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This can come up, for example, if the terminal receives a ^E character and has an empty answerback string configured. Without this early return, we append zero bytes to ldisc's ordinary bufchain input_queue, which is harmless; but we also append a zero-length record to ldisc's list of (type, length) chunks describing which parts of the input bufchain should be treated as interactive or as coming from special dedicated keystrokes (e.g. telling Return apart from ^M). That zero-length record is not _immediately_ harmful, but when the user next presses a key, it will have a different type from the empty answerback data, so that another chunk record is appended to the list after the zero-length one. And then ldisc_input_queue_callback goes into a tight loop, because it keeps trying to consume bytes from the start of the input bufchain but bounding the size at the length of the first (type, length) chunk, which is zero. So it consumes 0 bytes, finds the bufchain still isn't empty, and loops round again.
PuTTY source code README
========================
This is the README for the source code of PuTTY, a free Windows and
Unix Telnet and SSH client.
PuTTY is built using CMake <https://cmake.org/>. To compile in the
simplest way (on any of Linux, Windows or Mac), the general method is
to run these commands in the source directory:
cmake .
cmake --build .
These commands will expect to find a usable compile toolchain on your
path. So if you're building on Windows with MSVC, you'll need to make
sure that the MSVC compiler (cl.exe) is on your path, by running one
of the 'vcvars32.bat' setup scripts provided with the tools. Then the
cmake commands above should work.
To install in the simplest way on Linux or Mac:
cmake --build . --target install
On Unix, pterm would like to be setuid or setgid, as appropriate, to
permit it to write records of user logins to /var/run/utmp and
/var/log/wtmp. (Of course it will not use this privilege for
anything else, and in particular it will drop all privileges before
starting up complex subsystems like GTK.) The cmake install step
doesn't attempt to add these privileges, so if you want user login
recording to work, you should manually ch{own,grp} and chmod the
pterm binary yourself after installation. If you don't do this,
pterm will still work, but not update the user login databases.
Documentation (in various formats including Windows Help and Unix
`man' pages) is built from the Halibut (`.but') files in the `doc'
subdirectory. If you aren't using one of our source snapshots,
you'll need to do this yourself. Halibut can be found at
<https://www.chiark.greenend.org.uk/~sgtatham/halibut/>.
The PuTTY home web site is
https://www.chiark.greenend.org.uk/~sgtatham/putty/
If you want to send bug reports or feature requests, please read the
Feedback section of the web site before doing so. Sending one-line
reports saying `it doesn't work' will waste your time as much as
ours.
See the file LICENCE for the licence conditions.