[Bug runtime/26844] New: off-by-one error in the output of sprint_ubacktrace()

["agentzh at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=26844

            Bug ID: 26844
           Summary: off-by-one error in the output of sprint_ubacktrace()
           Product: systemtap
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: runtime
          Assignee: systemtap at sourceware dot org
          Reporter: agentzh at gmail dot com
  Target Milestone: ---

Consider the following C program for the target program:

```
int foo(void) {
    return 3;
}

int bar(void) {
    return foo();
}

int main(void) {
    bar();
    return 0;
}
```

Compile it to generate the executable file `./a.out`:

```
gcc -g a.c
```

And then run the following stap oneliner:

```
$ stap -e 'probe oneshot { printf("{%s}\n", sprint_ubacktrace()) }'
{foo+0x4 [a.out]
bar+0x9 [a.out]
main+0x9 [a.out]
0x7ffff7a3b11b [libc-2.27.so+0x2311b]}
WARNING: Missing unwind data for a module, rerun with 'stap -d
/usr/lib64/libc-2.27.so'
```

The warning can be safely ignored. What's unexpected here is the lack of
newline at the end of the last frame in the generated backtrace (between the
bytes `]` and `}` in the output above).

The current master of the upstream systemtap git repo also has the same
problem. We should commit any fixes to the upstream as well for this.

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