_tracking.exp         |  42 ++
>  15 files changed, 1184 insertions(+)
>  create mode 100644 gdb/testsuite/gdb.base/adjust_breakpoint-2.cpp
>  create mode 100644 gdb/testsuite/gdb.base/adjust_breakpoint-3.cpp
>  create mode 100644 gdb/testsuite/gdb.base/adjust_breakpoint-4.cpp
>  create mode 100644 gdb/testsuite/gdb.base/adjust_breakpoint-missing-source.exp
>  create mode 100644 gdb/testsuite/gdb.base/adjust_breakpoint.cpp
>  create mode 100644 gdb/testsuite/gdb.base/adjust_breakpoint.exp
>  create mode 100644 gdb/testsuite/gdb.base/source-tracking-inline-1.c
>  create mode 100644 gdb/testsuite/gdb.base/source-tracking-inline-2.c
>  create mode 100644 gdb/testsuite/gdb.base/source-tracking-inline.exp
>  create mode 100644 gdb/testsuite/gdb.base/test_source_tracking.c
>  create mode 100644 gdb/testsuite/gdb.base/test_source_tracking.exp
>
> diff --git a/gdb/NEWS b/gdb/NEWS
> index bce3b358c3f..13437acbcec 100644
> --- a/gdb/NEWS
> +++ b/gdb/NEWS
> @@ -47,6 +47,25 @@ info locals
>    The new "-shadowed on|off" option overrides the "set print shadowed"
>    setting for a single invocation of the command.
>  
> +* GDB now supports source-tracking breakpoints, which automatically
> +  adjust their location when source code changes between rebuilds.
> +  When enabled, file and line breakpoints capture the surrounding
> +  source code context and use it to adjust the breakpoint line if the
> +  source is modified.  Source tracking can be enabled with 'set
> +  breakpoint source-tracking enabled on'.

You've added this into the 'Changed commands' section.

> +
> +* New commands

You've added a second 'New commands' section now, which isn't correct.

> +
> +set breakpoint source-tracking enabled [on|off]
> +show breakpoint source-tracking enabled
> +  Enable or disable source-tracking for file and line breakpoints.
> +  When enabled, breakpoints capture surrounding source code and
> +  automatically adjust their location when the source changes between
> +  recompilations.

Still missing a blank line just here.

> +maintenance info source-tracking-context BPNUM
> +  Displays the captured source context that GDB uses to track and
> +  adjust a breakpoint when source changes.
> +
>  *** Changes in GDB 18
>  
>  * Support for the Common Trace Format (CTF) has been removed.  GDB now


> +  if (new_bp_line != bp_source->bp_line)
> +    {
> +      gdb_printf (_("Breakpoint %d adjusted from line %d to line %d.\n"),
> +		  number, bp_source->bp_line, new_bp_line);
> +      notify_breakpoint_modified (this);
> +    }
> +
> +  bp_source = std::make_unique<breakpoint_source>
> +    (breakpoint_source_capture (expanded, BREAKPOINT_SRC_CTX_LINES));
> +  if (!breakpoint_source_is_tracked (bp_source.get ())) {

The '{' needs to be on a line by itself.

If you can get the NEWS file sorted out, then I think we're basically
ready to get this merged.  I'd like to give this a little time so that
anyone who would like to weigh in has a chance to do so, but I plan to
give an approval for this next week.

Thanks,
Andrew