PuTTY is tiny, compared to many other Windows applications. And it's easy to install: it depends on no DLLs, no other applications, no service packs or system upgrades. It's just one executable. You install that executable wherever you want to, and run it.
We want to keep both these properties - the small size, and the ease of installation - if at all possible. So code contributions that depend critically on external DLLs, or that add a huge amount to the code size for a feature which is only useful to a small minority of users, are likely to be thrown out immediately.
We do vaguely intend to introduce a DLL plugin interface for PuTTY, whereby seriously large extra features can be implemented in plugin modules. The important thing, though, is that those DLLs will be optional; if PuTTY can't find them on startup, it should run perfectly happily and just won't provide those particular features. A full installation of PuTTY might one day contain ten or twenty little DLL plugins, which would cut down a little on the ease of installation - but if you really needed ease of installation you could still just install the one PuTTY binary, or just the DLLs you really needed, and it would still work fine.
Depending on external DLLs is something we'd like to avoid if at all possible (though for some purposes, such as complex SSH authentication mechanisms, it may be unavoidable). If it can't be avoided, the important thing is to follow the same principle of graceful degradation: if a DLL can't be found, then PuTTY should run happily and just not supply the feature that depended on it.
D.8 Single-threaded code
PuTTY and its supporting tools, or at least the vast majority of them, run in only one OS thread.
This means that if you're devising some piece of internal mechanism, there's no need to use locks to make sure it doesn't get called by two threads at once. The only way code can be called re-entrantly is by recursion.
That said, most of Windows PuTTY's network handling is triggered off Windows messages requested by WSAAsyncSelect(), so if you call MessageBox() deep within some network event handling code you should be aware that you might be re-entered if a network event comes in and is passed on to our window procedure by the MessageBox() message loop.
Also, the front ends (in particular Windows Plink) can use multiple threads if they like. However, Windows Plink keeps very tight control of its auxiliary threads, and uses them pretty much exclusively as a form of select(). Pretty much all the code outside windows/winplink.c is only ever called from the one primary thread; the others just loop round blocking on file handles and send messages to the main thread when some real work needs doing. This is not considered a portability hazard because that bit of windows/winplink.c will need rewriting on other platforms in any case.
One important consequence of this: PuTTY has only one thread in which to do everything. That ‘everything’ may include managing more than one login session (
section D.3
), managing multiple data channels within an SSH session, responding to GUI events even
when nothing is happening on the network, and responding to network requests from the server (such as repeat key exchange) even when the program is dealing with complex user interaction such as the re-configuration dialog box. This means that almost none of the PuTTY code can safely block.
D.9 Keystrokes sent to the server wherever possible
In almost all cases, PuTTY sends keystrokes to the server. Even weird keystrokes that you think should be hot keys controlling PuTTY. Even Alt-F4 or Alt-Space, for example. If a keystroke has a well-defined escape sequence that it could usefully be sending to the server, then it should do so, or at the very least it should be configurably able to do so.
To unconditionally turn a key combination into a hot key to control PuTTY is almost always a design error. If a hot key is really truly required, then try to find a key combination for it which isn't already used in existing PuTTYs (either it sends nothing to the server, or it sends the same thing as some other combination). Even then, be prepared for the possibility that one day that key combination might end up being needed to send something to the server - so make sure that there's an alternative way to invoke whatever PuTTY feature it controls.
D.10 640×480 friendliness in configuration panels
There's a reason we have lots of tiny configuration panels instead of a few huge ones, and that reason is that not everyone has a 1600×1200 desktop. 640×480 is still a viable resolution for running Windows (and indeed it's still the default if you start up in safe mode), so it's still a resolution we care about.