gdb/testsuite: rewrite capture_command_output proc

I noticed a test failure in gdb.base/completion.exp for RISC-V on a
native Linux target.  Upon investigation I discovered a couple of
reasons for the failure, this commit addresses one of them.  A later
commit will address the other issue.

The completion.exp test makes use of the capture_command_output proc
to collect the output of the 'maint print registers' command.  For
RISC-V this command produces a lot of output.

Currently the capture_command_output proc tries to collect the
complete command output in a single expect buffer, and what I see is
an error caused by the expect buffer becoming full.

This commit rewrites capture_command_output to make use of
gdb_test_multiple to collect the command output line at a time, in
this way we avoid overflowing the expect buffer.

The capture_command_output proc has some logic for skipping a prefix
pattern, which is passed in to the proc as an argument.  In order to
handle this correctly (only matching the prefix at the start of the
command output), I use two gdb_test_multiple calls, the first spots
and discards the echoed command and the (optional) prefix pattern, the
second gdb_test_multiple call then collects the rest of the command
output line at a time until a prompt is seen.

There is one slight oddity with the current implementation, which I
have changed in my rewrite, this does, slightly, change the behaviour
of the proc.

The current implementation uses this pattern:

  -re "[string_to_regexp ${command}]\[\r\n\]+${prefix}(.*)\[\r\n\]+$gdb_prompt $"

Now a typical command output will look like this:

  output here\r\n
  (gdb)

As the TCL regexp matching is greedy, TCL will try to match as much as
possible in one part of the pattern before moving on to the next.
Thus, when this matches against (.*)[\r\n]+, the (.*) will end up
matching against 'output here\r' and the [\r\n]+ will match '\n' only.

In short the previous implementation would leave the '\r' on the end
of the returned text, but not the final trailing '\n'.

Now clearly I could make a new version of capture_command_output that
maintained this behaviour, but I couldn't see any reason to do this.
So, my new implementation drops the final '\r\n' completely, in our
example above we now return 'output here' with no '\r'.

This change doesn't seem to affect any of the existing tests, but I
thought it was worth mentioning.

diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index 5ab4df1bcf3c606d88b35a7dd84b01248ae86f24..432ed5e34ca23e61868b6929db8fdd3dcc3e0bff 100644 (file)
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -7857,12 +7857,41 @@ proc capture_command_output { command prefix } {
     global gdb_prompt
     global expect_out
 
+    set code {
+       -re "^[string_to_regexp ${command}]\r\n" {
+           if { $prefix != "" } {
+               exp_continue
+           }
+       }
+    }
+
+    if { $prefix != "" } {
+       append code {
+           -re "^${prefix}" {
+               # Nothing, we just move onto the next gdb_test_multiple
+               # call, which actually collects the command output.
+           }
+       }
+    }
+
+    gdb_test_multiple "$command" "capture_command_output for $command" $code
+
     set output_string ""
-    gdb_test_multiple "$command" "capture_command_output for $command" {
-       -re "[string_to_regexp ${command}]\[\r\n\]+${prefix}(.*)\[\r\n\]+$gdb_prompt $" {
-           set output_string $expect_out(1,string)
+    gdb_test_multiple "" "" {
+       -re "^(\[^\r\n\]+\r\n)" {
+           if { ![string equal $output_string ""] } {
+               set output_string [join [list $output_string $expect_out(1,string)] ""]
+           } else {
+               set output_string $expect_out(1,string)
+           }
+           exp_continue
+       }
+
+       -re "^$gdb_prompt $" {
        }
     }
+
+    set output_string [regsub "\r\n$" $output_string ""]
     return $output_string
 }